Impacts of rising atmospheric CO2 concentrations and climate variability on air-sea CO2 fluxes in the Arabian Sea between 1980 and 2018

The Arabian Sea (AS) acts as a net source of CO₂ to the atmosphere primarily due to summer upwelling of deep, carbon-rich waters along the coastlines of Somalia and Oman. Recent environmental changes, such as rising atmospheric CO₂ concentrations, rapid warming, and changing summer monsoon winds, may affect air-sea CO₂ fluxes in the region. However, our understanding of the long-term variability of these fluxes and its drivers remains limited. To address this gap, we use an eddy-resolving model of the Indian Ocean to assess the evolution of air-sea CO₂ fluxes from 1980 to 2018 and explore the mechanisms underlying their long-term variations. Our analysis indicates a decline in AS carbon emissions to the atmosphere, estimated at 0.06 ± 0.05 PgC/yr, decreasing at a rate of -7% per decade over the study period. This decline primarily stems from rising anthropogenic CO₂ concentrations, resulting in a -13% reduction in outgassing per decade, partially offset by climate warming, which enhances outgassing at a rate of +6% per decade. The influence of warming and climate variability on CO₂ emissions is notably pronounced in the Arabian Gulf and the region off the coast of Somalia. In the former, rapid warming driven by enhanced downward radiation and weakened surface winds has intensified CO₂ emissions, resulting in a 40% increase in outgassing by the late 2010s compared to the early 1980s. In the latter, the decline in CO₂ outgassing driven by rising atmospheric CO₂ concentrations is counterbalanced by enhanced upwelling. These findings underscore the importance of local climatic factors, such as changes in winds and radiative forcing, in modulating the evolution of air-sea CO₂ fluxes at regional scales and highlight the necessity of improving their representation in climate models.