Evaluating the Arabian Sea as a regional source of atmospheric CO2: seasonal variability and drivers

The Arabian Sea (AS) was confirmed to be a net emitter of CO2 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in situ data have been collected, leaving data-based methods to calculate air-sea exchange with fewer and potentially out-of-date data.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 CO2 flux compared to other approaches. To address these shortcomings, we employ a high-resolution (1/24 degrees) regional model to quantify the seasonal cycle of air-sea CO2 exchange in the AS by focusing on two main contributing factors, pCO(2) and winds. We compare the model to available in situ pCO(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 pCO(2) because it captures the AS's intense monsoon dynamics. In the seasonal pCO(2) cycle, temperature plays the major role in determining surface pCO(2) except where DIC delivery is important in summer upwelling areas. Since seasonal temperature forcing is relatively uniform, pCO(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 CO2 release. The most intense air-sea CO2 exchange occurs during the summer monsoon when outgassing rates reach similar to 6 molCm(-2)yr(-1) in the upwelling regions of Oman and Somalia, but the entire AS contributes CO2 to the atmosphere. Despite a regional spring maximum of pCO(2) driven by surface heating, CO2 exchange rates peak in summer due to winds, which account for similar to 90 % of the summer CO2 flux variability vs. 6 % for pCO(2). In comparison with other estimates, we find that the AS emits similar to 160 TgCyr(-1), slightly higher than previously reported. Altogether, there is 2x 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 pCO(2), studies looking to predict CO2 emissions in the AS with ongoing climate change will need to correctly resolve their timing, strength, and upwelling dynamics.