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    Recent oxygenation of oxygen minimum zone in the Arabian Sea and possible causes

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    Date
    2025
    Author
    Narvekar, J.
    Kessarkar, P.
    Sreejith, K.S.
    Fernandes, L.L.
    Kumar, S.P.
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    Abstract
    Dissolved oxygen (DO) in the ocean is crucial for the survival of marine organisms and a prolonged exposure to sub-optimal levels impacts their life and species distribution. The oxygen minimum zones (OMZs) are the regions in the global ocean where the mid-depth oxygen concentrations are persistently low. These zones occur naturally in areas where high sinking flux of organic carbon co-exists with poor ventilation of waters. The Arabian Sea (AS) houses the second most intense OMZ after the eastern tropical Pacific. Climate change is expected to exacerbate the de-oxygenation within the OMZ, leading to either its intensification or its spatial expansion or both. In a warming ocean, DO concentrations are expected to decrease due to a reduction in the solubility with an increase in temperature and increase in the upper ocean stratification curtailing the ventilation. Contrary to this notion, the recent measurements in a rapidly warming AS during 2019 to 2023 showed an enhancement in the DO concentration within the OMZ and a reduction in its thickness. The average DO concentration increased by 2.5 times and 2 times in 2022 and 2023 respectively from 10.31 mol/kg in 2019, while the OMZ thickness decreased by 21 % in 2022 and by 35 % in 2023 from 1014 m in 2019. We show that the reasons for this were a combination of (1) advection of waters with increased DO from the source region of water masses that enter the AS at mid-depths and (2) a reduction in the in-situ production of organic carbon within the AS. Our findings contradict prevailing views on climate-driven de-oxygenation, prompting a reassessment of biogeochemical responses and water mass dynamics in the rapidly warming AS.
    URI
    https://doi.org/10.1016/j.pocean.2025.103600
    http://drs.nio.res.in/drs/handle/2264/9560
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