Spatial Distribution and Seasonal Variation of Hypoxic Zone in the Eastern Equatorial Indian Ocean

The spatial distribution and seasonal variations of the hypoxic zone in the eastern equatorial Indian Ocean were investigated using survey data collected from four cruises from 2013 to 2018. Results showed that hypoxic zone occurred all year round in the eastern equatorial Indian Ocean, and it spread southward in the shape of a double tongue at two depths with one at subsurface centered at a depth of 150 m and the other in intermediate water centered at a depth of 800 m. The southward expansion and maximum thickness of the hypoxic zone were greatest in the spring inter-monsoon and least in the summer monsoon. The hypoxic zone originated from the southward expansion of the hypoxic water in the Bay of Bengal and its spatial distribution was driven by southward output flux of mid-deep (100–1000 m) hypoxic water from the Bay of Bengal. The hypoxia southward expansion was blocked near the equator in the subsurface layer, because of mixing with multiple zonal circulations (e.g., Wyrtki Jets and the equatorial undercurrent), which meant that the hypoxic zone extended over a smaller area than in the intermediate water. These new findings will contribute to an improved understanding of the hypoxic zone and will contribute to circulation research, particularly about intermediate circulation in the eastern equatorial Indian Ocean.