Drivers of the Air-Sea CO2 Flux Seasonality and its Long-Term Change in the NASA-GISS CMIP6 submission.

Roughly 1/3 of the anthropogenic carbon emitted to the atmosphere has been taken up by the ocean since the industrial revolution. This has resulted in significant impacts to ocean properties and ecosystems, including ocean warming, ocean acidification, changes in ocean circulation, and changes to nutrient and species distributions (Doney et al., Ann. Rev. Mar. Sci. 2012). These impacts are expected to increase well into the 21^st century (Bopp et al., Biogeosciences 2012). Modeling studies have shown that not only are sea surface pCO₂ and the corresponding exchange of CO₂ across the air-sea interface increasing, but their seasonalities (difference between the maximum and minimum value within a seasonal cycle) are increasing as well (e.g., Hauck and Völker, GRL 2015). Additionally, recent observational evidence suggests that the increased uptake of anthropogenic CO₂ is resulting in enhanced seasonality in sea surface pCO₂ (Landschützer et al., Nat. Clim. Change 2018). These previous studies motivate the questions: how well do CMIP6 models represent the seasonal cycles of pCO₂ and the CO₂ flux, and how are these seasonal cycles predicted to change?

Here, we present the analysis of the seasonal cycles of the CO₂ flux, sea surface pCO₂, and their respective drivers, including temperature, salinity, wind speed, dissolved inorganic carbon (DIC), and alkalinity, in the NASA-GISS contribution to CMIP6. Seasonal cycles during the historical period are confronted against observations in different ocean regions. We find that the model most closely reproduces the seasonal cycles of the flux and pCO₂ in the subtropical gyres and Southern Ocean, with the largest source of bias in these regions being insufficient DIC-driven changes in pCO₂. Discrepancies in pCO₂ seasonal cycles in the equatorial regions and subpolar North Pacific and Atlantic are driven by a combination of biases in the seasonal cycles of alkalinity, DIC, and temperature. For the equatorial regions, the bias in wind speed also largely contributes to the bias in the CO₂ flux. Changes in the seasonal cycles of the CO₂ flux and pCO₂ under an idealized forcing scenario are largest in the subpolar and subtropical regions, and are driven mostly by changes in the sensitivity of pCO₂ to temperature and DIC.