Evaluating the Arabian Sea as a regional source of atmospheric CO ₂ : seasonal variability and drivers

The Arabian Sea (AS) was confirmed to be a net emitter of CO 2 to the atmosphere during the international Joint Global Ocean Flux Study program of the 1990s, but since then few in situ data have been collected, leaving data-based methods to calculate air–sea exchange with fewer and potentially out-of-date data. Additionally, coarse-resolution models underestimate CO 2 flux compared to other approaches. To address these shortcomings, we employ a high-resolution (1/24 ∘ ) regional model to quantify the seasonal cycle of air–sea CO 2 exchange in the AS by focusing on two main contributing factors, p CO 2 and winds. We compare the model to available in situ p CO 2 data and find that uncertainties in dissolved inorganic carbon (DIC) and total alkalinity (TA) lead to the greatest discrepancies. Nevertheless, the model is more successful than neural network approaches in replicating the large variability in summertime p CO 2 because it captures the AS's intense monsoon dynamics. In the seasonal p CO 2 cycle, temperature plays the major role in determining surface p CO 2 except where DIC delivery is important in summer upwelling areas. Since seasonal temperature forcing is relatively uniform, p CO 2 differences between the AS's subregions are mostly caused by geographic DIC gradients. We find that primary productivity during both summer and winter monsoon blooms, but also generally, is insufficient to offset the physical delivery of DIC to the surface, resulting in limited biological control of CO 2 release. The most intense air–sea CO 2 exchange occurs during the summer monsoon when outgassing rates reach ∼ 6 molCm-2yr-1 in the upwelling regions of Oman and Somalia, but the entire AS contributes CO 2 to the atmosphere. Despite a regional spring maximum of p CO 2 driven by surface heating, CO 2 exchange rates peak in summer due to winds, which account for ∼ 90 % of the summer CO 2 flux variability vs. 6 % for p CO 2 . In comparison with other estimates, we find that the AS emits ∼ 160 Tg C yr −1 , slightly higher than previously reported. Altogether, there is 2× variability in annual flux magnitude across methodologies considered. Future attempts to reduce the variability in estimates will likely require more in situ carbon data. Since summer monsoon winds are critical in determining flux both directly and indirectly through temperature, DIC, TA, mixing, and primary production effects on p CO 2 , studies looking to predict CO 2 emissions in the AS with ongoing climate change will need to correctly resolve their timing, strength, and upwelling dynamics.

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