Future Phase Reversal of the Seasonal Cycle of pCO2 in the Arctic Ocean

Previous studies of the ongoing changes in the seasonal cycles of ocean CO₂ system variables have generally focused on changes in amplitude. Here we focus on changes in seasonal phasing. To do so, we analyzed 9 CMIP5 models under the historical and RCP8.5 scenarios. All models project little change in the seasonal phasing of oceanic pCO₂ over this century throughout most of the ocean. But the Arctic Ocean is an exception. Its seasonal cycle in pCO₂ is presently dominated by a negative summertime anomaly relative to the annual mean. That negative excursion is driven by effects from ocean biological uptake of carbon outweighing those from the decline in CO₂ solubility due to summertime warming. Yet by the end of the century, the Arctic Ocean's summertime pCO₂ anomaly becomes positive in most models. Idealized CMIP5 simulations indicate that this sign reversal results from the combined action of the ocean's increase in anthropogenic carbon and climate-driven changes in the ocean. The sign reversal is mostly driven by a large increase in the seasonal amplitude of SST due to increased loss of summer sea ice, an effect that eventually overwhelms the non-thermal effects. This mechanism is clarfied with (1) a Taylor expansion of the drivers of oceanic pCO₂ seasonal variability and (2) a comparison of seasonal variations of oceanic pCO₂ and aqueous CO₂ concentration [CO₂*], i.e., the product of pCO₂ and the CO₂ solubility. Unlike for oceanic pCO₂, [CO₂*] does not exhibit a reversal in the summertime anomaly because being corrected by the solubility, it is insensitive to seasonal temperature changes. In all models, the biologically driven negative summertime anomaly in [CO₂*] is only intensified, not being countered by a growing temperature effect. Thus the change in the amplitude of the seasonal cycle of [CO₂*] (on average a tripling in the Arctic Ocean by 2100) can differ from that of oceanic pCO₂. These differences emphasize that when projecting seasonal changes in CO₂ system variables under climate change, one cannot assume that the seasonal variations in their drivers remain unchanged, particularly in the Arctic. Moreover, although oceanic pCO2 may be an appropriate variable when discussing air-sea CO₂ fluxes, it can be misleading to consider only pCO2 and not [CO₂*] when focusing on how ocean acidification affects marine organisms.