The Boreal summer intraseasonal oscillation (BSISO) of the Asian summer monsoon (ASM) is one of the most prominent sources of short-term climate variability in the global monsoon system. Compared with the related Madden-Julian Oscillation (MJO) it is more complex in nature, with prominent northward propagation and variability extending much further from the equator. In order to facilitate detection, monitoring and prediction of the BSISO we suggest two real-time indices: BSISO1 and BSISO2, based on multivariate empirical orthogonal function (MV-EOF) analysis of daily anomalies of outgoing longwave radiation (OLR) and zonal wind at 850 hPa (U850) in the region 10S-40N, 40-160E, for the extended boreal summer (May-October) season over the 30-year period 1981-2010.

It has been well recognized that the tropical intraseasonal oscillation (ISO) exhibits prominent seasonal variation (Madden1986, Wang and Rui 1990; Salby and Hendon 1994; Zhang and Dong 2004; CLIVAR Madden-Julian Oscillation (MJO) working group 2009; Kikuchi et al. 2012). Compared to boreal winter, during boreal summer the main centers of convective variability associated with the ISO are shifted away from the equator to 10-20N, and the propagation patterns are considerably more complicated. While the boreal winter ISO (also known as the MJO) shows predominantly eastward propagation, the boreal summer ISO (BSISO) also exhibits northward/northeastward propagation over the Indian summer monsoon (ISM) region (Yasunari 1979, 1980; Krishnamurti and Subrahmanyan 1982; Lau and Chan 1986; Wang et al. 2005; Annamalai and Sperber 2005), and northward/northwestward propagation over the WesternNorth Pacific-East Asian (WNP-EA) region (Murakami 1984, Chen and Chen 1993; Kemball-Cook and Wang 2001; Kajikawa and Yasunari 2005; Yun et al. 2009, 2010), often inconjunction with MJO-like propagation along the equator (Lawrence and Webster 2002). Whereas the MJO has been regarded as applicable in all seasons, albeit with generally weaker variability in boreal summer (Madden and Julian 1972, 1994; Wheeler and Hendon 2004; Zhang 2005), the BSISO has been regarded as a specific mode of the tropical ISO that prevails in boreal summer (Wang and Xie 1997). Thus, for many applications it is instructive to consider the tropical ISO as described by two modes, the MJO and BSISO. The MJO dominates during boreal winter (December-April) and the BSISO dominates during boreal summer (June-October) with May and November being transitional months during which either mode may prevail (Kikuchi et al. 2012). Given the extreme importance of the BSISO, having real-time indices of it can assist immensely in monitoring and forecasting applications.

For the MJO, the Real-time Multivariate MJO (RMM) index developed by Wheeler and Hendon (2004) is the most widely used for such applications (e.g. Leroy and Wheeler 2008; Wheeler et al. 2009; Gottschalck et al. 2010; Rashid et al. 2011). It is defined by the first two principal component time series of the multivariate empirical orthogonal function (MV-EOF) modes of the equatorial mean (between 15S and 15N) outgoing longwave radiation (OLR; a good proxy for convection), and zonal winds at 850 hPa (U850) and 200 hPa (U200). The equatorial symmetric nature of the RMM index makes it an excellent measure of the equatorial eastward propagating mode, the MJO. However, since the RMM index is designed to depict all year round MJO activity, it is not expected to fully represent the seasonality of the ISO,especially during the peak of the boreal summer when ISO activity is furthest from the equator. It is noted that the RMM index is capable of capturing a large fraction of the intraseasonal OLR variance during boreal winter over the major convective regions. During boreal summer, on the other hand, the variance that is captured by the RMM is primarily confined between 5S and 18N, and does not reach as far north into the Asian summer monsoon region as has been doccumented for the BSISO. Improving upon the RMM index for real-time monitoring and prediction of the BSISO is the gole of the current work.

Annamalai H, Sperber KR (2005) Regional heat sources and the active and break phases of boreal summer intraseasonal (30-50 day) variability. J Atmos Sci 62:2726-2748.

