The International Pacific Research Center (IPRC) was established in the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaii (UH) in October, 1997, under the “U.S.-Japan Common Agenda for Cooperation in Global Perspective” as a collaborative global-change research effort between Japan and the United States. Its purpose is to provide an international research environment dedicated to improving understanding of the nature and predictability of climate variability in the Asia-Pacific sector, including the influences of global environmental change within the region.

The IPRC Science Plan defines the Center’s overall structure. It states the IPRC mission, presents four scientific themes and goals, describes specific objectives, and outlines strategies for attaining them. The goals of each theme are:

· To understand climate variations in the Pacific and Indian Oceans on interannual-to-interdecadal time scales

· To determine the influences on Asia-Pacific climate of western-boundary currents, the Kuroshio/Oyashio Extension system, marginal seas, and the Indonesian Throughflow

· To understand the processes responsible for climatic variability and predictability of the Asia-Australian Monsoon System and its hydrological cycle at intraseasonal through interdecadal time scales

· To identify the relationship between global environmental change and Asia-Pacific climate

These goals contribute to international efforts under the World Climate Research Programme and the International Geosphere-Biosphere Programme.

The Center’s overall research strategy emphasizes diagnostic studies, modeling, and data assimilation, and the IPRC will ensure that the necessary computational resources and data-management capabilities are available for these activities. Formal mechanisms of external guidance and oversight are provided by the IPRC’s Scientific Advisory and Implementation Committees.

II. INTRODUCTION

This Science Plan is intended to be a guidebook for the maintenance and development of scientific activities at the International Pacific Research Center (IPRC). In this section, the IPRC mission is defined, scientific themes and goals are presented, and specific objectives are listed. The overall strategy for attaining these objectives, which emphasizes numerical modeling and data analysis, is presented in Section II. The objectives, their scientific rationales, and specific strategies are then discussed in detail in Sections III-VI. Personnel and infrastructure requirements are presented in Section VII. Mechanisms for internal management and external guidance are outlined in Section VIII. Finally, IPRC activities are placed in the context of other institutions and research programs in Sections IX and X.

A. Mission

The Asia-Pacific sector is subject to substantial short-term natural climate variability, and is both affected by and contributes to global environmental change. Currently, about 57% of the world’s population lives in this region, and these climatic signals cause considerable stress on its inhabitants. Thus, understanding the Asian-Pacific climate system is of great societal importance, and warrants a concentration of scientific effort and resources to accelerate progress. The IPRC is devoted to this research effort. Its mission is:

To provide an international, state-of-the-art research environment to improve understanding of the nature and predictability of climate variability in the Asia-Pacific sector, including regional aspects of global environmental change.

At the present time, the IPRC is the only research organization dedicated to this important mission. As such, it is a necessary component of a complete, global-change research strategy, which must rely on a network of cooperating institutions throughout the globe.

B. Themes

While the Mission of the IPRC is expected to endure, the particular research activities conducted through the IPRC will likely evolve, as deemed appropriate by IPRC scientists, and its Scientific Advisory and Implementation Committees. Current activities are divided into four basic themes:

THEME 1: Indo-Pacific ocean climate

THEME 2: Regional-ocean influences

THEME 3: Asian-Australian Monsoon system

THEME 4: Impacts of global environmental change

Consistent with the IPRC Mission, these themes are focused on Asia-Pacific climate issues, each emphasizing different subregions or processes. They are also important for global climate issues, since some Asia-Pacific phenomena are known to remotely force climate variability outside the region.

The IPRC will strive to contribute to each of these themes. The level of activity within each theme, however will likely vary, however, depending on the areas of expertise of IPRC scientists and on the availability of resources.

C. Goals

Each theme has a goal that broadly outlines its subject matter. Here, the goals of each theme are presented, together with lists of questions that illustrate key scientific issues.

GOAL 1: To understand climate variations in the Pacific and Indian Oceans on interannual-to-interdecadal time scales

Questions to be considered within this broad goal are:

· What are the oceanic and atmospheric processes that determine midlatitude Pacific decadal variability?

· Does the atmosphere respond to midlatitude sea-surface temperature (SST) anomalies, and if so how?

· What are the pathways by which heat and salt are exchanged among the tropical, subtropical and subpolar regions of the Pacific Ocean?

· How do the wind-driven and buoyancy-forced circulations interact to determine these pathways?

· What are the oceanic and atmospheric processes that cause ENSO decadal variability?

· How is the tropical thermocline maintained and perturbed by subtropical forcing? Are equatorial SST anomalies determined in part by subtropical forcing?

· What are the nature and causes of Indian-Ocean SST variability on interannual-to-decadal time scales.

GOAL 2: To determine the influences on Asia-Pacific climate of western-boundary currents, the Kuroshio/Oyashio Extension system, marginal seas, and the Indonesian Throughflow

This goal complements the preceding one. Like Goal 1, it involves understanding fundamental oceanic processes, but it focuses on the western-boundary region of the Pacific Ocean and its marginal seas. Questions that relate to this goal are:

· What is the nature of the ocean-atmosphere interactions in these regional oceans? How strongly do they influence Asia-Pacific climate?

· What determines variations of the Pacific western-boundary currents?

· What role do these currents play in the interactions among the subarctic, subtropical and tropical circulations?

· How does the Pacific general circulation interact with coastal and East Asian marginal seas?

· What factors determine the strength, vertical structure, and water-mass composition of the Indonesian Throughflow?

· What are the dynamic and thermodynamic processes that cause variability in the Kuroshio/Oyashio Extension region? How do the ocean and atmosphere interact in this region, which is known to exhibit large SST and oceanic heat-content anomalies.

· What processes are involved in the formation and circulation of North Pacific Intermediate Water?

GOAL 3: To understand the processes responsible for climatic variability and predictability of the Asia-Australian Monsoon System (A-AMS) and its hydrological cycle at intraseasonal through interdecadal time scales

Questions for this goal can be divided into two major groups, depending on time scale. Questions involving intraseasonal-to-seasonal time scales are:

· Why is the Asian monsoon characterized by sudden changes (singularities) at specific geographic locations?

· What causes active-break sequences of the monsoons?

· How are intraseasonal oscillations (ISOs) related to monsoon onset and withdrawal?

· Why does the equatorial Indian Ocean appear to be a source region for boreal summer ISOs?

· Why do summertime ISOs propagate northward in the Indian Ocean while northwestward over the western North Pacific?

· What are the linkages between ISOs and monsoon interannual variability?

· Are intraseasonal variations of the monsoon predictable?

· What roles do land-atmosphere and ocean-atmosphere interactions have in determining the A-AMS hydrological cycle?

· What roles does the hydrological cycle play in the monsoon active and break periods?

Those that involve interannual-to-interdecadal times scales are:

· What are the dominant modes of natural climatic variability of the A-AMS?

· Are there coupled ocean-atmosphere-land modes of interannual-to-interdecadal variability that are intrinsic to the A-AMS?

· What mechanisms cause interannual-to-interdecadal variations of the monsoons?

· Are remote influences from the Pacific and Indian Oceans important for these variations?

· If so, which oceanic regions are most closely linked with monsoon variability?

· Do monsoonal variations in turn affect remote oceanic regions?

· What are the limits of predictability of the A-AMS climate variability on interannual-to-interdecadal time scales?

· How do monsoon variations affect predictability of global climate variations?

GOAL 4: To identify the relationships between global environmental change and Asian-Pacific climate

Questions for this goal are:

· How does the mean state and variability of the A-AMS depend on the mean state of the global climate system?

· How is Asia-Pacific climate affected by changes in levels of atmospheric CO2 and aerosols, land cover, and land-use practices?

D. Objectives

Each of the themes and associated goals has objectives that define specific scientific activities that are possible to achieve within a finite time interval and with well-defined resources. Objectives for Theme 1 and Goal 1 are:

OBJ. 1.1: Identify the ocean-atmosphere processes that cause decadal variability in the extratropical North Pacific

OBJ. 1.2: Identify the oceanic and atmospheric processes that modulate ENSO on decadal time scales

OBJ. 1.3: Determine the oceanic processes that maintain the Pacific-Ocean general circulation and that cause its climatic variability

OBJ. 1.4: Determine the nature and causes of Indian-Ocean SST variability on interannual-to-decadal time scales

Objectives for Theme 2 and Goal 2 are:

OBJ. 2.1: Determine the processes that maintain the Kuroshio, Oyashio and their Extensions and that cause their climatic variability.

OBJ. 2.2: Identify the processes that maintain the low-latitude western-boundary currents of the Pacific Ocean and that cause their climatic variability.

OBJ. 2.3: Determine the influences of the East-Asian marginal seas and the Indonesian Throughflow on the Asia-Pacific climate system

Objectives for Theme 3 and Goal 3 are:

OBJ. 3.1: Understand the monsoon annual cycle and intraseasonal variability

OBJ. 3.2: Determine the causes of interannual-to-interdecadal variability of the monsoon

OBJ. 3.3: Understand the role of atmosphere-ocean-land interactions in monsoon predictability

OBJ. 3.4: Understand the monsoon hydrological cycle and its impact on Asia-Pacific climate

Objectives for Theme 4 and Goal 4 are:

OBJ. 4.1: Determine the impacts of changing external forcing on Asia-Pacific climate

OBJ. 4.2: Determine the sensitivity of the Asia-Pacific climate system and hydrological cycle to changes in land-surface characteristics

These objectives summarize research activities that are ongoing, or expected to be implemented, at the IPRC. As such, they provide a foundation for planning and decision-making. Their scientific rationales and strategies for achieving them are discussed in Sections III-VI.

III. RESEARCH STRATEGY

A complete strategy for climate research includes: i) short-duration, intensive process studies; ii) long-term observations; iii) diagnostic studies; iv) modeling; and v) prediction. Process studies increase understanding of particular mechanisms believed to be of critical importance. Long-term observations are essential for identifying important modes of climate variability and for model validation. Diagnostic studies look for relationships among different elements of the climate system, providing evidence for or against existing theories or leading to new ones. Models provide a means for testing hypotheses, for identifying key processes, and for suggesting areas of further observational study. Prediction, the raison d’être for climate research, provides the ultimate test of success in all the research areas. Progress is most rapid when there is active interaction among scientists working in all of these areas.

A. IPRC focus

The IPRC research strategy emphasizes two of the above categories, namely, diagnostic studies and modeling, relying on research carried out elsewhere for support in the other areas. For this reason, a continuous communication between IPRC and external scientists working in these other areas is important for the Center’s success (Section IX). Indeed, it is expected that the IPRC will provide a valuable resource for the other climate activities, helping to identify the need for future process studies, to assist in the development of the long-term climate observing system, and to improve climate models used for prediction.

B. Diagnostic studies

Goals of the IPRC diagnostic studies are: i) to augment the basic description of the A-AMS; and ii) to identify basic modes of climate variability. The success of this effort requires easy access to relevant datasets, including historical data (also proxy data), new in-situ and space-based observations, and model-based analyses of observations obtained by data assimilation. To ensure their availability, the IPRC will maintain the Asian-Pacific Data Research Center (see Section VII).

C. Modeling

Overall goals of the IPRC modeling activity are: i) to identify the processes that cause Asia-Pacific climate variability; and ii) to determine how much of this variability is predictable. Modeling activities therefore include: a) development of models that can realistically simulate the oceanic and atmospheric general circulations and their variability; b) the development of coupled models; c) design and implementation of numerical process experiments; and d) use of existing models in collaboration with scientists at other institutions, which will hasten the development of IPRC model development.

This broad range of modeling activities, and the complexity of the phenomena being studied, requires the use of a hierarchy of models of varying types and dynamical sophistication. Such models include:

· Simple and intermediate, oceanic, atmospheric and coupled models to develop and refine hypotheses and to verify results from general circulation models (GCMs)

· High-resolution, limited-domain, oceanic models with specified atmospheric forcing to study regional processes, particularly the role of marginal seas in the A-AMS

· Basin-scale oceanic GCMs with specified atmospheric forcing to examine the effects on Asia-Pacific climate of processes such as the thermohaline circulation, water-mass formation, and remote circulation variations

· Global atmospheric GCMs forced by specified SST fields

· High-resolution, limited-domain atmospheric models embedded in a global GCM

· Coupled, global atmospheric and limited-domain oceanic GCMs to refine parameterizations, determine and reduce systematic errors, and study the role of surface processes

· Coupled, global oceanic and atmospheric GCMs (land, atmosphere, ocean, ice) to study coupled interactions, global/regional processes, and long-term variability

The IPRC will seek to use actively as many of these model types as possible. In the near future, however, the IPRC will not likely have either sufficient manpower or computing resources to maintain all these models, particularly those near the end of the list. (Section VIIC describes the existing computer system and its capabilities.) The IPRC researchers will therefore seek to collaborate with external modeling groups to expand its resources.

D. Data assimilation

Solutions to coupled models imply that slowly evolving oceanic processes are an essential part of the dynamics of long-term variability. It is therefore important to estimate the oceanic state as precisely as possible both to identify key processes and to enhance climate predictability. A large number of improved data sources (e.g., TAO array, satellite-based SST, SSH, winds and VOS XBT/XCTD) and data sets (e.g., Levitus, 1994), da Silva, 1995) have emerged in recent years. Nevertheless, most of these data provide reliable estimates only of the directly measured fields like temperature and salinity, whereas the values of some crucial quantities like sea surface fluxes and transports remain quite uncertain. Furthermore, accuracy of the data (especially ocean-atmosphere fluxes) on climatological time scales is often inadequate to force ocean models reliably during long-term integrations.

Data assimilation (i.e., model/data synthesis) presents a natural way to reduce these data deficiencies, by fitting the data into a dynamically consistent framework using oceanic and atmospheric models. Its benefits include:

· Interpolation and extrapolation of sparse data sets

· Providing a “best” estimate of the oceanic or atmospheric state given the available data

· Validation of models within a framework that allows statistical testing of hypotheses

In addition to these direct benefits, data assimilation often leads to

· Improvement of model parameterizations, by identification of regions of significant model/data error

· Identification of relationships (teleconnections) between aspects of climate system that are not directly observed, by correlation analyses of optimally synthesized fields

· Efficient design of field experiments, by providing an accurate estimation of the oceanic state

The IPRC will utilize data assimilation as a basic research tool in several of its planned activities.

IV. INDO-PACIFIC CLIMATE (THEME 1)

A. Identify the ocean-atmosphere interactions that cause decadal variability in the extratropical North Pacific

1. Observations: At decadal time scales, there are clear associations between oceanic and atmospheric variables in the North Pacific. For example, decadal SST anomalies have their largest amplitudes in the midlatitude North Pacific rather than the tropics, and are linked with the Pacific-North American (PNA) atmospheric pattern (Graham et al., 1994; Deser et al., 1996; Latif and Barnett, 1994). In the northwestern Pacific, the ocean thermal structure lags changes of atmospheric forcing in the central Pacific by up to five years (Miller et al., 1998, Deser et al., 1999). In addition, distinct modes of decadal variability have been identified using a variety of statistical techniques (e.g., Latif and Barnett, 1994, 1996; Latif et al., 1997, Solomon et al. 2002). Recently, two different modes of Pacific decadal variability have been identified by Nakamura et al. (1997) and Nakamura and Yamagata (1999). One is confined to the North Pacific with SST anomalies centered on the Subarctic Front; the other extends into the tropics of both hemispheres with largest SST anomalies on the Subtropical Front. (Nakamura and Yamagata, 1999, also discuss a third mode, but it is concentrated in the midlatitude marginal seas, and so more relevant to the research in Section IVC.)

