Need for high-resolution oxygen profiles from the eastern Arabian Sea

The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest among the three major oceanic OMZs and accounts for 30-50% of oceanic nitrogen loss. It is hypothesized that OMZs will expand due to global climate change. In the Eastern Arabian Sea (EAS), our understanding of OMZ dynamics is primarily based on model simulation, highlighting the need for direct observations. This study compares ship-based oxygen dataset from the Marine Ecosystem Dynamics of eastern Arabian Sea (MEDAS) programme, collected onboard FORV Sagar Sampada and ORV Sagar Kanya (10 repeat observations across seven transects along the EAS between December 2017 and January 2019), with the World Ocean Atlas 2018 (WOA18) dataset. MEDAS data show that the OMZ (oxygen concentrations ≤22µM) is found between ~60 and 1350m, similar to the WAO18's estimate (between ~100 and 1300m). However, the WAO18 failed to depict the denitrifying zone (defined as DO≤5µM) observed between 190 and 1060m at northern EAS (~22°N) and its southward extension (up to ~12°N), underscoring the need for high-resolution oxygen data. Furthermore, the widespread use of WOA climatology in biogeochemical models may lead to inaccuracies, particularly for parameters sensitive to low oxygen levels, as WOA lacks data from regions like the Indian EEZ,  where BIOARGOs cannot capture complex biogeochemical features. Therefore, ship-based datasets are critical for accurately validating models in such regions