Abstract:

Drivers of future seasonal cycle changes in oceanic pCO₂

Recent observation-based results show that the seasonal amplitude of surface ocean partial pressure of CO (pCO₂) has been increasing on average at a rate of 2-3 µatm per decade. Future increases in pCO₂ seasonality are expected, as marine CO₂ concentration ([CO₂]) will increase in response to increasing anthropogenic carbon emissions. Here we use seven different global coupled atmosphere-ocean-carbon cycle-ecosystem model simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5) to study future projections of the pCO₂ annual cycle amplitude and to elucidate the causes of its amplification. We find that for the RCP8.5 emission scenario the seasonal amplitude (climatological maximum minus minimum) of upper ocean pCO₂ will increase by a factor of 1.5 to 3 over the next 60-80 years. To understand the drivers and mechanisms that control the pCO₂ seasonal amplification we develop a complete analytical Taylor expansion of pCO₂ seasonality in terms of its four drivers: dissolved inorganic carbon (DIC), total alkalinity (TA), temperature (T), and salinity (S). Using this linear approximation we show that the DIC and T terms are the dominant contributors to the total change in pCO₂ seasonality. To first order, their future intensification can be traced back to a doubling of the annual mean pCO₂, which enhances DIC and alters the ocean carbonate chemistry. Regional differences in the projected seasonal cycle amplitude are generated by spatially varying sensitivity terms. The subtropical and equatorial regions (40°S-40°N) will experience a ∼30-80 µatm increase in seasonal cycle amplitude almost exclusively due to a larger background CO₂ concentration that amplifies the T seasonal effect on solubility. This mechanism is further reinforced by an overall increase in the seasonal cycle of T as a result of stronger ocean stratification and a projected shoaling of mean mixed layer depths. The Southern Ocean will experience a seasonal cycle amplification of ∼ 90-120 µatm in response to the mean pCO₂-driven change in the mean DIC contribution and to a lesser extent to the T contribution. However, a decrease in the DIC seasonal cycle amplitude somewhat counteracts this regional amplification mechanism.

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