2.Models: Prominent aspects of Pacific decadal variability have already been simulated in coupled ocean-atmosphere models. In the central North Pacific, decadal SST changes appear to respond primarily to air-sea heat fluxes, and hence are largely forced by internal atmospheric variability (Barnett et al., 1999; Pierce et al., 2001; Schneider et al., 2002). In the Kuroshio-Oyashio extension region, both SST and thermocline-depth anomalies are influenced by ocean dynamics (Pierce et al., 2001), reflecting the delayed adjustment of the thermocline to changes in the basin-wide wind stress (Xie et al., 2000; Seager et al., 2001; Schneider et al., 2002). In a number of coupled model simulations, the passive, non-local oceanic response to stochastic atmospheric forcing accounts for the simulated decadal variability without midlatitude ocean-to-atmosphere feedback (Frankignoul et al., 2000; Schneider et al., 2002). This is in contrast to earlier studies that suggest active ocean to atmosphere coupling (Latif and Barnett, 1994, 1996; Münnich et al., 1997; Xu et al., 1998; Weng and Neelin, 1998; Jin et al., 1998). In this scenario, ocean-atmosphere coupling in the northwestern ocean provides positive feedback, an atmospheric teleconnection (like the PNA) is established that forces the northeastern ocean, and Rossby waves are excited in that region that then radiate back to the northwestern ocean to provide negative feedback.

Other studies suggest that decadal variability originates in the tropics, with higher-latitude variability resulting from ENSO-like atmospheric teleconnections (Yukimoto et al., 1996, 1998; Knutson and Manabe, 1998). A third idea is that the northeastern subtropics is an important region of positive feedback (Kleeman et al., 1999; Solomon et al., 2003), an idea that is supported by the "wave-action" analysis reported in Nakamura and Yamagata (1999). A fourth idea is that global coupled ocean-atmospheric interactions are important for decadal climate variability (Meehl et al., 1998).

3. Hypotheses: Several specific processes have been hypothesized to be involved (Miller and Schneider, 2000). The ‘null’ hypothesis explains the preponderance of low frequency variance as the oceanic response to air-sea fluxes of heat, fresh water or momentum due to intrinsic atmospheric variability. Other hypotheses postulate coupling of the subpolar and subtropical ocean circulations with storm-track variability. Another is changes in the shallow, meridional circulation cells that link the subtropical, tropical and subpolar regions (see Sections IIIB and IIIC). A third invokes coupling to the atmosphere associated with the formation, circulation, and dissipation of North Pacific Intermediate Water (NPIW). A fourth is anomalous oceanic heat storage that feeds back with some delay to produce subtle but persistent atmospheric forcing (Lau and Nath, 1990; Alexander, 1992a,b), possibly with heat-content anomalies being advected around the Pacific gyre circulations (Meehl et al., 1998). In fact, Pacific decadal variability likely involves a number of these, and other, processes that interact in complex ways.

4. Strategy: The IPRC strategy for exploring Pacific decadal variability includes both diagnostic studies and a variety of modeling experiments. Diagnostic studies, which extend previous work like that of Nakamura et al. (1997) and Nakamura and Yamagata (1999), will locate more precisely key regions of ocean-atmosphere interaction and describe modes of variability in greater detail. Based on the historical development of ENSO theory and modeling, it is likely that advances in understanding will emerge from a suite of solutions to a hierarchy of coupled models: Solutions to simple and intermediate models will isolate key processes; solutions to highly resolved, coupled GCMs will validate the assumptions inherent in the simpler systems, and simulate climate variability more accurately. In addition, it will be necessary to conduct experiments with uncoupled models in order to elucidate the individual roles of atmospheric and oceanic processes. For example, how the atmosphere responds to midlatitude SSTAs (if at all) is not well understood (Section C). Studies with atmosphere-only models forced by specified SSTA distributions or surface heat fluxes are needed to resolve this issue. Likewise, studies with ocean-only models are needed to clarify how the ocean responds to midlatitude wind-stress and heat-flux anomalies. In particular, it is important to determine the remote influences of Rossby waves generated by these midlatitude forcings in the eddy-rich North Pacific.

B. Identify the oceanic and atmospheric processes that modulate ENSO on decadal time scales

1. Observations: Decadal variations of ENSO have been extensively documented in recent years (Trenberth and Hurrell, 1994; Graham, 1994; Kleeman et al., 1996; Latif et al., 1997). This interest has been stimulated by the long-lasting negative excursion of the Southern Oscillation in the early 1990s, which among other things has been linked to the devastating drought in Northeast Australia during the same period (Kleeman et al., 1996). At the same time, decadal variability in the North-Pacific midlatitudes has attracted much attention (Trenberth and Hurrell, 1994; Mann and Park, 1996; Nakamura et al., 1997; Nakamura and Yamagata, 1999). Data analyses suggest further that decadal variations in both regions are linked (Latif et al., 1997; Nakamura et al., 1997; Nakamura and Yamagata, 1999; Kleeman and Power, 1998; Meehl et al., 1998), implying that ENSO decadal variability involves interactions between the midlatitudes and tropics.

2. Tropical/midlatitude interactions: At interannual time scales, there is strong evidence that the primary interactions are directed from the tropics to midlatitudes: ENSO is known to be generated primarily by tropical ocean-atmosphere coupling, and this variability is then communicated to higher latitudes largely by atmospheric teleconnections. Indeed, during ENSO events variability of the Pacific North American (PNA) atmospheric teleconnection pattern is remotely forced mainly by anomalous tropical-atmospheric heating in this way. Although the PNA atmospheric teleconnection is the most important pathway, oceanic pathways are likely to be regionally important: One such pathway is poleward of Kelvin waves along the eastern boundary; another is anomalous poleward Ekman transport during ENSO events.

At decadal time scales, the generation region of Pacific climate variability is not yet clearly defined (Section IIIA). Most studies, however, support the idea that it is generated at midlatitudes, and if so then the decadal interactions are directed from midlatitudes to the tropics. Barnett et al. (1999), for example, conclude that a midlatitude-to-tropics teleconnection accounts for the ENSO decadal variability in their coupled solution; specifically, wind anomalies generated at midlatitudes extend far enough into the tropics to force the ocean circulation there. Other researchers have pointed toward oceanic teleconnections as being the interaction mechanism (Gu and Philander, 1997; Kleeman et al., 1999). As discussed next, the basic idea in these studies is that midlatitude processes alter the circulation that carries cool, subsurface water into the tropics, namely, the North Pacific Subtropical Cell (STC).

3. North Pacific STC: Sverdrup theory provides a vertically averaged (barotropic) picture of the Pacific general circulation that nicely illustrates the large-scale horizontal gyres, but it completely masks the baroclinic exchange of water masses between them. A useful way to illustrate these exchanges is by zonally averaging the flow. In the North Pacific, the zonally averaged circulation exhibits two shallow (z < 1000 m) meridional overturning cells. One of them, the North Pacific STC, consists of subduction in the subtropics, equatorward transport of salty thermocline water into the tropics, equatorial upwelling, and a return flow of near-surface water to the subtropics (McCreary and Lu, 1994; Liu et al., 1994; Lu and McCreary, 1995; Hirst et al., 1996; Lu et al., 1998). The other, the North Pacific Subpolar Cell (SPC) involves subtropical subduction, subsurface flow into the subpolar ocean, upwelling there via Ekman suction, and southward flow of near-surface water (see Section IIIC3; McCreary and Lu, 1994; Lu et al., 1998). Zonal averaging is conceptually revealing, but it greatly oversimplifies the actual three-dimensional pathways associated with the various branches of these cells (Lu et al., 1998); see Section IIIC for a brief description of some of these pathways.

4. Mechanisms: In the coupled solution of Kleeman et al. (1999), midlatitude wind-stress anomalies change the transport of the STC. Specifically, they excite Rossby waves that reflect from the western boundary as coastal and equatorial Kelvin waves, thereby propagating midlatitude signals into the tropics; there, they alter the tropical thermal structure, and in particular modify the size and/or strength of the equatorial cold tongue. A similar oceanic teleconnection is present in the coupled solution of Yukimoto et al. (1998). Alternately, Gu and Philander (1997) hypothesized that midlatitude heat-flux anomalies alter the temperature of the equatorward-flowing branch of the STC, and that these anomalies are eventually advected to the equator to affect the cold tongue. This idea is supported by Deser et al. (1996) who reported a subsurface temperature anomaly circulating within the North Pacific STC, and by Bingham and Lukas (1994) who discussed anomalous advective transports of heat and salt by the NEC and the Mindanao Current. On the other hand, Schneider et al. (1999a,b) argue otherwise, commenting that the Gu and Philander (1997) mechanism does not explain much of the observed tropical temperature variability. Their analysis, however, leaves open the possibility that some of the temperature variance is compensated by salinity anomalies such that density remains unchanged: In contrast to the dynamically active temperature anomalies cited above, such salinity-compensated temperature anomalies are a passive tracer in the ocean (Liu and Shin 1999), but, upon upwelling to the surface, they can initiate a coupled adjustment of the tropics (Schneider 2000).

5. Strategy: To understand ENSO decadal variability, it is perhaps most important to obtain solutions to coupled models that can simultaneously simulate both decadal and ENSO variability, and then to diagnose the causes of each oscillation and of the interactions between them. Given the differences among existing coupled solutions it is highly unlikely (at least initially) that any single model will adequately simulate all aspects of Pacific decadal variability. The IPRC strategy will therefore be to utilize a variety of coupled systems, of varying types and degrees of sophistication.

C. Extratropical air-sea interactions and feedbacks

1. Introduction: Ocean-atmosphere interaction and feedback are of fundamental importance for understanding and modeling the climate and its variability. Development of a general understanding for these coupled dynamics is still in an infant stage. For example, processes determining the partition between atmospheric and oceanic transports of heat and fresh water are not understood. Simple balance relations (e.g., Cessi, 2000) give some guidance, but have not been rigorously tested. Decadal variability of the climate system, besides being an important topic in its own right (Sections IIIA and IIIB), offers a window into these coupled dynamics, as low frequency variability is possibly associated with a shift in the roles of the atmosphere and ocean in balancing the earth’s heat budget. This Objective is also closely related to Objectives IIID and IIIE, as its achievement requires knowledge of how the oceanic circulation is maintained and of the processes that drive its variability.

2. Extratropical ocean-to-atmosphere feedback: The atmospheric response to SST or surface heat-flux anomalies lies at the heart of understanding the ocean’s role in climate. Since the ocean performs a significant fraction of the poleward heat transport, this response must be important for the coupled state, at least at low frequencies such as decadal. Most of the published work, however, suggests that the response of the extratropical atmosphere to the ocean is weak and/or difficult to detect (see Kushnir et al., 2002). Recent satellite observations, however, show a strong and ubiquitous response of the atmospheric boundary layer to SST variability associated with oceanic mesoscale activity (Xie et al., 1998; Chelton et al., 2001; Nonaka and Xie, 2002), hinting at an ocean-to-atmosphere feedback. The underlying hypothesis relies on increased vertical mixing in the atmosphere in response to warm anomalies of SST. On the planetary scale, idealized GCM experiments indicate that the extratropical storm tracks respond strongly to SST anomalies, resulting in changes in near-surface baroclinicity (Inatsu et al. 2002). The exact dynamics for the atmospheric adjustments and their influence on the large-scale coupled system are unknown.

3. Coupled response to ocean heat transport: To determine the low-frequency atmospheric response, atmospheric models are typically forced by prescribed SST patterns. This methodology is successful in the tropics, where SST largely determines the state of the atmosphere. In the extratropics, however, this approach is questionable for two reasons. First, mid-latitude SST anomalies are largely a response to atmospheric internal variability, and only in special areas such as the Kuroshio extension does the ocean appear to force low frequency anomalies of SST. Second, the lack of SST adjustment to air-sea heat fluxes unrealistically constrains the atmosphere to the least damped mode of variability (Barsugli and Battisti 1998). An alternative approach prescribes perturbations to the mixed-layer heat budget that reflect anomalies due to ocean heat transports. SST anomalies are allowed to interact freely with the atmosphere through air-sea heat exchange. Yulaeva et al. (2001) and Sutton and Mathieu (2002) utilized this approach in atmospheric GCMs coupled to slab-ocean models, by prescribing anomalous oceanic heat flux convergences. Both found a significant atmospheric response with non-local adjustments of surface temperatures.

4. Strategy: Available data sets, in particular satellite derived products with high time and space coverage, will be used to seek further observational evidence for ocean-to-atmosphere coupling. Fundamental dynamics of the observed feedbacks between SST and the atmospheric boundary layer will be explored using high resolution atmospheric general circulation models forced by observed and idealized, SST or heat-flux anomalies. Implications of this coupling for coupled ocean atmosphere modes and decadal climate variability will be explored using a hierarchy of model integrations, varying from coupled model simulations (albeit of low resolution) to oceanic models that include effects of this coupling in an idealized way. As an example of a model of the latter sort, Behringer et al. (1979) explored how oceanic advection might force the Gulf Stream recirculation through the SST effect on the wind stress; this effect, closely related to the aforementioned satellite observations, leads to a sharpening of the front between the subpolar and subtropical gyres.

The coupled responses to ocean heat fluxes, including the storm track response, will be explored using experiments with AGCMs coupled both to slab and full-physics ocean models. Coupled atmosphere-slab ocean models will be employed to explore the sensitivity of the atmosphere to the changes of the mixed-layer heat budget, for example by including anomalous mixed-layer Ekman advection, a process implicated in decadal North Pacific variability (Miller et al., 1994; Seager et al., 2001; Schneider et al., 2002). These models will also be used to explore the coupled response to prescribed ocean heat fluxes in the Kuroshio-Oyashio extension region, an area of interest to Theme II. Finally, drastic sensitivity experiments will be performed, for example, by removing ocean heat transports altogether (Seager et al. 2002).

Coupled AGCM-slab ocean model experiments will be contrasted with similar solutions to fully coupled ocean-atmosphere GCMs, to help to identify feedbacks of ocean dynamics that are important to coupled variability. For example, is variability in the subtropical gyres or the shallow Subtropical Cells (STCs) essential? Oceanic heat transports (Trenberth and Caron 2001) are largest equatorward of 30 degree of latitude, suggesting an important role of the STCs. In the tropics, the latent-heat flux is the major means for the ocean to balance the solar radiation, resulting in fresh water flux at the surface. The STCs play an important role in the fresh-water transport that is needed to balance this surface flux. Is this fresh-water transport important for the behavior of the coupled system?

D. Determine the oceanic processes that maintain the Pacific-Ocean general circulations and that cause its climatic variability

1. Introduction: As noted in Sections IIIA and IIIB, hypotheses for Pacific decadal variability involve changes in oceanic circulation that extend at least to the depth range of upper-intermediate water (NPIW). Specifically, circulations that have been suggested to be of particular importance are the shallow meridional cells that link the subtropics to tropical and subpolar oceans (the STC and SPC, respectively) and the western-boundary currents associated with them. To achieve the first three objectives of Theme 1, then, it is necessary to understand the processes that account for decadal variations of these basic features of the Pacific general circulation. This, in turn, requires knowledge of the processes that maintain the mean flows as well.

