Drivers of future seasonal and interannual changes in pCO2

Anthropogenic CO₂ emissions are altering the ocean's underlying carbonate chemistry; increasing its CO₂ concentration and lowering the buffering capacity. As a consequence, the ocean's pCO₂ response to any temporal change in its main drivers, temperature (T) and dissolved inorganic carbon (DIC), could be amplified. We use 6 different global coupled atmosphere/ocean model simulations, from the Coupled Model Intercomparison Project Phase 5 (CMIP5), to study future projections of the pCO₂ annual cycle and interannual variations. We find that for the Representative Concentration Pathway 8.5 (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; and the global mean interannual variability of pCO₂ (calculated as 1 standard deviation) could increase by 62 ± 22 % by the end of the 21^st century.

To understand the mechanisms that control the pCO₂ variability amplification we develop a complete analytical Taylor expansion of pCO₂ in terms of its four drivers: DIC, T, total alkalinity (TA), and salinity (S). Using this linear approximation we show that the amplification is mainly caused by the increased response of pCO₂ to T and DIC variations, due to a larger background pCO₂ and a lower capacity to buffer DIC. However, there are large regional differences; the ocean's buffering capacity is reduced the most in the high latitudes, exposing them to larger pCO₂ amplification rates. The DIC’s seasonal and interannual variations are projected to decrease in the equatorial Pacific, which counteracts the regional seasonal amplification and even reduces the future pCO₂ interannual variability for some models. The intra-model differences in interannual projections indicate that the potential changes in water carbonate chemistry are simulated with higher consistency than those in ocean circulation and biology.