Drivers of future seasonal cycle changes in oceanic p CO 2

Recent observation-based results show that the seasonal amplitude of surface ocean partial pressure of CO2 (pCO(2)) has been increasing on average at a rate of 2-3 mu atm per decade (Landschutzer et al., 2018). Future increases in pCO(2) seasonality are expected, as marine CO2 concentration ([CO2]) will increase in response to increasing anthropogenic carbon emissions (McNeil and Sasse, 2016). 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 pCO(2) 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 pCO(2) 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 pCO(2) seasonal amplification we develop a complete analytical Taylor expansion of pCO(2) 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 pCO(2) seasonality. To first order, their future intensification can be traced back to a doubling of the annual mean pCO(2), 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 degrees S-40 degrees N) will experience a approximate to 30-80 mu atm 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 approximate to 90-120 mu atm in response to the mean pCO(2)-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.