2. Subtropical mean circulation: To illustrate the scope and complexity of the mean flows in the subtropical ocean, consider the actual pathways followed by water in the equatorward branch of the STC as it moves from the subtropics to the equator. After water subducts into the thermocline in the subtropics, it first circulates about the perimeter of the Subtropical Gyre, and then flows westward as part of the North Equatorial Current (NEC). At shallower depths, some of it crosses the latitude band of the ITCZ to flow directly to the equator in the interior ocean (Liu et al., 1994; Rothstein et al., 1998). Most of it, however, flows completely across the basin where it bifurcates near the Philippine coast, feeding the Kuroshio to the north and the Mindanao Current to the south (see Section IVB; Lu and McCreary, 1994; Lu et al., 1998). Some of the Mindanao-Current water joins the NECC, some exits the basin via the Indonesian Throughflow, and some joins the EUC, eventually upwelling in the eastern, equatorial ocean. Note that the Mindanao Current plays a key role in the exchange between the Subtropical and Tropical Gyres; indeed, it is analogous to the upper branch of the Hadley circulation associated with the A-AMS that ties together the atmospheric subtropical and tropical circulations. Some NPIW also participates in this circulation, apparently with most of it leaving the basin in the Throughflow (McCreary and Lu, 1999). The STCs have slowed down for the last 20 years (McPhaden and Zhang), in response to changes of the wind stress (Nonaka et al. 2002).

3. Subpolar mean circulation: Less is known about the three-dimensional structure of the SPC, but like the STC it involves zonal circulations that extend throughout the subpolar and subtropical oceans (Lu et al., 1998). Essentially, the SPC is driven by southward Ekman drift across the southern boundary of the Subpolar Gyre (near the line of zero wind curl). A subsurface, northward flow is generated to balance this surface mass loss, and the SPC is closed via open-ocean upwelling due to Ekman suction by negative wind curl. The subpolar gyre can be roughly divided into the two gyres, namely, the Western Subarctic Gyre to the west of the dateline, and the Alaska Gyre to the east; the two gyres are linked by the Alaskan Stream to the north and (a northern part of) the North Pacific Current to the south. Just where the subsurface flow crosses into subpolar ocean is an open question. In the McCreary and Lu (1994) and Lu et al. (1998) modeling studies, it all occurs via the western-boundary current. Their models, however, lacked a realistic Mixed Water Region and Kuroshio Extension (Section IVA). It is more likely that the actual exchange takes place via eddy processes in the Mixed Water Region, or farther to the east where the Kuroshio Extension jet weakens and broadens to merge with the large-scale, interior Sverdrup circulation (Qu et al., 2000a). Strong evidence in support of the northward-flowing, subsurface SPC branch is the existence of a wintertime temperature inversion, with warm saline water beneath cold fresh surface water (Watanabe, 1998); moreover, the warm subsurface water has a density of 26.7-27.2 sigma theta throughout the Western Subarctic Gyre, a range consistent with that in the northern part of the North Pacific Current (Ueno and Yasuda, 2000). Finally, the mixed-layer thickness has a large annual cycle in the subpolar ocean because of the strong wintertime cooling there. Mixed-layer processes are therefore likely to be an important part of SPC dynamics and thermodynamics. An impediment to progress is that the mixed layer is poorly represented in models of the subpolar region.

4. Variability: Little is known about the nature or causes of decadal variability in such intergyre circulations. Analogies with annual and interannual variations point toward the importance of Rossby-wave propagation, and remote forcing. For example, TOPEX observations show energetic annual Rossby waves west of the dateline (Chelton and Schlax, 1996), likely forced by local trade wind variations and in the eastern ocean; moreover, year-to-year differences in their strength are reflected in the magnitude of the sea-level anomalies associated with them (Mitchum and Lukas, 1990). Likewise, there is evidence that Rossby-wave propagation from the eastern boundary of the basin is involved in Pacific interannual variability (Jacobs et al., 1994). Modeling studies also point to the importance of Rossby-wave propagation in decadal variability (see Sections IIIA and IIIB). As noted in Section IIIB, there are also indications that advection of subsurface temperature and salinity anomalies by the STC, presumably generated earlier by surface heat-flux variations at midlatitudes, is involved.

5. Strategy:

5.1 Types of models: A hierarchy of Pacific-Ocean (and Indo-Pacific) models will be used to address this objective. A major part of the effort, however, will be to develop a highly resolved oceanic GCM that is able to simulate accurately the Pacific currents that are believed to be climatologically important. In particular, GCM solutions will obtained that are able to simulate all possible exchange circulations among the subpolar gyre, subtropical gyre and tropical circulation (such as the STC and SPC noted above). The model will have fine enough resolution to be able to investigate the influences of mesoscale variability on transportation of heat and fresh water fluxes in the surface and intermediate layers.

5.2 Model improvements: Based on prior experience, we expect that solutions to the models will be able to simulate fundamental aspects of the Pacific general circulation, but will likely misrepresent subtler aspects of the flow (such as the subsurface branches of the STC and SPC). A well-known cause of such problems is the parameterization of mixing processes (both vertical and horizontal mixing) and the surface mixed layer, and a concerted effort will be made to improve mixing schemes. In addition, data assimilation will be used to produce a "best" representation of the Pacific general circulation. Solutions with and without assimilated data will then be compared to help identify specific model weaknesses. This effort will require development of data-assimilation methods.

E. Determine the nature and causes of Indian-Ocean SST variability on interannual-to-decadal times scales

1. Introduction: The influence of the Indian Ocean on Asia-Pacific climate is less clear than it is in the Pacific. It is generally felt that it must be important, but there is no dominant Indian-Ocean ocean-atmosphere phenomenon (like ENSO in the Pacific) that irrefutably demonstrates its importance.

2. SST anomalies: Recently, however, there have been a few studies that suggest the importance of Indian Ocean SST anomalies. Nicholls (1989) linked Australian rainfall variability with an SST anomaly dipole pattern, with one center of action over Indonesia and the other in the southeastern tropical Indian Ocean. Strong southern Australian winter rainfall occurs when SST is anomalously warm (cold) in Indonesia (the southeastern Indian Ocean) and vice versa. The time series associated with the dipole is not correlated with the Southern Oscillation Index, and so it is independent of ENSO. There are also considerable interannual SST variations in the Arabian Sea, with cool (warm) SST anomalies corresponding to strong (weak) monsoons. Modeling work, however, suggests that these SSTAs are a response to the monsoon variability, and do not feedback significantly to affect it. Most recently, Krishnan and Mujumdar (1999) have demonstrated that Indian rainfall variability is significantly correlated with SST anomalies in the southwestern Indian Ocean, and Goddard and Graham (1999) have shown that these anomalies affect rainfall in East Africa.

3. Dynamics:

3.1 Climatological situation: The likely reason there is not a prominent ENSO-like response in the Indian Ocean is that the climatological winds are weak and westerly near the equator. Typically, then, the Indian Ocean does not develop a cold tongue in the eastern, equatorial ocean. Indeed, the shallow, meridional circulation cell in the Indian Ocean that corresponds to the Pacific STC is a cross-equatorial cell that involves coastal, rather than equatorial, upwelling along the coasts of Somalia, Oman and India (McCreary et al., 1993).

3.2 Extreme events: There are events, however, when easterly winds do appear along the equator. Their appearance seems to be part of a general northward shift of the southern-hemisphere trades. Occasionally, the easterly anomalies are large enough to produce a remarkably large oceanic response, associated with strong upwelling off Somalia and in the eastern, equatorial ocean, and low sea level throughout the eastern ocean. There have been three major events of this sort in the past 50 years, during 1961, 1994 and 1997, and they were associated with droughts in Indonesia and high rainfall in the western Indian Ocean and Africa (Vinayachandran et al., 1999; Murtugudde et al., 1999). Smaller events have occurred throughout this time period, but were not strong enough to draw the attention of the scientific community. One explanation for such events is that they are a response to forcing external to the Indian Ocean that requires the Southeast Trades to shift northward. Murtugudde et al. (1999) and Yamagata et al. (1999) have suggested that they might also be a natural mode of the climate system intrinsic to the Indian Ocean itself.

4. Strategy: There are beginning to be a number of modeling studies that look at the causes of Indian-Ocean SST anomalies (Murtugudde and Busalacchi, 1999; Behera et al., 1999). The IPRC will participate in this effort using ocean-only and coupled models. The ocean-only models will be forced with a variety of interannual wind and heat-flux products. In addition, it is well established in solutions forced by climatological forcing (e.g., Han et al., 1999) that Indian Ocean SST is sensitive to the parameterization of the surface mixed layer. Therefore, solutions will be obtained in models with different mixed-layer schemes. Coupled systems that explore the influence of Indian-Ocean SST anomalies will be developed using regional Indian-Ocean and global-ocean models.

V. REGIONAL-OCEAN INFLUENCES

The overall goal of this theme is to investigate regional oceanic phenomena that are known (or believed) to be important in the variability of the large-scale oceanic gyres and in climate. These phenomena include eddy-mean current interactions in western-boundary currents, cross-frontal exchange, water-mass formation (e.g., Subtropical Mode Water and North Pacific Intermediate Water), and interactions with coastal/marginal seas. These processes are all involved in intergyre exchange, and inter-ocean exchange in the case of the Indonesian Throughflow. It is expected that this research effort will lead to enhanced predictability of the Pacific large-scale circulation, as well as of the regional ocean circulations themselves (e.g., the Kuroshio meander).

A. Determine the processes that maintain the Kuroshio, Oyashio and their Extensions and that cause their climatic variability

1. Introduction: Western-boundary currents are of special interest for climate issues because they transport so much heat and salt, and because they are primary pathways of intergyre communication. In the North Pacific basin, the Kuroshio and the Oyashio are the western-boundary currents associated with the subtropical and the subpolar gyre, respectively. They join off the coast of Japan, forming the Mixed Water Region where a large amount of intergyre exchange appears to take place (Yasuda et. al., 1996; Ueno and Yasuda, 2000). They then flow eastward to form the Kuroshio and Oyashio Extensions that export heat, salt, and tracers into the ocean interior. Decadal SST variations have been found to be most pronounced in the midlatitude North Pacific, rather than the tropics, particularly in the Kuroshio/Oyashio Extension regions, and they are linked to the PNA (Deser et al., 1996; Yasuda and Hanawa, 1997; Nakamura et al., 1997).

2. Kuroshio: The Kuroshio flows poleward from the NEC bifurcation, transporting a large amount of heat and salt into the midlatitude ocean. Where it flows off the edge of the East China Sea, its transport is about 30 Sv and its variability is relatively small (Kagimoto and Yamagata, 1997). However, where it enters into the Shikoku basin south of Japan, its transport increases to be about 60 Sv on average, and it can sometimes be as large as 100 Sv (Imawaki et al., 1997), indicating the formation of a recirculation gyre there. It is well known that the Kuroshio exhibits a peculiar decadal variability south of Japan, having either a large meander or a straight path. Recently, it was proposed that internal nonlinear dynamics of the subtropical gyre, involving the recirculation gyre, may cause such bimodality at decadal time scales (Qiu and Miao, 2000). Besides this long-term variability, large transport fluctuations are also caused by cyclonic and anticyclonic eddies that originate in the Kuroshio Extension region (Ebuchi and Hanawa, 2000; Mitsudera et al., 2000a) in addition to eddies that originate from the south (Ichikawa, 2000). The eddies collide with the Kuroshio, causing large fluctuations in the Tokara Strait south of Kyushu (Feng et al., 2000), and sometimes trigger meanders (Yoshikawa et al., 1998; Mitsudera et al., 2000a). A recent numerical simulation has reproduced all the observed phases of such meander formation (Waseda et al., 2000a, b, c).

The warm water advected by the Kuroshio produces a sharp ocean-land thermal contrast, thus causing a severe wintertime East-Asian monsoon. Heat is released to the atmosphere in the Kuroshio and adjacent regions, and during the winter this release results in the formation of Subtropical Mode Water (STMW). Recent analysis shows that the core temperature of the STMW has a close relationship with the East Asian monsoon at interannual time scales, while at decadal time scales it is linked to the Subtropical Gyre strength with lag of several years (Hanawa, pers. comm.).

3. Oyashio: The Oyashio transports cold and fresh water from the north. Its water mass originates in part from the Bering Sea, flowing southward as the East Kamchutska Current (approximately 10 Sv), and some from the Sea of Okhotsk (about 3 Sv). The exchange of water between the Pacific Ocean and the Sea of Okhotsk is likely to involve both mesoscale eddies (Yasuda et al., 2000) and strong tidal mixing (Nakamura et al., 2000). An important property of the Okhotsk outflow is its low potential vorticity. The low-potential-vorticity water flows into the Mixed Water Region across the boundary between the subpolar and subtropical oceans, subducts below the Kuroshio Extension, and is an important element in the formation of NPIW (Yasuda et al., 1996). Indeed, in the climatological mean, the Oyashio overshoots the gyre boundary southward by more than 5º (Qu et al., 2000a). The dynamics of this remarkable southward penetration involves a density current associated with the fresh and low-potential-vorticity Okhotsk water over continental slope (Mitsudera et al., 2000b).

The Oyashio also exhibits decadal-to-interdecadal variations in its southward penetration scale, which causes large anomalies in SST and subsurface temperature off the eastern coast of Japan (Sekine, 1988; Miller et al., 1998). These temperature anomalies further impact biological activity and fisheries in the western North Pacific. This variability is linked to the basin-wide wind-stress field, and hence the Oyashio is hypothesized to provide an essential feedback mechanism in the dynamics of North Pacific decadal variability (Section IIIA).

4. Kuroshio and Oyashio Extensions: The Kuroshio Extension is formed when the Kuroshio separates from the Japanese coast at about 35ºN and enters the open basin of the North Pacific. It consists of an eastward-flowing inertial jet accompanied by large-amplitude meanders and vigorous pinched-off eddies. In this region, the Sverdrup constraint on the integrated mass transport no longer holds, due to deep recirculation gyres located south (and possibly north) of the Kuroshio Extension, and the eastward transport of the Kuroshio Extension can attain values three times larger (approximately 130 Sv) than the surface baroclinic transport (Wijffles et al., 1998). Long-term satellite altimeter measurements during the past decade have revealed that its eastward surface transport and zonal-mean position changed coherently on interannual time scales, with a larger eastward transport consistently corresponding to a more northerly position of the Kuroshio Extension and vice versa. In addition, both of these changes appear to be related to the strength of the southern recirculation gyre (Qiu, 2000).

The Oyashio Extension (or the Subpolar Front) extends northeastward from 38ºN near the coast of Japan to 40-45ºN offshore (Qu et al., 2000a), its offshore location being close to the line of zero wind curl, consistent with Sverdrup theory (Hurlbert et al., 1996). Mixed-layer drifters reveal that eastward jet of the Oyashio Extension has a width and surface mean speed similar to those of the Kuroshio Extension, but it is more steady and probably more barotropic (Maximenko et al., 1998, 2000). In contrast to the front across the Kuroshio Extension, the Subpolar Front is density-compensated across the Oyashio Extension, in spite of the large difference in water properties (Yuan and Tally, 1995; Zhang and Hanawa, 1993).

