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The highly saline, oxygen-saturated waters of the Arabian Gulf (hereafter the Gulf) sink to intermediate depths (200-300 m) when they enter the Arabian Sea, ventilating the World's thickest oxygen minimum zone (OMZ). Here, we investigate the impacts of a warming of the Gulf consistent with climate change projections on the intensity of this OMZ. Using a series of eddy-resolving model simulations, we show that the warming of the Gulf waters increases their buoyancy and hence limits their contribution to the ventilation of intermediate depths. This leads to an intensification of the OMZ and an increase in denitrification that depletes subsurface nitrate and limits deoxygenation at depth. The projected future concomitant increase of Gulf salinity only partially reduces the OMZ intensification. Our findings highlight the importance of the Arabian marginal seas for the biogeochemistry of the North Indian Ocean and stress the need for improving their representation in global climate models.

Plain Language Summary Dissolved oxygen in the ocean is fundamental for marine life. While relatively abundant in surface waters, oxygen generally declines with depth as it is consumed by organisms' respiration. In certain regions like the Arabian Sea, oxygen concentrations are too low at depth to support marine animals. These are known as "oxygen minimum zones" (OMZs). At their core, extreme oxygen depletion known as suboxia can also cause a loss of bioavailable nitrogen, essential for phytoplankton growth. Using a series of computer simulations, we show that the sinking of oxygen-saturated dense waters formed in the Arabian Gulf contributes to oxygen replenishment of the intermediate depths (200-300 m) in the northern Arabian Sea, reducing the intensity of the OMZ and limiting the volume of its suboxic core. We also show that a warming of the Gulf waters consistent with recent observations and future climate projections limits their ability to sink and ventilate the intermediate depths. This results in a strong intensification of the OMZ and an important loss of bioavailable nitrogen. Our findings highlight the importance of semienclosed seas like the Arabian Gulf for the ventilation of the ocean and hence stress the need for improving their representation in climate models.