Evaluation of model representations of the Southern Ocean seasonal air-sea carbon dioxide flux

The Southern Ocean is a region of significant importance to the global carbon cycle, absorbing over 40% of the total anthropogenic carbon emitted to the atmosphere each year. Recent observations by profiling floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project indicate a stronger wintertime outgassing of carbon dioxide in the Southern Ocean than prior estimates. Combining these observations with shipboard measurements of surface pCO₂ has yielded an improved understanding of the seasonal cycle of the Southern Ocean CO₂ flux, indicating the importance of both summertime uptake and wintertime outgassing near the Polar Front to the overall flux south of 35°S. In this study we examine the Southern Ocean seasonal cycle from a neural network product combining ship observations and float estimates of surface pCO₂ and use this product to evaluate how well 12 CMIP5 models and one state estimate reproduce the seasonal cycle in this critical area of the Southern Ocean. While many of the models analyzed have similar total annual carbon fluxes, the seasonal cycles vary widely. Five of the models have an approximately correct seasonal cycle and flux magnitude. Four models have outgassing and uptake magnitudes that are similar to observations, but have an out-of-phase seasonal cycle. Finally, three models have a significantly exaggerated, but in phase seasonal cycle, with very strong summertime uptake balancing a massive wintertime outgassing. For accurate future projections of climate and how the seasonal cycle changes, it is important to properly represent current seasonal cycles in addition to the annual flux magnitude. We find that several models do this well and future work will explore the mechanistic differences between these models and those that display different seasonal CO­₂­ fluxes.