In the Mixed Water Region, subtropical and subpolar waters are mixed and spread in the subtropical gyre to form NPIW. In addition, the Kuroshio Extension broadens as it flows east, consistent with the Sverdrup constraint (Qu et al., 2000a). A part of this water may flow poleward across the gyre boundary to form a subsurface temperature and salinity maximum (mesothermal layer) in the subpolar gyre (Ueno and Yasuda, 2000), providing another pathway of intergyre exchange.

5. Importance: Obtaining a better understanding of the Kuroshio and Oyashio system is important to climate research for a number of reasons.

5.1 Large heat anomalies: The Kuroshio Extension and its recirculation gyre exist in a region where the oceanic transfer of heat to the atmosphere is extremely large. Thus, any large-scale changes in the Kuroshio Extension system are likely to modify the air-sea heat fluxes as well as SST (Qiu, 2000).

5.2 Strong oceanic variability: The Kuroshio/Oyashio region contains strong jets and eddies. In addition to the interannual variability in the eddy field, the Kuroshio, the Oyashio, their Extensions, and recirculation gyres undergo large-scale variations in paths and transports on interannual-to-decadal time scales. The Kuroshio bimodality is one of these events. These variations may be caused by basin-wide wind anomalies (Yasuda and Hanawa, 1997), self-sustained nonlinear dynamics associated with the recirculation gyre (Qiu and Miao, 2000), buoyancy fluxes due to winter cooling (Huang, 1990), or stochastic forcing by mesoscale eddies.

5.3. The STMW heat reservoir: The STMW provides a large heat reservoir, which may influence SST when the STMW is ventilated during winter.

5.4 Exchange pathways: The Mixed Water Region is a major crossroads of oceanic currents, which allows two-way (i.e., both north-to-south and south-to-north) exchange of water between the subtropical and subpolar oceans.

5.5 Mixing processes: Large amplitude fluctuations observed in the vertical profiles of temperature and salinity in the Mixed Water Region suggest that a broad spectrum of processes are active in modifying water properties. These processes include instabilities, breaking waves, geostrophic adjustment, lateral interleaving, and cabbeling (Talley et al., 1995; Maximenko and Shcherbina, 1996).

5.6 Air-sea interactions: There is evidence that interannual climate variations in East Asia and the western North Pacific may be controlled in part by local and simultaneous air-sea interactions over the Kuroshio and Oyashio Extensions, with an atmospheric pattern resembling the Western Pacific pattern (Hanawa et. al., 1989; Wallace et. al., 1990). Recent studies further indicate the importance of air-sea interactions in the Kuroshio-Extension region.

6. Strategy:

6.1. Data analysis: Relatively large amounts of data are accessible in the Kuroshio/Oyashio region. A high-resolution, isopycnally averaged, hydrographic data set is proving to be very helpful in analyzing water-mass properties there (e.g., Qu et al., 2000a). We will combine this historical data set, with XBT data, WOCE data, and various satellite products to investigate interannual-to-decadal variations of water masses, eddies, fronts, and gyres. The combined data set will also be great use for numerical modeling and data assimilation.

6.2. Numerical modeling: The Kuroshio and Oyashio are narrow and strong flows whose variability is influenced by local nonlinear processes as well as by basin-wide, remotely forced variability. Furthermore, the western rim of the Pacific Ocean has complex coastline and bottom-topographic features associated with marginal seas and ridges, so that the interaction with such topographic features and water exchanges are key elements to understand the variability of this region. These complexities require a hierarchy of models, varying from highly simplified, regional systems to highly resolved GCMs that resolve the bathymetry and the narrow jets and cover at least the Pacific Ocean in its entirety. Idealized experiments as well as hindcast and predictability experiments will be conducted. The Kuroshio bimodality and the intensity of the Oyashio southward penetration may be useful measures for evaluating model performance. These studies will allow us not only to assess potential predictability of the ocean, but also lead to model improvements and the identification of key processes.

6.3 Data assimilation: Data synthesis and ocean-state estimation incorporating various assimilation methods with hierarchy of models will also be carried out. The linkage between data and models in data-assimilation studies provides a quantitative means for assessing model performance, for improving model physics (e.g., mixing physics, air-sea interaction processes) and for estimating parameter values.

B. Identify the processes that maintain the low-latitude western-boundary currents of the Pacific Ocean and that cause their climatic variability

1. Introduction: In the Pacific Ocean, the low-latitude western boundary currents (LLWBCs) consist of the Mindanao Current (MC), the New Guinea Coastal Current (NGCC), and the New Guinea Coastal Undercurrent (NGCUC). They have been shown to play an important role in the development of El Niño/Southern Oscillation (ENSO) events (Lukas and Lindstrom, 1991; Webster and Lukas, 1992), and for this reason, considerable attention was paid to them during the TOGA and WOCE programs. Since the early 1990s, interest in LLWBCs has increased further due to the long-lasting negative excursion of the Southern Oscillation, which among other things has been linked to Pacific decadal variability (Trenberth and Hurrel, 1994; Kleeman et al., 1996; Nakamura et al., 1997). Several hypotheses to account for this variability involve changes in the shallow, meridional circulation cells that allow exchange of waters between the main oceanic gyres, namely the STCs (McCreary and Lu, 1994; Liu et al., 1994; see IIIC). Variability in their equatorward-flowing branches have been proposed as a slow oceanic process that provides a link between subtropical and equatorial variability (Gu and Philander, 1997; Kleeman et al., 1999; Nonaka et al., 2000; see IIIA).

2. Overview: In the North Pacific, much of the subsurface STC branch flows to the equator via an LLWBC, namely, the Mindanao Current (MC). This current is fed by the North Equatorial Current (NEC), which bifurcates at the Philippine coast, some flowing equatorward as part of the STC and some returning northward to join the Kuroshio and eventually to recirculate in the Subtropical Gyre. A recent data analysis suggests that the subsurface Luzon Undercurrent along the east coast of Luzon also participates in the equatorward branch of the intermediate layer (Qu et al., 1997, 1999). The latitude of the NEC bifurcation thus provides a possible measure of the strength of the North Pacific STC.

In the southern hemisphere, the LLWBC consists of the South Equatorial Current (SEC) and the NGCC/NGCUC. About half of the subsurface branch flows to the equator via the NGCC/NGCUC, while the rest flows through an interior pathway (Johnson and McPhaden, 1999). The location of the bifurcation determines how much SEC water bends equatorward to flow into the equatorial region and how much turns poleward to return to the subtropics via East Australian Current. More detailed properties of the southern-hemisphere bifurcation are not yet known.

The water in the LLWBCs converges equatorward, and is redistributed in the western equatorial Pacific by the Equatorial Undercurrent (EUC) to form the equatorial thermocline and by the North Equatorial Counter Current (NECC). Another important aspect of the region’s circulation is that some of the water is lost to the Indian Ocean in the Indonesian Throughflow (IT), making the LLWBCs a key component of the global thermohaline circulation (Gordon, 1986; Godfrey et al., 1993). The IT contributes to the heat budget of the Indian Ocean because of its warm temperature (e.g., Qu et al., 1994), and recent studies have revealed its importance in the evolution of “dipole mode” events there (Saji et al., 1999).

3. Goals:

3.1. Intergyre interactions: The latitude of the NEC bifurcation provides a possible measure of the strength of the intergyre interactions. A number of factors will influence the location of the bifurcation latitude, for example, the winds throughout the interior of the Pacific Ocean (McCreary and Lu, 1994), ENSO (Qiu and Lukas, 1996), the Indonesian Throughflow, the South China Sea circulation (e.g., Qu et al., 2000b; Lebedev and Yaremchuk, 2000), and mixing (Miyama et al, 2000). Furthermore, bifurcation of the SEC in the southern hemisphere is much less known.

Under this topic, goals are therefore to: i) describe how fluxes of properties are driven across gyre boundaries, and assess how “leaky” the gyres are; ii) determine how the NEC and SEC bifurcations vary in depth and time; iii) understand how the tropical and subtropical gyres interact near western boundaries where the geometry and mixing processes are highly complicated; and iv) examine how bifurcation variability is related both to local forcing such as the Australasian Monsoon and to remote forcing such as variations in STC strength.

3.2. LLWBC variability: Generally, the accumulated historical hydrographic observations have allowed us to gain a rather reliable view of the mean circulation in the western equatorial Pacific (e.g., Gouriou and Toole, 1993; Fine et al., 1994; Qu et. al., 1999; Lebedev and Yaremchuk, 2000). Their variability, however, has not been adequately observed or modeled with respect to interannual variability, and in some areas not even with respect to the annual cycle. Until recently, there have been almost no moored, direct current measurements in the LLWBCs south of 20°N and west of 147°E, the sole exception being observations of intraseasonal variations of the IT reported by Kashino et al. (1999). To date, there have been no direct time-series measurements of the MC, and even the phase of its annual cycle is unknown. Our best estimate of the mean transport of the MC is based on only 8 quasi-synoptic sections poorly distributed with respect to seasonal and interannual variability (Wijffels et al., 1995). Clearly, further observations and analyses are necessary.

Here, goals are to: i) determine to what extent the steady circulation is consistent with Sverdrup dynamics; ii) study variations in LLWBC transport, determining what drives them and how their dynamics differ in character between seasonal and decadal events; iii) study variations in properties (salinity, tracers) advected by the LLWBCs, in order to determine what role the LLWBCs play in exporting heat and freshwater from the tropics; iv) determine how LLWBC water-mass properties are altered by mixing and/or entrainment; and 5) determine how important mesoscale eddies are in the redistribution of heat, salt and tracers in the western equatorial Pacific and to the IT.

4. Strategy: Developing a better understanding of LLWBCs is thus essential for understanding tropical climate variability, and it has been identified as a key CLIVAR objective. We will carry out a suite of data analyses, numerical modeling and data assimilation studies designed to develop a comprehensive description of the upper- and intermediate-ocean circulation in the tropical western Pacific. Additionally, we will seek to identify key oceanic processes involved in LLWBC variability.

4.1. Data analysis: Data analyses will be conducted to develop a three-dimensional picture of the mean circulation and water mass distribution in the tropical western Pacific (using historical data), to examine their time variability across particular latitude or longitude lines (using repeated hydrographic and/or XBT measurements), and to provide a basis for subsequent model validation. In contrast to previous climatological analyses (e.g., Levitus, 1982), we will: i) average the data along isopycnal rather than pressure surfaces to avoid artificial smoothing of properties at depths near the pycnocline (Gouriou and Toole, 1993); and ii) use a 0.5° by 0.5° grid with an e-folding smoothing scale generally less than 200 km to resolve the LLWBCs adequately. This approach has proven to yield improved results in comparison to averaging along pressure surfaces particularly in the western boundary regions (Qu et al., 2000a).

4.2. Numerical experiments: In order to identify processes that maintain the LLWBCs and that cause their variability, we will carry out various numerical modeling experiments. We are currently implementing POM and several different layer ocean models (LOMs) in an Indo-Pacific basin configuration. Taking advantage of the LOMs’ efficiency, we will conduct a large number of experiments using the LOMs to identify key forcing regions and parameters. The POM version of the model has 41 sigma levels and a telescoping grid as fine as 0.33º in the North Pacific western boundary region and Indonesian Archipelago; the model is designed to focus on western boundary processes. The STC theory of McCreary and Lu (1994), which separates the ocean into various dynamically distinct regimes, provides a guide for these experiments. Furthermore, sensitivity tests to surface forcing fields will be conducted using reanalysis products (ECMWF and NCEP) and satellite products (ERS, QuickSCAT, SSM/I and TRMM). Validation against subsurface observations is initially planned on the subsurface salinity maximum in the subtropical South Pacific Ocean. This maximum extends all the way into the western equatorial Pacific under the surface warm pool where TRITON buoys are making salinity measurements from surface to 750 m. Salinity profiles obtained by ARGO will also be used when available.

4.3. Data assimilation: To estimate and analyze LLWBC variability, we will implement an advanced version of the Singular Evolutive Extended Kalman (SEEK; Pham et al., 1998) filter that is capable of feeding the model with information from various data sources. Wavelet-based error estimation methods will also be incorporated (Jameson and Waseda, 2000). The data sets to be assimilated into the model include SSH from satellite altimeters (TOPEX/JASON); SST from AVHRR and TRMM; temperature, salinity and ADCP data from TRITON/TAO buoys in the equatorial Pacific; drifting-buoy data; and upper-ocean data from XBT/CTDs, ARGO buoys and WOCE sections. The data assimilation will constrain the model toward a more realistic representation of the Pacific circulation, enabling a better understanding of the STCs, bifurcation latitudes, and LLWBCs, and of their influence on the equatorial ocean.

C. Determine the role of the East-Asian marginal seas and of the Indonesian Throughflow on the A-AMS

The North Pacific marginal seas include the Okhotsk Sea, the Japan Sea, the East China and Yellow Seas, and the South China Sea. The variability of these marginal seas is generally as poorly known as for the western-boundary currents. Recent observational programs are beginning to alleviate this situation. Among them, we are primarily interested in the Okhotsk Sea and the South China Sea because these two seas are thought to be potentially important for climate changes.

The Indonesian archipelago, where the Indonesian Through Flow exists, is the most important adjacent sea in the Indo-Pacific region with respect to the global ocean circulation.

1. Marginal seas:

1.1 Okhotsk Sea: The Okhotsk Sea has peculiar water properties compared with other marginal seas of the western Pacific. There is a temperature minimum at about 100m deep due to winter convection on the shelf, when relatively fresh water caused by river influx occupies the upper 100m, followed by sharp halocline. Drifting ice covers the sea for almost six months because of the upper fresh water and severe winter monsoon. Knowledge of the circulation in this sea is still incomplete. Variability is almost completely unknown except for that of ice coverage observed from satellites.

The Okhotsk Sea is an important sea in relation to climate from several reasons. First, it is thought to provide a source water of NPIW, which is an important component of inter-gyre exchange between the subtropical and the subarctic gyres. The Sea is connected to the Pacific Ocean through several deep straits, among which the deepest one is the Bussol’ Strait at 46.5N which accounts for 40% of the outflow transport across the Kuril islands. Low potential vorticity (PV) mode water is formed in the vicinity of the islands that flows out from the straits forming eddies, leading to a density (or PV anomaly)-driven current of Oyashio (Yasuda et. al., 1999). The low potential vorticity water is hypothesized to form due to either winter convection or tidal mixing. Second, SST near the Bussol Strait becomes as low as 5ºC even during the summer. This low temperature is likely to be an appearance of subsurface water due to tidal mixing (Awaji, personal communication). Coupling with northeasterly winds associated with an atmospheric blocking high, called the Okhotsk High, is often situated over the sea during summer. This low SST may cause cold summers, called Yamase, in the Northeastern area of Japan (Rikiishi and Iida, 1990). Third, there are substantial variability in sea ice coverage in the Okhotsk Sea due to wind stress changes. This can cause a large heat flux anomaly from the ocean to the atmosphere in winter, which can then feedback to cause changes in the atmospheric circulation (Honda et. al., 1997).

