Eastern Arabian Sea Is a Perennial Source of CO 2 With High Coastal Fluxes: Quantifying Key Drivers

Abstract Global atmospheric carbon dioxide (CO 2 ) concentrations have increased rapidly in recent years, raising concerns about air‐sea CO 2 exchange. The Arabian Sea, being a major CO 2 source with uncertain coastal fluxes, necessitates assessing its governing mechanisms and key drivers. To address this issue, nine basin‐scale observations were conducted in the eastern Arabian Sea (EAS) between January 2018 and January 2019. Surface p CO 2 (289–1,310 μatm) exhibited the lowest mean in the early spring (428 ± 30 μatm) and the highest in the late summer monsoon (606 ± 97 μatm). Strong upwelling coupled with cyclonic eddies elevates surface p CO 2 in the south and central EAS during the summer monsoon (SM), while moderate upwelling and strong winds drive CO 2 enrichment in the north. Coastal stratification and benthic production lower p CO 2 during non‐monsoons. Annually, mixing accounted for 36%–52% of p CO 2 variability, followed by air‐sea exchange (5%–34%), biological processes (10%–29%) and temperature (3%–26%); freshwater influence was significant only in the south (12%) during the peak SM. Consistently higher surface p CO 2 than the atmosphere makes the EAS a perennial source (4.7 ± 8.4 mmolC m −2 d −1 ), with maximum efflux during the peak SM (15.9 ± 19.5 mmolC m −2 d −1 ). The SM contributes 66% of annual emissions (9.9 TgC y −1 ) as upwelled CO 2 exceeds biological uptake. The north EAS, though productive in winter and summer, emits the most (6.5 TgC y −1 ), with substantial input from turbid macro‐tidal nearshore regions (2.1 TgC y −1 ), making it a hotspot for CO 2 emissions. Although no clear p CO 2 trend emerged over two decades, climate cycles and monsoon intensity strongly modulate interannual variability.