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

Alain de Verneil (1) , Zouhair Lachkar (1) , Shafer Smith (2) , Marina Lévy (3)

1 New York University [Abu Dhabi]
2 CIMS - Courant Institute of Mathematical Sciences [New York]
3 PROTEO - Processus et interactions de fine échelle océanique

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 little in situ data has been collected, leaving data-based methods to calculate air-sea exchange with fewer data and potentially out-of-date. Additionally, coarse-resolution models underestimate CO₂ flux compared to other approaches. To address these shortcomings, we employ a high-resolution (1/24 o) regional model to quantify the seasonal cycle of air-sea CO₂ exchange in the AS by focusing on two main contributing factors, pCO₂ and winds. We compare the model to available in situ pCO₂ 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 pCO₂ because it captures the AS's intense monsoon dynamics. In the seasonal pCO₂ cycle, temperature plays the major role in determining surface pCO₂ , except where DIC delivery is important in summer upwelling areas. Since seasonal temperature forcing is relatively uniform, pCO₂ differences between the AS's sub-regions 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 where outgassing rates reach ∼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 pCO₂ driven by surface heating, CO₂ exchange rates peak in summer due to winds, which account for ∼90% of the summer CO₂ flux variability versus 6% for pCO₂ in a Reynolds decomposition. In comparison with other estimates, we find that the AS emits ∼160TgCyr⁻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 pCO₂ , studies looking to predict CO₂ emissions in the AS with ongoing climate change will need to correctly resolve their timing, strength, and upwelling dynamics.

Domaines: Sciences de l'environnement

Alain de Verneil, Zouhair Lachkar, Shafer Smith, Marina Lévy. Evaluating the Arabian Sea as a regional source of atmospheric CO₂: seasonal variability and drivers. 2021. ⟨hal-03607931v1⟩