There are several hydrographic surveys adjacent to the Kuril islands conducted by Japanese Fisheries Agency associated with SAGE. A few current meter measurements also take place in the western boundary of the Okhotsk sea by Japanese researchers.

1.2 South China Sea: The South China Sea is believed to have a particularly important influence on the A-AMS. The combination of complex geometry of the basin, together with the overlying reversing monsoon, gives rise to complicated, highly time-dependent circulations with several eddy features. Furthermore, its circulation is influenced strongly by the western Pacific Ocean circulation. Until recently, even the seasonal cycle of its circulation is not fully described (Qu et al., 2000b). Since the South China Sea is an important sea for the monsoon onset, which strongly influences the jet-stream position over East Asia and the Northwest Pacific during summer, mixed-layer dynamics and variability of the circulation due to local and remote forcings should be further explored.

SCSMEX has been conducted in 1998 to investigate air-sea interaction during the monsoon onset in the South China Sea. ATLAS buoys are being moored in the sea by Taiwan researchers as a part of the program.

2. Indonesian Throughflow:

2.1 Importance: The complex straits and seas of the Maritime Continent allow a strongly variable flow from the Pacific to the Indian Ocean, the Indonesian Throughflow (see Lukas et al., 1996, and Godfrey, 1996, for reviews). The Throughflow is dynamically and thermodynamically important because its heat transport alters the heat budget in the western Pacific warm pool as well as in the Indian Ocean. Furthermore, it is an important component of the global ocean thermohaline circulation (Gordon, 1986). The observed link between summer-monsoon rainfall and SST anomalies in the eastern Indian Ocean north of Australia (Nicholls, 1995) suggests a linkage between the monsoons and Throughflow variations.

2.2 Modeling background: A number of modeling studies have explored the influence of the Indonesian Throughflow on Pacific and Indian Ocean circulations. Hirst and Godfrey (1993) and by Vershell et al. (1995) conducted numerical experiments using global ocean models with and without the Throughflow and reported significant differences in both basins. Schneider (1998) compared solutions to a coupled ocean-atmosphere GCM with and without the Throughflow. With the Throughflow, SST increased in the eastern Indian Ocean and decreased in the equatorial Pacific. These changes shifted the western Pacific warm pool and centers of atmospheric deep convection westward, affecting atmospheric pressure throughout the tropics and, via atmospheric teleconnections, in midlatitudes as well. As a result, the surface wind stress changed in the tropics, strengthening the tropical currents in the Pacific Ocean and weakening them in the Indian Ocean. However, the precise role of the Indonesian Throughflow in causing these changes was not addressed because of the model limitation.

3. Strategy: There are factors peculiar to the marginal seas and the Indonesian Throughflow that make them particularly difficult to model. First, since one goal of this research topic is to study interactions with the Pacific large-scale flows, the models must include the Pacific basin and in the case of the Indonesian Throughflow the Indian Ocean as well. Second, they must all have a high horizontal resolution in order to resolve the complex bathymetry of the marginal seas. (The recent experiments of Metzger and Hurlburt, 1996, underscore the potentially important influence that complex bathymetry can have on oceanic circulations.) Third, they must also have sufficient vertical resolution to be able to handle properly the strong vertical shears that can develop due to complex topography and small straits. High vertical resolution is also needed to adequately model the very thin surface mixed layer that can result from intense rainfall and river runoff. Both high vertical and horizontal resolution is needed to realistically simulate the intense coastal upwellings that occur in various locations during different phases of the monsoon. Fourth, the ocean model must include a realistic surface mixed layer, that is able to model effects due to salinity/freshwater flux variations, which are known to influence the upper-ocean heat budget and thus air-sea interaction in these highly convective regions. Finally, exceptionally strong tidal mixing which is observed to occur within the Maritime Continent and neighboring boundary regions, will require the use of a special mixing scheme.

Given this complexity, it is anticipated that model development will be a particular challenge. The IPRC modeling strategy will therefore be to develop several types of models for this purpose. The strategy will also involve close comparison of observations from such field programs as ARLINDO (Indonesian Throughflow program), SCSMEX, and the developing Pacific North Equatorial Current bifurcation experiment. In parallel, marginal-sea SST sensitivity studies using AGCMs, such as the work of Ose et al. (1996) for the South China Sea, will help determine the potential impact of improved marginal sea modeling on the predictability of regional climate variability

VI. ASIAN-AUSTRALIAN MONSOON SYSTEM

The A-AMS is the most energetic component of the Earth’s climate system. It covers the vast monsoon regions of South Asia, East Asia, Australia, East Africa, the tropical Indian and western Pacific Oceans, and the Asian marginal seas (e.g., South China Sea). Its aperiodic and large-amplitude variability has considerable societal and economic impacts on the local inhabitants. There is also increasing evidence that the A-AMS prominently influences global climate. Fluctuations of monsoon rainfall affect the midlatitude jet stream of the winter hemisphere (Lau and Boyle, 1987), and these impacts ripple all the way around the planet.

The East Asia-western Pacific monsoon has unique features and a disproportionate influence on the global climate. The western Pacific is a major heat source for the interannual variations of the global climate system associated with ENSO. East Asia is the core region of the most powerful winter monsoon on the Earth that involves strongest tropical-extratropical interaction. In view of its specific importance to global climate variability and regional economics, the study of the East Asian-Western Pacific monsoon is a distinctive research focus of A-AMS at IPRC.

This section outlines the four objectives of the IPRC monsoon research program. They concern intraseasonal-to-seasonal and interannual-to-interdecadal monsoon variability, and influences of land-atmosphere-ocean interactions and the hydrological cycle.

A. Understand the monsoon annual cycle and intraseasonal variability

1. Annual cycle:

1.1 Background: A thorough description and understanding of the A-AMS climatological annual cycle is necessary background information for the study of monsoon variability. The broad-scale features of the monsoon annual cycle including onset, active and break periods and associated migration of organized convection are well documented, and the roles of the Tibetan Plateau (Li and Yanai, 1996) and of Indian-Ocean SST variations (Shukla and Fennessy, 1994) in the evolution of the South Asian monsoon have also been recognized. Yet, the physical processes that determine many specific and regional features still remain unknown, especially regarding the hydrological cycle.

Heating gradients associated with convection and longwave radiation are the primary driving force for the monsoon circulation. Particular attention should therefore be paid toward identifying the processes that determine the annual march of heat and moisture sources in the region (Yanai and Tomita, 1998). During the onset, development, and withdrawal of the summer monsoon, the convection centers migrate along preferred pathways (Meehl, 1987; Wang, 1994), sometimes continuously but at other times with sudden “jumps” (Tao and Chen, 1987; Nitta, 1987; Lau and Yang, 1996). In some variables and at some locations, the amplitude and phase of these jumps are stable enough from year-to-year to emerge as “Climatological Intraseasonal Oscillations” (Nakazawa, 1992; Ueda et al., 1995; Wang and Xu, 1997). The extreme phases of the phase-locked intraseasonal oscillation are referred to as “singularities” in monsoon climatology (Wang and Xu 1997). Generally, regional convection centers vary in intensity on several time scales, but it is not clear what controls their preferred location or intensity. Webster et al. (1998) proposed that individual centers can reinforce or compete with each other, for example, subsidence associated with one convection center may suppress convection in a neighboring location.

1.2 Questions: A number of related questions need to be addressed in the study of the monsoon annual cycle. They include:

· What controls the migration of the large-scale convective heat source from the equatorial western Indian Ocean across Bay of Bengal toward Indian subcontinent during the monsoon onset? How does this progression differ between weak and strong monsoons?

· Why is the Asian monsoon characterized by sudden changes (singularities) at specific geographic locations?

· What are the radiative impacts of clouds, especially cirrus, on monsoon evolution and intensity? Is cirrus coverage during strong monsoons (strong flow aloft) significantly greater than during weak monsoons?

· How does SST affect the monsoon annual cycle in various regions?

· How important are the East-Asian marginal seas in determining the mean monsoon structure?

· Does the structure of convection, which depends on vertical wind shear and therefore coupling with the ocean, vary significantly between weak and strong monsoons?

2. Intraseasonal variability:

2.1 Background: Dominant modes of monsoon variability during the wet season are “break” and “active” periods with weak and heavy rainfall, respectively (Krishnamurti and Bhalme, 1976; Murakami et al., 1986; Hendon and Liebmann, 1990a,b; Drosdowsky, 1996; Lau and Yang, 1996). These oscillations are closely linked to large-scale atmospheric events that involve northward and/or eastward propagation of rain bands (Yasunari, 1979, 1980; Krishnamurti et al., 1985). These events are commonly referred to as intraseasonal oscillations (ISOs).

The ISOs strongly interact with the monsoon. On the one hand, they exhibit strong seasonality, and hence are regulated by the annual cycle (Madden, 1986; Wang and Rui, 1990a). During boreal winter, the eastward-propagating Madden-Julian (1971, 1972) equatorial mode dominates ISOs. During boreal summer, however, ISO variability is much more complex, appearing to involve the coexistence of several distinct modes: the aforementioned eastward-propagating equatorial mode; a northward-propagating signal primarily in the Indian monsoon region; a westward-propagating off-equatorial mode in the western Pacific and southeast Asia; and a standing oscillation between the equatorial Indian Ocean and western Pacific (Wang and Rui, 1990a; Zhu and Wang, 1993). On the other hand, ISOs also feedback to influence the monsoon on a variety of time scales, including the annual cycle (Murakami et al., 1986). For example, there is an increasing amount of evidence suggesting that ISOs are involved in interannual monsoon variations (Webster, 1998). In the ECMWF model, the dominant EOF pattern of interannual variability is strikingly similar to that for the intraseasonal variability (Ferranti et al., 1997), suggesting that they may be dynamically related. The solution also indicated that year-to-year variations of the intensity and frequency of ISOs contributed to interannual variations of monsoon rainfall. Such multiple-time scale interactions seem to be a basic characteristic of the A-AMS, but the nature of the processes that cause them is not well understood.

2.2 Hypotheses: Various hypotheses have been put forward to account for ISOs of A-AMS.

2.2.1 Local processes: Webster et al. (1983) emphasized the role of the land-surface hydrological cycle in sustaining the northward propagation of the rain bands, but northward propagation also takes place over the ocean.

2.2.2 Tropical-midlatitude interactions: Tropical-midlatitude atmospheric events (such as cold surges) have been linked to ISOs, suggesting they are involved in their generation (Sumathipala and Murakami, 1987; Magana and Yanai, 1991; Meehl et al., 1996).

2.2.3 Ocean-atmosphere interactions: Active ocean-atmosphere coupling on intraseasonal time scales was evident in the TOGA/COARE data (Flatau and Flatau, 1997). The subsequent theoretical analysis of Wang and Xie (1998) and numerical experiments of Waliser et al. (1999) showed that this coupling can enhance ISOs. In addition, the cross-equatorial Ekman transport in the Indian Ocean was shown to oscillate in-phase with the active-break monsoon cycles of the monsoon, suggesting that the monsoon ISO may control upper-ocean heat-content variations at intraseasonal time scales (Loschnigg and Webster, 1996).

2.3 Questions: Fundamental questions that need to be addressed in ISO studies include:

· How are the ISOs related to monsoon onset and withdrawal?

· Why does the equatorial Indian Ocean appear to be a source region for boreal summer ISOs?

· Why do summertime ISOs propagate northward in the Indian Ocean while northwestward over the western North Pacific?

· How do synoptic disturbances interact with ISOs?

· Are ISOs externally forced by tropical-extratropical interactions, or are they an intrinsic mode of the tropical atmosphere?

· What are the linkages between ISOs and monsoon interannual variability?

3. Strategy: To address the questions raised in this subsection, a strategy of combining diagnostic and modeling studies is adopted. Many of the fundamental features of the boreal summer ISOs and its relation to monsoon annual cycle have been ambiguously defined. This calls for a thorough observational analysis. The observational analysis will focus on revealing fundamental characteristics of the boreal summer ISOs and their relation to monsoon subseasonal variability. The NCEP/NCAR and ECWMF reanalysis datasets, the CPC merged analysis of precipitation and other measures of rainfall, and the NCEP SST, will be our primary data sources. Results obtained from these analyses will provide clues for the development of simplified monsoon models and for the validation of existing model control runs. The observational analyses will also facilitate formulation of hypotheses for theoretical study and the design of future numerical experiments.

On the other hand, the nonlinear nature of the A-AMS and its interaction with lower boundary forcing limits the capability of empirical approaches in revealing the cause-effect mechanisms of the complex interaction. Therefore, it is necessary to use numerical models. The modeling effort will focus on increasing our understanding of the physical processes responsible for both the atmospheric internal dynamics and the roles of the lower boundary forcing. The primary models suitable for addressing the problems raised in this subsection are intermediate atmospheric and oceanic models (Wang and Fang, 1996) and atmospheric general circulation and high-resolution regional monsoon models. To identify the variability associated atmospheric internal dynamics, the intermediate atmospheric models and atmospheric GCMs will be run in uncoupled mode with prescribed lower boundary conditions; solutions will be compared for various boundary forcings, to identify roles of SST or land surface forcing in various regions. To understand better specific mechanisms that may at work in a particular phenomenon, simplified theoretical models will also be developed. These simplified models include anomaly models that are suitable for study monsoon dynamics, and two-dimensional coupled models that are suitable for elaborate the northward propagation of the ISO in the monsoon domain.

B. Determine the causes of interannual-to-interdecadal monsoon variability

1. Tropospheric Biennial Oscillation:

1.1 Background: The Tropospheric Biennial Oscillation (TBO) is one of the most pronounced components of Asia-Pacific climate.

1.1.1 Large-scale coherence: It is apparent in rainfall records in India (Mooley and Parthasarathy, 1984), Indonesia (Yasunari and Suppiah, 1988), and East Asia (Chen et al., 1992), appearing as prominent spectral peaks with periods ranging from 2.2 to 3 years. Moreover, rainfall is coherent over large spatial scales in the TBO frequency band, as shown by analyses of global precipitation (Lau and Sheu, 1988). Anomalies of SST and surface winds in the tropical Pacific and Indian Oceans also exhibit coherent variations at TBO time scales (Barnett, 1991; Rasmusson et al., 1990; Ropelewski et al., 1992), but with centers of maximum variability located near the equator not in the summer monsoon regions.

1.1.2 Propagation: The TBO rainfall anomalies have been shown to originate in the South Asia summer monsoon region, and then to propagate southeastward to the western South Pacific (Meehl, 1987). Tropospheric wind and SST anomalies also propagate from the Indian Ocean to the Pacific Ocean (Barnett, 1983), and consequently Indian summer rainfall is best correlated with the central-eastern Pacific SST at the following winter (Yasunari, 1990). These properties suggest that the TBO is an intrinsic mode of oscillation that is tightly phase-locked to the annual cycle.

1.1.3 Relation to ENSO: Finally, the spatial structures of the TBO and of ENSO are extremely similar. This similarity leads to the speculation that the TBO is a component of ENSO in the tropical Indo-Pacific Ocean.

