The Oxygen Minimum Zone (OMZ) in the northern Indian Ocean is among the world's largest naturally occurring low-oxygen regions and is experiencing progressive deoxygenation under climate change. This study investigates the spatial and temporal variability of the OMZ and its relationship with water masses and ocean circulation in the Arabian Sea and the Bay of Bengal using GLODAP v2.2023 observations and CMEMS reanalysis data from 1993–2025. Vertical profiles, temperature–salinity analyses, spatial distributions, and long-term dissolved oxygen trends were used to characterize OMZ dynamics. The Arabian Sea hosts the most intense OMZ between 289 and 623 m depth, where anoxic conditions are maintained by high biological productivity and the intrusion of oxygen-depleted waters from the Persian Gulf and the Red Sea. In contrast, lateral ventilation by the Southwest Monsoon Current (SMC) supplies oxygen to the Bay of Bengal, restricting the OMZ to hypoxic conditions despite its greater vertical extent (59–744 m). However, the Bay of Bengal exhibited the fastest deoxygenation rate during the study period, with dissolved oxygen declining by 0.476 mmol m⁻³ yr⁻¹. Across the northern Indian Ocean, dissolved oxygen at the OMZ core (~411 m) decreased by 32.6% between 1993 and 2023, indicating that enhanced stratification and ocean warming are intensifying OMZ expansion, while seasonal monsoon circulation remains a key natural ventilation mechanism.
 

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