Oxygen minimum zone along the eastern Arabian Sea: Intra-annual variation and dynamics based on ship-borne studies

Abstract

The oxygen minimum zone (OMZ) in the eastern Arabian Sea (EAS, ∼6° to 21°N), within Indian Exclusive Economic Zone (EEZ), is mapped, for the first time, for one year through ten repeated ship-based observations between December 2017 and January 2019 at seven to ten stations along the 2000 m depth contour. On an annual basis, the OMZ (<20 µM oxygen) in the EAS varied between 60 and 1350 m; its thickness decreased from north to south. During the winter monsoon, the upper boundary of the OMZ in the north and south was deeper (150–160 m) than the central EAS (∼110 m). Consequent to the summer monsoon upwelling and resultant Ekman transport, this upper boundary shoaled to 50–70 m in the south and central EAS. In contrast, relatively weaker upwelling in the north restricted the shoaling to ∼ 110 m. The core-OMZ (<5 µM oxygen), representing the core-denitrifying zone, has been found to have a perennial southern boundary in the central EAS at ∼ 15°N, which was thinner (150–750 m) than in the north (190–1000 m). The restriction of perennial core-OMZ to the central EAS is augmented by the relatively stronger upwelling south of it during the summer monsoon; the resultant upliftment of the water column has facilitated the progression of poleward undercurrent that ventilates this region. The aeration due to this poleward undercurrent enhanced the oxygen levels of OMZ (150 to 500 m) by ∼ 5 µM between 6.5°N and 13°N, and further north it increased by ∼ 2.5 µM up to ∼ 15°N. But this core-OMZ was extended to the south up to 12°N when a weak equatorward undercurrent acted in the region during the non-monsoon seasons. The study affirms the importance of sustained ship-based studies on OMZ in the EEZs, where no autonomous observatories like Bio-Argo floats are available, as such studies are critical to assess not only the future changes in the extent and behaviour of OMZ (core-OMZ) and its consequent advection onto the western continental shelf of India developing severe hypoxic to anoxic conditions but also its implications in modifying their biogeochemistry and food chain.