1.2 Hypotheses: A number of hypotheses have been proposed for the cause of the TBO. Meehl (1997) proposed that it is an intrinsic oscillation of the coupled atmosphere-ocean-land system. His hypothesis not only involves local ocean-atmosphere interactions, but also emphasizes the key roles of land-surface feedback and tropical-midlatitude interactions (Meehl, 1994). Tomita and Yasunari (1998) hypothesized that SST anomalies in the South China Sea and the winter monsoon maintained the TBO via tropical-extratropical interactions. A number of authors have proposed that the TBO arises through interactions between the monsoon annual cycle and slowly evolving coupled physics of the tropical ocean-atmosphere-land system (Nicholls, 1978; Barnett, 1991; Clarke et al., 1997; Chang and Li, 1999; Li and Chang, 1999). In their coupled ocean-atmosphere model Wang et al. (1999) reported that stochastic forcing associated with the ISOs enhanced interactions between the annual cycle and ENSO, resulting in quasi-biennial oscillations.

1.3 Questions: Key questions for understanding the nature and dynamics of the TBO are:

· What are essential characteristics of the TBO in terms of its coherent spatial structure and temporal evolution?

· What sets the time scale of the oscillation (2.3-3 years), and why is it irregular?

· What determines its amplitude modulation on decadal time scales?

· Is the TBO simply the quasi-biennial component of ENSO, or an independent mode of oscillation?

· What is the influence of the monsoon annual cycle?

· How does tropical-extratropical interactions contribute to the generation and maintenance of the TBO?

· Do TBO dynamics involve remote forcing from either the eastern central Pacific or the extratropical land surface?

2. Interannual monsoon variability:

2.1 Background: The development of some anomalous states of the Indian monsoon is clearly associated with ENSO (e.g., Shukla and Palino, 1983; Shukla, 1987). On the other hand, other anomalous monsoons appear to have nothing to do with ENSO, suggesting that there are intrinsic modes of interannual climate variability within the A-AMS. Moreover, there is now evidence indicating that the Asian monsoon may in turn influence ENSO and other seasonal-to-interannual climate signals (Yasunari, 1990; Webster and Yang, 1992; Mantua and Battisti, 1995; Wainer and Webster, 1996; Lau and Yang, 1996).

2.1 Hypotheses:

2.1.1 Tropical interactions: Monsoon-ENSO interactions may be realized through changes in the location or intensity of convective heat sources within the tropics. The large-scale monsoon heat source migrates northwest-southeast across the equator following the land distribution (Meehl, 1987), whereas the one associated with ENSO migrates east-west along the equator with the shift of the warm pool during warm and cold events (Webster and Lukas, 1992). The interaction of these two convective systems is believed to be partly responsible for the complexity of interannual variability in the A-AMS region.

2.1.2 Extratropical interactions: The evolution of monsoon circulation in the western North Pacific and South China Sea may also play an important role in linking the Asian monsoon and ENSO. Yasunari (1990) showed a connection between Indian monsoon rainfall and variation of the subsurface water temperature in the western North Pacific. Wind-stress variability over the western North Pacific has important impacts on the Pacific basin-wide thermocline adjustment and ENSO evolution (Wang et al., 1999). The western Pacific monsoon has received little attention in the past. Understanding the interannual variation of the monsoon in this area, and its relationships to the other regional Asian monsoons, is an urgent need.

2.3 Questions: Questions that need to be addressed are:

· What are the dominant modes of A-AMS interannual variability, and what processes generate them?

· Are remote influences from the Pacific and Indian Oceans important for these variations? If so, which oceanic regions are most closely linked with monsoon variability in a particular subregion?

· How important are SST variations in the western Pacific and Asian marginal seas in the interannual variations of the East Asian summer monsoon?

· Why do Pacific SST anomalies affect the Indian summer monsoon in some years but not in others? One possibility is that Indian Ocean SST anomalies are also effective in influencing the Indian monsoon. What processes determine interannual SST variability in the Indian Ocean?

· What are the dominant modes of monsoon-ENSO coupling? It is known that ENSO affects the monsoon. To what extent and how do monsoon variations in turn impact the Pacific trades and ENSO?

3. Interdecadal monsoon variability:

3.1 Background:

3.1.1 Climate shift in the 1970s: The North Pacific climate shift in the late 1970s (Nitta and Yamada, 1989; Trenberth and Hurrell, 1994) significantly changed the onset characteristics of the Pacific basin-wide warming (Wang, 1995). There is now evidence that its impacts are also reflected in the A-AMS. Nitta and Hu (1996) showed a sudden change in summer rainfall and temperature over the southern China (22-26oN, 110-120oE) that occurred nearly in phase with the North-Pacific climate shift. Furthermore, the position of the ridge of the western North Pacific Subtropical High exhibited a coherent change: The North-Pacific cooling after the late 1970s was accompanied by a southwestward extension of the subtropical ridge and by drought and hot summer conditions in southern China. Australian temperatures also exhibited prominent change after the late 1970s.

3.1.2 Decadal monsoon-ENSO interactions: There are also indications that the monsoon and ENSO interact at decadal time scales. Torrence and Webster (1998) conducted a wavelet analysis of monsoon and ENSO variability during the 20th century, finding considerable decadal fluctuations in the variance of both phenomena. Kripalani and Kulkarni (1996) showed that extrema of Indian-monsoon rainfall tended to occur when ENSO and decadal variations were in phase, with El Niño events tending to result in floods during above-normal decadal epochs. The mechanisms that might account for this decadal monsoon-ENSO connection are not known. In a modeling study, Meehl et al. (1998) note a close correspondence between decadal variations of tropical Pacific SSTs, the South Asian monsoon, and global decadal climate variability.

3.2 Questions: Our knowledge of the nature of A-AMS interdecadal variability is extremely limited, and it needs to be documented systematically. Fundamental questions like the following need to be addressed:

· What is the relationship between the interdecadal variation of the A-AMS monsoon and the interdecadal variation of global SST?

· To what extent and how does interdecadal shift of Pacific Ocean climate affect the Asian summer monsoon? In general, how does Indo-Pacific decadal variability affect the A-AMS?

· Does the winter monsoon in turn affect Pacific decadal variability, as suggested by Namias et al. (1988)? Specifically, What role does A-AMS winter variability (particularly, regarding the Siberian High and the Aleutian Low) play in Pacific decadal SST variability?

· How are ISOs, and the onset of ENSO, modulated by Pacific decadal variability?

4. Synoptic and climate variability of tropical storms in the A-AM region

4.1 Background: The western North Pacific (WNP) is one of the most important regions for the tropical cyclone (TC) genesis in the world, with about 40% of all TCs being generated in the region each year (Gray, 1968). Observations show that more than 80% of TCs in the WNP form in the ITCZ/monsoon trough, where both the large-scale confluent flow and cumulus convection are favorable for the formation of synoptic-scale disturbances and TCs (Gray, 1968). Ritchie and Holland (1998) summarized 5 environmental flow regimes associated with the TC genesis in the WNP: the monsoon shear line, monsoon confluence region, easterly waves, Rossby-energy dispersion, and monsoon gyre: Over an 8-year analysis period, percentages of the TC genesis associated with each regime were 42%, 29%, 18%, 8%, and 3%, respectively. The monsoon shear lines may provide dynamical instability to initiate the CISK and WISHE mechanisms. The confluence zone between easterly trades and monsoon westerlies has been shown to be important for a range of scale interactions and energy accumulations by atmospheric internal waves (e.g., Holland, 1995, Kuo et al., 2001).

The impact of ENSO on the genesis and intensification of tropical cyclones in the western North Pacific was investigated using observational data (Wang and Chan 2002). During an El Niño most tropical storms form in the southeast quadrant (5-17ºN, 140-180ºE), whereas during a La Niña most form in the northwest quadrant (17-30ºN, 120-140ºE). In addition, during strong El Niño years, 2.5 times more tropical storms in the fall recurve northward across 35ºN than during strong La Niña years. These properties imply that El Niño events substantially increase the poleward transport of heat and moisture, which may have a significant impact on the extratropical general circulation. The authors hypothesized that the properties were attributable to changes in the large-scale circulation associated with ENSO forcing, such as the zonal shift of low-level vorticity associated with meridional shear of the zonal flow and the change of strength of the midlatitude subtropical high.

4.2 Questions: Questions that need to be addressed are:

· What are the important dynamic and thermodynamic factors regulating the climate variability of the TC genesis and movement in the A-AM region?

· How do climate changes of TC tracks impact the tropical-midlatitude heat and moisture exchange and global energy budget?

· What are the synoptic atmospheric processes that lead to tropical cyclogenesis in the WNP?

5. Strategy: Addressing these questions requires studies on the interaction between ENSO, the Asian monsoon, and the interdecadal variations through both diagnostic analysis and numerical experiments with a hierarchy of numerical models. In principle, the strategy for study of the interannual to interdecadal variability of the A-AMS is similar to that previously proposed for study of the monsoon annual cycle and ISOs. Models that will be used include conceptual and intermediate systems, atmospheric GCMs and high-resolution regional monsoon models, and coupled GCMs. The AGCM and regional monsoon climate models, and their coupled versions, will be discussed in more detail in the next subsections. In addition to numerical modeling, data analysis with modern satellite products (e.g., TRMM and QuikSCAT data) will be conducted to identify the processes that give rise to cyclogenesis in the A-AM region

C. Understand the role of atmosphere-ocean-land interactions in monsoon predictability

1. Atmosphere-ocean-land interactions:

1.1 Background: The A-AMS is arguably the best example of tight coupling of ocean-atmosphere-land processes (Webster, 1987). Seasonal land heating and ocean heating alone are not able to account for the observed monsoons, both being needed (Shukla and Fennessy, 1994; Li and Yanai, 1996).

1.1.1 Orographic effects: The response of the tropics to the annual cycle of solar forcing is strongly influenced by the distribution of land, with pronounced zonal variability in both the annual cycle and the mean state. Orographic effects are important in determining diabatic heating over the Tibetan Plateau (Murakami, 1987; He et al., 1987; Ding, 1994; Li and Yanai, 1996), and are likely important to the atmospheric circulation and the diabatic heating over the maritime continent (Kitoh and Yamazaki, 1991). Land-locked convection in the Australasian sector is associated with intense diabatic heating, which drives large-scale atmospheric circulations, such as the Walker Circulation (Cornejo-Garrido and Stone, 1977; Stone and Chervin, 1984), and the monsoons (Webster, 1987).

1.1.2 Oceanic effects: The associated surface winds extract moisture from the ocean by evaporation, drive ocean currents that redistribute heat, and stir the upper ocean resulting in diabatic cooling. These upper-ocean processes appear to be important in both the western-Pacific warm pool and the northern Indian Ocean (Yasunari, 1989; Lukas and Lindstrom, 1991; Meehl, 1993). The resulting SST variations can feedback to affect winds and rain, both positively and negatively. These feedback processes are an important aspect of the tropical climate, and they are critical to the existence of interannual modes of variability (Hirst and Lau, 1990; Wang and Xie, 1998). In the tropical Pacific, ocean-atmosphere processes are also essential for the development of prominent features of the annual cycle as well (Mitchell and Wallace, 1992; Wang, 1994; Xie and Philander, 1994; Philander et al., 1996).

1.2 Questions: Questions like the following need to be addressed:

· Are there coupled ocean-atmosphere-land modes of interannual-to-interdecadal variability intrinsic to the A-AMS?

· How do ocean-atmosphere and land-atmosphere interactions contribute to the generation and maintenance of the TBO and variability of the ENSO-monsoon system?

· What roles do atmosphere-ocean interactions play in generating and maintaining ISOs?

· How does topography (e.g., the Tibetan Plateau) influence the onset and decay of the mean monsoon?

· Is ocean-atmosphere interaction actively involved in the monsoon annual cycle?

· What roles does the atmosphere-ocean-land interaction play in the annual march of the convection centers and the formation of the Asian monsoon singularities?

2. Monsoon predictability:

2.1 Background: Predictability of the A-AMS has been an important research area for many years (e.g., Walker, 1923; Normand, 1953; Charney and Shukla, 1981; WCRP, 1993). It is a particularly difficult problem because the monsoons involve interactions on such a wide range of time scales. It is nevertheless believed that some aspects of A-AMS variability are predictable. This predictability arises due both to regional processes that contribute to low-frequency boundary forcing and to global influences that modulate the smallest space and time scales (hydrological processes).

2.1.1 ENSO interactions: Webster and Yang (1992) and Lau and Yang (1996) discuss monsoon/ENSO interaction as a means for enhancing monsoon predictability. Webster and Yang (1992) hypothesized that the seasonal variation of the SST gradient along the equator in the Pacific Ocean is the primary factor in variations of the coupling between the Pacific basin and the A-AMS, whereas Ju and Slingo (1995) noted that variations in the ITCZ provided a mechanism for linking the two regions.

2.1.2 Other interannual interactions: Other interannual modes of the coupled system seem to exist, and they offer hope of additional predictive skill beyond what might be gained from ENSO prediction alone. For example, the correlation pattern between SST and Australian winter monsoon rainfall shows a dipole, with a maximum centered near New Guinea, a minimum in the tropical southern Indian Ocean, and a node stretching from Southeast Asia through Indonesia to northwest Australia (Nicholls, 1989). A third center of action is found under the South Pacific Convergence Zone. Enhanced rain is associated with warmer SST north and east of Australia.

2.1.3 ISO interactions: On the other hand, it must be recognized that monsoon interannual variability may be intrinsically linked to higher-frequency monsoon variations, such as the ISOs. Thus, monsoon predictability may be limited by its own internal dynamics (Palmer et al., 1992; Palmer, 1993; Brancovic et al., 1994).

2.2 Questions: Questions that need be considered in this topic include:

· Are the monsoon intraseasonal oscillations chaotic? Or to what extent they are predictable?

· To what extent the monsoon predictability is prohibited by the chaotic atmospheric internal dynamics?

· Are there geographic differences in the monsoon predictability? And why if any?

· What factors determine the predictability of the A-AMS?

3. Strategy: Addressing the questions raised in this subsection requires exclusively the use of coupled atmosphere-ocean-land models.

3.1 Model development: Our working hypothesis is that there are seasonal-to-interannual A-AMS variations that are partially predictable due to slowly varying regional boundary conditions and remotely forced signals. It is therefore essential to be able to model the monsoon system realistically. Yet, existing climate models have difficulty in producing realistic simulations of monsoon rainfall, especially in East Asia, and they cannot accurately simulate ISOs. Moreover, the present coupled AGCMs are not able to determine either the sensitivity of the monsoon to SST anomalies or the processes that cause the anomalies. A key IPRC research effort, then, is to develop coupled GCMs, and nested regional climate models, that are able to simulate accurately the mean monsoon, ISOs, and interannual monsoon variability (see Section VD). This development will be a difficult task, given the well-known sensitivity of coupled system to the background climate state (e.g., Battisti and Hirst, 1989; Davey et al., 1996), initial conditions (e.g., Palmer et al., 1992), and ensemble averaging (e.g., Stern and Miyakoda, 1995).