Chen TC, Chen JM (1993) The 10-20day mode of the 1979 Indian monsoon: its relation with the time variation of monsoon rainfall. Mon Weather Rev 121: 2465-2482

CLIVAR Madden-Julian Oscillation Working Group (2009) MJO simulation diagnostics. J Clim 22:3006-3030

Gottschalck J, Wheeler M, Weickmann K, Vitart F, Savage N et al (2010) A framework for assessing operational Madden-Julian Oscillation forecasts: A CLIVAR MJO Working Group project. Bull Amer Meteor Soc 91: 1247-1258

Kajikawa Y, Yasunari T (2005) Interannual variability of the 10-25- and 30-60-day variation over the South China Sea during boreal summer. Geophys Res Lett 32:L04710

Kemball-Cook S, Wang B (2001) Equatorial waves and air-sea interaction in the boreal summer intraseasonal oscillation. J Clim 14:2923-2942

Kikuchi K, Wang B, Kajikawa Y (2012) Bimodal representation of the tropical Intraseasonal oscillation. Clim Dyn. 38:1989-2000

Krishnamurti TN, Subramanian D (1982) The 30-50 day mode at 850 mb during MONEX. J Atmos Sci 39: 2088-2095

Lau WKM, Chan PH (1986) Aspects of the 40-50 day oscillation during the northern summer as inferred from outgoing long wave radiation. Mon Wea Rev 114:1354-1367

Lawrence DM, Webster PJ (2002) The boreal summer intraseasonal oscillation: Relationship between northward and eastward movement of convection. J Atmos Sci 59:1593-1606.

Leroy A, Wheeler MC (2008) Statistical prediction of weekly tropical cyclones activity in the Southern Hemisphere. Mon Wea Rev 136: 3637-3654

Madden RA (1986) Seasonal-variations of the 40–50 day oscillation in the tropics. J Atmos Sci 43:3138–3158

Madden RA, Julian PR (1972) Description of global-scale circulation cells in tropics with a 40-50day period. J Atmos Sci 29:1109-1123

Madden RA, Julian PR (1994) Observations of the 40-50-Day Tropical Oscillation - a Review. Mon Wea Rev 122:814-837

Murakami M (1984) Analysis of deep convective activity over the western Pacific and southeast Asia. Part II: seasonal and intraseasonal variations during northern summer. J Meteor Soc Jpn 62: 88-108

Rashid HA, Hendon HH, Wheeler MC, Alves O (2011) Prediction of the Madden-Julian oscillation with the POAMA dynamical prediction system. Clim Dyn 36: 649-661

Salby ML, Hendon HH (1994) Intraseasonal behavior of clouds, temperature, and motion in the tropics. J Atmos Sci 51:2207-2224

Wang B (2005) Basic theories. In: Lau KM and Waliser DE (eds) Intraseasonal Variability of the Ocean and Atmosphere. 307-360, Springer, Heidelberg, Germany

Wang B, Rui H (1990) Synoptic climatology of transient tropical intraseasonal convection anomalies: 1975-1985. Meteor Atmos Phys 44:43-61

Wang B, Xie X (1997) A model for the boreal summer intraseasonal oscillation. J Atmos Sci 54: 72-86

Wheeler MC, Hendon HH (2004) An all-season real-time multivariate MJO index: Development of an index for monitoring and prediction. Mon Wea Rev 132:1917-1932

Wheeler MC, Hendon HH, Cleland S, Meinke H, Donald A (2009) Impacts of the Madden-Julian oscillation on Australian rainfall and circulation. J Clim 22: 1482-1498

Yasunari T (1979) Cloudiness fluctuations associated with the northern hemisphere summer monsoon. J Meteor Soc Jpn 57: 227-242

Yasunari T (1980) A quasi-stationary appearance of 30 to 40 day period in the cloudiness fluctuation during the summer monsoon over India. J Meteor Soc Jpn 58: 225-229

Yun KS, Ha KJ, Ren B, Chan JCL, Jhun JG (2009) The 30-60 day oscillation in the East Asian summer monsoon and its time-dependent association with the ENSO. Tellus A 61A:565-578

Yun KS, Seo KH, Ha KJ (2010) Interdecadal change in the relationship between ENSO and the intraseasonal oscillation in East Asia. J Clim 23: 3599-3612

Zhang CD, Dong M (2004) Seasonality in the Madden-Julian oscillation. J Clim 17:3169-3180

Zhang CD (2005) Madden-Julian oscillation. Rev Geophys 43:RG2003.   Doi:10.1029/2004RG 00015