Drivers of future seasonal and interannual changes in pCO2

Angeles Gallego, University of Hawaii, Honolulu, United States, Axel Timmermann, Center for Climate Physics, Institute for Basic Science, Busan, South Korea, Tobias Friedrich, IPRC-SOEST, Honolulu, HI, United States and Richard E Zeebe, Univ Hawaii Manoa, Honolulu, HI, United States
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 21st 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.