3.2 Model intercomparison: In addition to model development, the IPRC will make use of model intercomparison projects, such as AMIP, ENSIP and CMIP. These projects bring together different models that have been initialized from the same point and integrated for a common period. (In the case of the uncoupled AMIP, all models were forced with the same SST boundary conditions.) Comparison of the monsoon circulation among the members of these model ensembles can be very revealing not only of model deficiencies, but also of model strengths (Sperber and Palmer, 1996).

3.3 Systematic approach: The model will then be used in a systematic numerical experimentation program, designed to quantify the relative roles of atmosphere-ocean and atmosphere-land interactions in A-AMS climate variability. First, the global GCM will be integrated with climatological SST and land conditions. Then, additional integrations will be obtained in which land processes and/or SST are allowed to vary. Intercomparison of the various integrations will provide a quantitative basis for estimating the relative importance of internal dynamics, atmosphere-land interactions, and atmosphere-ocean interaction. Identification of the roles of an individual interaction process in mean monsoon and monsoon variability can be achieved by adequately designed numerical experiments with coupled GCMs or hybrid coupled GCM (regional monsoon model) with an intermediate ocean model. In many cases of monsoon studies, our concern is confined to the feedback of slowly evolving SST rather than deep-ocean circulation. Use of hybrid coupled model can be more efficient provided the intermediate ocean model well simulates SST variations.

3.4 Hindcast-mode predictability studies: One of the approaches that is useful for determining monsoon predictability is to perform ensemble hindcasts. Obviously, the hindcast-mode predictability studies will be compared with historical observations. Forecast-mode predictability studies can benefit from the abundance of satellite observations that are now becoming available. For example, we will have the opportunity to compare model predictions in the future with observations of oceanic rainfall from TRMM.

D. Understand the monsoon hydrological cycle and its impact on Asian-Pacific climate

1. Background: Essential factors in A-AMS dynamics are continental-scale land-surface hydrological processes. Latent heat release through moisture convergence is a critical process providing positive feedback to the monsoon circulation. In addition to its obvious involvement in diabatic heating, the hydrologic cycle also plays an important role in terms of soil moisture processes (Webster, 1983, 1987), snow cover (Hahn and Shukla, 1976; Barnett et al., 1989; Yanai and Li, 1994), and vegetation. Studies of the large-scale land-atmosphere interaction and hydro-meteorological processes are particularly relevant to A-AMS studies.

2. Questions: The objectives of the IPRC research effort in this area are to understand the hydrological cycle in the atmosphere, at the land surface, and in the upper oceans, and to determine its impacts on the A-AMS. Questions that need to be addressed are:

· What roles do land-atmosphere and ocean-atmosphere interactions have in determining the A-AMS hydrological cycle?

· What roles does the hydrological cycle play in the monsoon active and break periods?

· Does the land surface play an active role in monsoon hydrological variations on intraseasonal to interdecadal variations?

· Does the ocean play an active role?

3. Strategy:

3.1 Nested system: To address such questions, an essential part of IPRC strategy is to use high-resolution models of the A-AMS region that incorporate hydrology. It will be necessary to imbed the regional models within global GCMs that can produce realistic simulations of the atmospheric general circulation, including the planetary-scale signals known to influence the A-AMS. The circulation from this “external” model will then provide lateral boundary conditions or background conditions to drive a high-resolution regional model. If desirable, the regional model may in turn be used to force yet more highly resolved hydrological models. Several preliminary modeling studies have indicated the potential of such a nested modeling approach for improving regional climate forecasts. For example, NCEP’s ETA model has been successfully nested within the COLA AGCM, and the resulting solutions compared better with Asian summer monsoon rainfall observations (Vernekar et al., 1995).

3.2 Choice and development of nested system: The IPRC will not attempt to build its own AGCM or regional system from ground up, but seek to utilize previously developed ones (such as ETA). It is anticipated that this choice will not be easy, given the phenomenological complexity of the A-AMS, and several candidate models will be evaluated initially.

3.3 Systematic approach: The regional model will be used in a systematic numerical experimentation program, designed to quantify the relative roles of atmosphere-ocean and atmosphere-land interactions in the variability of the A-AMS and associated hydrological cycle. The principles of the approach is simile to that discussed in section C.

3.4 Other activities: Finally, this objective will also be pursued through data analysis, and will be closely associated with observational efforts such as GAME (GEWEX Asian Monsoon Experiment) and SCSMEX (South China Sea Monsoon Experiment).

VII. IMPACTS OF GLOBAL ENVIRONMENTAL CHANGE

A. Determine the impacts of changing external forcing on Asia-Pacific climate

It is recognized that changes in external forcing can profoundly influence the earth's climate system and have significant effects in the Asia-Pacific region. These forcings can include greenhouse gases, anthropogenic aerosols, solar variability, soot, mineral dust, and volcanic aerosols. Prominent among these, and of first priority for the IPRC to address, are possible changes from increasing greenhouse gases and sulfate aerosols. Each has a significantly different effect on radiative forcing over the Asian region. Greenhouse-gas increases provide positive radiative forcing to the climate system with no particular regional pattern. Sulfate aerosols, on the other hand, provide a strongly regional negative radiative forcing over and around South Asia (Mitchell et al., 1995).

The way these two forcings combine has dramatic implications for the South Asian monsoon. Global coupled GCMs run with CO2 increase and no change in aerosols generally show an increase in the strength of the South Asian monsoon (e.g., Meehl and Washington, 1993). This is because the Asian land mass heats faster than the ocean in these experiments and the resulting increased land-sea temperature gradient then drives a more intense monsoon circulation. With the addition of the regional negative radiative forcing from sulfate aerosols, however, usually there is a decrease in monsoon intensity (e.g., Meehl et al., 1996; Lal et al., 1995; Mitchell and Johns, 1997). This is because the negative forcing from the sulfate aerosols locally dominates the positive forcing from the increased greenhouse gases; consequently, the Asian land area does not warm as fast as the Indian Ocean, the land-sea temperature gradient decreases, and the monsoon weakens.

This opposite-sign outcome for the South Asian monsoon (depending on how future aerosol concentrations are included in the models) is one of the most glaring and outstanding uncertainties for the future climate of South Asia. The IPRC will address several questions aimed at reducing uncertainty in this key area. First, the nature and radiative characteristics of the anthropogenic aerosols currently being produced needs to be determined. An essential first step in that direction is the recent collection of aerosol data and radiation properties by INDOEX (1999). The IPRC will rely on this crucial dataset and associated analyses for improved estimates of the radiative effects of sulfate aerosols in the Asian region. Preliminary results show that aerosols produced over South Asia are “darker” than anthropogenic aerosols over North America or western Europe. This darkening is thought to be the result of less “scrubbing” of emissions over South Asia. A darker aerosol over Asia will have profound effects on the magnitude of the negative radiative forcing thought to be associated with anthropogenic sulfate aerosols. Another uncertainty is how to account for the direct (clear sky) vs. indirect (enhanced cloud reflectivity) effects of sulfate aerosols (Iwasaki and Kitagawa, 1997). The IPRC scientists will make use of current data (e.g., from INDOEX, the US Atmospheric Radiation Measurement (ARM) program, or the 3rd Aerosol Characterization Experiment (ACE-3) southeast of Japan) to test various parameterizations of aerosol effects in GCM integrations and to look specifically at the details of the response over South Asia. Second, future projections of emissions and aerosol concentrations over Asia need to be refined. The four recent scenarios for future emissions formulated by IPCC (1999) are a step in that direction. These latest scenarios can be included in model simulations performed by IPRC staff in both global models and embedded regional models over South Asia.

Another aspect of uncertainty concerning the South Asian monsoon is that some global coupled models show an increase of interannual variability of the South Asian monsoon with increased CO2 (Meehl and Washington, 1993; Kitoh et al, 1997; Bhaskharan and Mitchell, 1998). This effect is of interest because of the possibility of increased extremes of drought and flood in future monsoon seasons. A focus for IPRC scientists therefore will be not only to investigate changes in the mean monsoon climate due to increasing greenhouse gases and sulfate aerosols, but also changes in its variability. For this purpose, IPRC scientists will perform on-site modeling experiments with global or embedded regional models, and can analyze model output from various international centers archived by the Coupled Model Intercomparison Project (CMIP) or the IPCC Data Distribution Centre (DDC).

El Niño is associated with some of the most pronounced climatic variability in the Asia-Pacific region. In connection with future climate warming, several aspects of El Niño need to be assessed that could affect future climate extremes: Will the long-term mean Pacific climate shift toward a more El Niño-like or La Nina-like regime? Will El Niño variability (the amplitude and/or the frequency of temperature swings in equatorial Pacific) increase or decrease? How will El Niño’s impact on weather in the Pacific basin and Asia change?

Several global climate models indicate that as global temperatures warm due to increased greenhouse gases, the Pacific climate will tend to resemble an El Niño-like state more (Meehl and Washington 1996, Knutson and Manabe, 1998; Mitchell et al., 1995; Timmermann et al., 1999). That is, in the mean the eastern Pacific would warm relatively more than the west, with an associated eastward shift of precipitation from Australasia to the central and western Pacific. This conclusion could be sensitive to model representations of cloud feedbacks (Meehl et al., 1999), and indeed some models show a La-Nina-like response (Noda et al., 1999).

So far, attempts to address the question of future changes in El Niño variability using climate models have shown conflicting results, varying from slight decreases or little amplitude change (Tett, 1995; Knutson et al., 1997) to a substantial amplitude increase (Timmermann, et al., 1999). Concerning El Niño’s effects on Asia-Pacific weather, Meehl et al.'s (1993) results indicate that future seasonal precipitation extremes associated with a given El Niño are likely to be more intense due to the warmer mean base state in a future climate. Specifically, for the tropical Pacific and Indian Ocean regions, anomalously wet areas could become wetter, and anomalously dry areas become drier during future El Niño events. In addition, changes in the extratropical base state in a future warmer climate may cause a shift in the teleconnection pattern to midlatitudes, particularly over the North Pacific, which may result in an associated shift of precipitation and drought conditions in future El Niño events (Meehl et al., 1993).

When assessing changes in El Niño, it must be recognized that an "El Niño-like" pattern can occur at a variety of timescales ranging from interannual to interdecadal (Zhang et al., 1997; Lau and Weng, 1999) or even as a response to external forcings such as increased CO2 noted above (Meehl and Washington, 1996; Knutson and Manabe 1998). Making conclusions about “changes” in future El Niño events will be complicated by these factors. Additionally, since substantial internally generated variability of El Niño statistics occurs on multi-decadal to century time scales occurs in long unforced climate model simulations (Knutson et al., 1997), the attribution of past and future changes in El Niño amplitude and frequency to external forcing may be quite difficult, perhaps requiring use of ensemble climate experiments. Thus, IPRC scientists will make use of climate change experiments with global coupled models from other centers made available internationally as mentioned above, and run ensemble experiments with coupled models at the IPRC specifically designed to study changes of El Niño, the El Niño-like response pattern, and the associated affects on the A-AMS.

B. Determine the sensitivity of the Asia-Pacific climate system and hydrological cycle to changes in land-surface characteristics

Land-cover and land-use practices have changed dramatically over the past century and will continue to change in Asia, with potentially important impacts on the region’s climate and hydrology. For example, the development of roads in northern Thailand has been shown to have significantly impacted the hydrology of the region (Giambelluca, 1996). Deforestation resulting from logging has been shown to alter local climatic (Giambelluca et al., 1996; Zhang et al., 1996) and hydrologic (Ziegler and Giambelluca, 1996) conditions, and vegetation changes over South Asia have been shown to directly affect monsoon characteristics (Chase et al., 1996). Understanding these climate and hydrology impacts is a prerequisite for making numerical predictions about other global and regional climate changes given projections of future states of land cover.

This objective can only be achieved in the framework of numerical models which successfully simulate the monsoon system and its natural variability. Such models must have sophisticated land-surface components. A hierarchy of climate models will be used to address this objective, including atmosphere/land surface models run with observed SSTs, global coupled GCMs, and embedded high-resolution regional models with detailed hydrologic formulations. Experiments will be conducted to determine the sensitivity of the model’s climate statistics to specified changes from a control run. For example, land-surface characteristics observed by Landsat and by AVER have changed in the 20+ years of observation. Do discernible changes in the observed characteristics correspond to significant changes in the model simulations? Because the monsoon system may include instabilities and therefore be chaotic, it will be necessary to conduct ensemble integrations. In addition to experiments with global atmosphere/land surface models with observed SSTs and integrations with global coupled GCMs, control runs in the global framework can be used to provide external boundary conditions to an embedded high resolution regional model with more detailed hydrology to conduct local impacts experiments.

The ultimate goal will be to perform model experiments with an interactive land-surface model. That is, as the external forcing changes due to increased greenhouse gases or sulfate aerosols, how does the surface vegetation change and feed back onto the regional climate changes. In a similar way, specified land use changes could be included to determine the feedbacks on the climate system, which would then influence the conditions of the land surface. An embedded regional land-use and land-cover-change model could be used for this purpose.

VIII. RESOURCE REQUIREMENTS

The IPRC will ensure that adequate resources are available to pursue its mission. Above all, the IPRC’s success requires participation of highly qualified scientists. It also requires a foundation of stable financial support, state-of-the-art computing facilities, easy access to climate data sets, and a sufficient infrastructure.

A. Funding

Joint research activities at the IPRC are conducted as a part of the Frontier Research System for Global Change (FRSGC) on the Japanese side, and as a part of the Global Change Research Program of the United States on the US side. Its annual budget is submitted to the Implementation Committee (see Section VIIIB) for review and approval. The IPRC is currently funded by the FRSGC through JAMSTEC and NASDA and by NASA on the US side. Additional support from the US participating parties (NSF, NASA, NOAA, DOE) is expected.

B. Scientists

To maintain a viable research program that can attain IPRC objectives requires a team of stably funded, resident researchers with expertise in a variety of fields (e.g., numerical experimentation, oceanic and atmospheric GCM development, and data and model analysis). Several types of scientists make up the IPRC research staff: tenure-track faculty, non-tenure-track research scientists, Frontier researchers, post-docs, and students.

1. Tenure-track faculty: As permanent members of the UH faculty, these scientists provide the backbone of the IPRC research staff. The Interim Cooperative Agreement between UH and JAMSTEC/NASDA calls for up to 10 tenure-track UH faculty, funded jointly by UH and JAMSTEC/NASDA. To date, 5 of these positions have been filled.

2. Research scientists: These non-permanent positions are renewed annually, but with no upper limit for the duration of employment. Currently, 10 of these positions are filled for 2001, funded both by FRSGC and NASA through the Research Corporation of the University of Hawaii (RCUH). Three more research scientists will be hired during 2001 using NASA funds.

3. Frontier researchers: These scientists are employees of JAMSTEC through the FRSGC, and at UH have the status of “non-compensated visiting colleagues.” As the “visits” of these scientists are multi-year, they are an important (essential) part of the IPRC research program. Currently, there are 3 such scientists at the IPRC.

4. Post-doctoral fellows and students: Currently, the IPRC supports 6 post-doctoral fellows. Four of them will leave during 2001, and will be replaced by 5 new postdocs. The IPRC also supports several graduate students, who came to the IPRC together with new IPRC faculty. Funds for postdocs are currently all from the IPRC FRSGC or NASA grants. Future support is also expected to arise from individually funded proposals.

C. Computing Facilities

Much of the IPRC modeling activities require the use of state-of-the-art oceanic and atmospheric GCMs. These models can only be run on extremely fast computers with large internal memories. They also require substantial mass storage, a technical support staff, supportive workstations, and a high-speed local network. The IPRC computing facilities must be capable of routinely carrying out long integrations of models of intermediate complexity, as well as shorter integrations using highly resolved, state-of-the-art GCMs. Production runs using the latter models will likely remain beyond IPRC capability, and require computational resources at national or international supercomputer centers.

Computational resource needs are expected to change often. To ensure that the IPRC responds to these changing needs, the computing facilities will be reviewed periodically, but at least once each year.

1. Central system: The IPRC central computing system requires one or more compute servers, a file server, and nearly on-line storage capability. It must be able to service all IPRC researchers (eventually as many as 10 tenure-track faculty and 30 or more other scientists), and allow access by a suitable level of collaborating scientists at other institutions. Finally, it must be "balanced" in that the components have comparable levels of capability, so that no single one of them represents a serious bottleneck.

The central components currently at the IPRC are:

Compute Servers: The IPRC has five compute servers: i) a shared-memory, vector-parallel system Cray SV-1 with 24 CPUs, 14 GB RAM, a 152 GB disk, and a peak speed of 28.8 GFLOPS, ii) an SGI Origin 2000 with 32 CPU system with 18 GB RAM, a 198 GB disk system, and a peak speed of 16 GFLOPS; and iii) an SGI Origin 2000 with 24 CPU system with 8.5 GB RAM, 27 GB disk, and 11 GFLOPS peak speed, iv); a Sun Enterprise 450 with 4 CPU’s and 2 GB memory and a peak speed of 3.2 GFLOPS and, v) an Aspen Systems 2 CPU Alpha based system with a peak speed of 3 GFLOPS. A sixth compute server, an Origin 3400 with 24 CPU’ and a peak speed of 19.2 GFLOPS should be online in the near future. With these machines, the IPRC has local access to both shared-memory, distributed-memory systems, and vector machines. This availability enables IPRC researchers to develop code for all modern computer architectures.

File Server: The file server is a cluster of computers or processing elements that are well equipped with memory, disk capacity and input/output bandwidth for post-integration analysis and visualization of solutions obtained on the compute servers. The current IPRC file server is a 4 CPU SGI Origin 200 with 1 GB RAM and a peak speed of 1.4 GFLOPS. The server controls magnetic disks (1028 GB, RAID 5) and a data storage library. In the near future, a switched Fibre Channel storage area network will allow direct 100 MB/s access to the RAID storage from all of the SGI servers.

Data Storage: The concept of nearly online data storage is that of a large-capacity data repository that allows data to be stored and retrieved easily without human intervention. The IPRC system is a StorageTek 9710 automatic tape library with 6 tape drives. Capacity is 14.7 TB, and the software includes automatic file migration from the compute servers and backup of files.

These resources should meet IPRC needs for the next few years.

2. Technical support: The current staff consists of a systems manager, a scientific programmer, and a data specialist. Requested positions for additional programmers, a second data specialist and a second system administrator will be filled soon.

3. Workstations: Each researcher has a workstation and/or a personal computer (PC) available for visualization, data analysis, communication and word processing. Currently, the IPRC has 31 workstations and 49 personal computers.

4. Networking: The various units within the IPRC computing facility must be networked at a bandwidth high enough to provide a level of connectivity consistent with the rate at which the compute servers can output results. The computing facility itself must have a relatively high bandwidth connectivity to provide adequate remote access. The existing network meets these requirements.

D. Asia-Pacific Climate Data Center

All IPRC research activities require extensive use of climate data. The IPRC Asia-Pacific Climate Data Center (APDRC) is envisioned to assist scientists in this regard. Its Mission Statement is:

The Mission of the Asia Pacific Data Research Center is to increase understanding of climate variability in the Asia-Pacific region by developing the computational, data-management, and networking infrastructure necessary to make data resources readily accessible and usable by researchers, and by undertaking data-intensive research activities that will both advance knowledge and lead to improvements in data collection and preparation.

The linkage of research activities with data management in one center is novel, and we expect the combination to lead to increased data usage, to improvements in data products, and hence to more rapid scientific progress.

To achieve its Mission, specific APDRC goals are to: 1) develop and implement a user-friendly data server system (DSS); 2) establish efficient DSS communication in an international setting; 3) maintain a data archive focussed on Asia-Pacific issues; and 4) undertake data-intensive research projects that will both advance scientific knowledge and lead to improvements in data collection and preparation. Data-management tasks to be undertaken at the APDRC include collection, quality control, synthesis, and interpolation onto useful grids. Research activities include both data analysis and data assimilation. Data assimilation is a particularly important activity for the APDRC, and a close working relationship with the Global Ocean data Assimilation Experiment (GODAE) project will be sought.

It is hoped and expected that the APDRC will develop into a powerful research resource, not just for IPRC scientists, but for the international climate community as well. In addition to the obvious scientific benefits of such an international resource, the collection and distribution of data between the IPRC and other countries in the Asia-Pacific region (and elsewhere) will provide a means for strengthening international collaboration.

The APDRC does not yet exist owing to insufficient funds from the FRSGC grant. Funds from the NASA grant will support much of the APDRC’s research part. Funds for its data-management part are being sought from NOAA.

E. Infrastructure

Everyday IPRC activities require adequate space and administrative support. Space requirements include:

· Offices

· Computing center

· Data Center

· Library

· Conference room(s)

· Seminar/lecture room(s)

Administrative requirements include personnel to assist with:

· Visa acquisition

· Conference/meeting/workshop support

· Visitor program (visa requirements, travel, lodging, etc.)

· Public relations

· Purchasing and other fiscal matters

At the present time, the IPRC physical plant and support personnel are adequate to meet these needs.

IX. MANAGEMENT STRUCTURE

The purpose of the IPRC management structure is to ensure that the Center develops into an international focal point for Asia-Pacific climate research. The management structure includes mechanisms both for internal review and for external guidance.

A. Internal management

The internal management structure is designed to ensure the active participation of all IPRC scientists in decision making. Mechanisms to ensure this participation include regular meetings with the entire research staff, Research Teams with Team Leaders, a Steering Committee, and annual review of the Science and Work Plans.

1. IPRC scientists: The IPRC researchers are an information resource that should be tapped. So, the research staff will be consulted on all substantive IPRC issues before a decision is reached.

2. Research Teams: The IPRC Themes, Goals, and Objectives outline an extensive and varied research agenda. To facilitate progress, IPRC activities are divided into a number of smaller Research Teams. Each Team will focus on one (or more) of the IPRC objectives or activities. They will be led by Team Leaders, who will usually be selected from the long-term scientific staff (i.e., either from the tenure-track faculty or Frontier visiting scientists). Team Leaders will annually review the status of their Research Team.

3. Committees: The IPRC Steering Committee consists of the IPRC Director, Executive Associate Director, Liaison Officer, and Team Leaders. It meets regularly to discuss all IPRC issues. The other IPRC Committees are charged with providing advice to the IPRC Director concerning the maintenance and improvement of specific IPRC activities. Currently, these committees are the Data, Computer, Visitors, Meetings and Workshops, Library, and Public Relations Committees. Membership on these committees is renewed annually.

4. IPRC administrators: The IPRC administrators are the Director, Executive Associate Director, and Liaison Officer. The Director has the overall responsibility for all aspects of the IPRC. His(her) duties are outlined in the Implementation Guide. Among other duties, the Executive Associate Director is responsible for Center’s everyday management. The Liaison Officer is appointed jointly by JAMSTEC and NASDA to facilitate communication and coordination among UH, JAMSTEC and NASDA, and to oversee management of Japanese funds.

B. External management

Mechanisms of external review and guidance are provided by the Science Advisory Committee (SAC), the Implementation Committee (IC), and the Joint Council (JC).

1. Scientific Advisory Committee: The SAC advises the IPRC Director and the IC on overall scientific direction of the IPRC. Its purpose is to ensure that the highest possible quality of research is conducted at the IPRC. A primary SAC function of the SAC is therefore to recommend the addition or deletion of scientific themes, goals and objectives. The SAC is composed of internationally respected scientists, with expertise in the areas of research being conducted by the IPRC. The SAC members are appointed by the JC upon recommendation from either the IC or existing SAC memberships.

2. Implementation Committee: The Implementation Committee (IC) was established as one of the joint subsidiary bodies of the Joint Council (ref. Article III of the Implementing Arrangement). The IC is a joint US-Japan body charged with ensuring that the IPRC is effectively implemented. Among other things, it annually reviews the IPRC Science Plan, Work Plan, and budget. It also makes recommendations to the JC concerning the IPRC Director and Executive Associate Director. The members of the IPRC-IC consist of representatives of the current participating parties of the IPRC (UH, JAMSTEC, NASDA, NSF, NASA, NOAA, and DOE).

3. Joint Council: The Joint Council for Global Change Research and Prediction oversees and coordinates Joint Activities between US and Japan, which are defined in Article II of the Implementing Arrangement. Research activities at the IPRC are regarded as one of the Joint Activities. In principle, the JC meets once a year and operates by consensus.

X. COLLABORATIONS

Because the IPRC (or any other single institution) cannot do everything itself, it is important that the IPRC develop close working relationships with institutions and individual scientists conducting research in related areas. The IPRC programs discussed below are intended to foster such interactions.

A. Institutions

The IPRC research should complement and facilitate, rather than duplicate, research efforts being conducted at other institutions. Therefore, the IPRC should coordinate its activities with related academic institutions, government laboratories, and non-governmental organizations in Japan, the US, and other countries as much as possible. If appropriate, it should enter into formal agreements with other institutions, to facilitate cooperation on problems of mutual interest. The following example illustrates the usefulness of such agreements. One goal of IPRC research is to improve predictability of the Asia-Pacific climate. Since the validation of this research can only be accomplished by actually doing predictions, cooperation with research and operational institutions that regularly carry out predictions is essential. Obvious candidate institutions are the JMA, NCEP, the IRICP and ECMWF.

B. Affiliates

The IPRC Affiliates program is intended to expand the intellectual resources of the Center, by facilitating close working relationships with scientists at other institutions. IPRC Affiliate Members are individuals who have a continuing working relationship with IPRC researchers on topics of relevance to IPRC objectives, or who actively support IPRC objectives in some other way. Affiliate members are appointed by the IPRC Director for three-year renewable terms. Affiliate members will receive preferential consideration for IPRC Visitor status and use of IPRC computers, and will regularly obtain IPRC information. IPRC funds, however, will not otherwise be used to support Affiliates’ research activities that take place outside the IPRC.

C. Visitors

A Visitor program is essential to the intellectual vitality of the IPRC. The program will support visits by external scientists for various durations (from a week to a year or more). Requests for Visitor status should be submitted in writing to the IPRC Director. They should include a brief description of the planned research, and a statement of how the work is related to IPRC objectives. All requests must be approved by the IPRC Director. Visits longer than one week must also be approved by the IPRC Visitors Committee.

D. Travelers

Travel support for IPRC scientists employed by UH and RCUH is available from two sources: i) externally funded contracts and grants; and ii) internal IPRC funds, if external funds are not available. If possible, sufficient funds will be built into the IPRC annual budget to allow IPRC scientists employed by UH and RCUH to attend scientific international meeting (or two national meetings) each year, and tenure-track faculty to attend two international meetings. Requests for travel funds should be submitted to the IPRC Executive Associate Director, together with a brief description of planned activities and a statement of how they support IPRC objectives. Travel support for IPRC scientists who are Frontier researchers are generally available from JAMSTEC or NASDA.

E. Meetings and workshops

Funds will be built into the IPRC annual budget to support meetings and workshops (“functions”) that involve groups external to the IPRC. Such functions are useful for the IPRC because they serve to keep IPRC scientists abreast of the latest developments and planning. All functions supported by the IPRC should have themes that advance IPRC objectives, and at least one IPRC scientist must be a member of its planning committee. Requests for IPRC support are submitted to the IPRC Director, together with an outline of the planned activities, a discussion of how they support IPRC objectives, and a preliminary budget. They are reviewed and approved by the IPRC Meetings and Workshops Committee, and final approval is by the IPRC Director. Since all such functions will benefit IPRC scientists (otherwise they won’t pass the internal review process), it is reasonable that IPRC funds to support them in part. It is also reasonable to ask that functions be supported by external agencies, since they will also benefit the community at large. Generally, it is expected that they will be supported roughly equally between IPRC and external funding sources.

XI. RELATIONSHIP TO OTHER PROGRAMS

The IPRC research activities should be clearly related to existing international research programs and explicitly contribute to them. This is true for the current ones, which map onto a number of the programs sponsored by the World Climate Research Programme (WCRP), the International Geosphere-Biosphere Programme (IGBP), and global-change research initiatives within Japan and the United States. Given the close relationship of its research efforts to these international programs, the IPRC should interact regularly with their advisory bodies to determine its unique contributions.

Current IPRC activities are perhaps most closely related to those of the Climate Variability and Predictability (CLIVAR) and Global Energy and Water Cycle Experiment (GEWEX) programs sponsored by WCRP.

The overall objectives of CLIVAR research are to improve understanding of the dominant modes of seasonal-to-centennial climate variability, to assess anthropogenically-induced climate changes, and ultimately to develop a system of coupled models and long-term observations that allow skillful predictions of seasonal-to-centennial climate variability. With respect to the A-AMS, the CLIVAR Monsoon Panel has adopted the following objectives for the A-AMS Implementation Plan:

· To document the spatial structure and temporal variability of the AA-monsoon system on intraseasonal, annual, interannual and interdecadal time scales

· To identify specific mechanisms in the complex evolution of the annual cycle in the coupled ocean-atmosphere-land system in the monsoon regions

· To unravel the mechanisms of the intraseasonal oscillations (ISO) affecting the monsoon regions and the Indian and western Pacific warm pool

· To determine the fundamental modes and mechanisms in ENSO-Monsoon coupling, including the Tropospheric Biennial Oscillations (TBO) and interdecadal modulations

· To quantify the relative roles of oceanic processes in different ocean basins, and land processes, in determining monsoon variability

· To determine the relative contribution of the chaotic vs. deterministic, as well as local vs. remote forcing in contribution to monsoon predictability

· To identify external influences including tropical-extratropical and tropospheric-stratospheric interactions affecting monsoon variations

These CLIVAR objectives mesh well with all the IPRC themes, goals and objectives.

The overall objective of the GEWEX program is to observe, understand and model the hydrological cycle and energy fluxes globally within the atmosphere, at the land surface, and in the upper ocean. The ongoing GEWEX Asian Monsoon Experiment (GAME) is responsible for obtaining necessary observations in the A-AMS region. The objectives of GAME are to understand the role of the Asian monsoon as a major component of the global energy and water cycle, to determine the feedback processes associated with monsoon variability, and thereby to improve the seasonal forecasting of monsoon and regional water resources. The planned South China Sea Monsoon Experiment (SCSMEX) has similar objectives, but is focussed on the South China Sea. The IPRC activities under Themes 3 and 4 will complement and enhance GAME and SCSMEX observational programs in the areas of data analysis and modeling.

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