Ventilation of Oxygen to Oxygen Minimum Zone Due to Anticyclonic Eddies in the Bay of Bengal

Abstract

Intense oxygen minimum zone (OMZ) occurs in the middepth of the Eastern Tropical Pacific (ETP), Arabian Sea (AS), and Bay of Bengal (BoB). However, the occurrence of anammox/denitrification was reported only in the ETP and AS and its absence in the BoB is attributed to presence of traces of dissolved oxygen (DO). Anticyclonic eddies (ACEs) supply high-nutrient, organic-rich, and oxygen-poor waters from the coastal upwelling regions leading to strengthening of OMZ in the offshore of AS and ETP. In the absence of western boundary upwelling, we hypothesize that the ACEs supply DO-rich water leading to weakening of OMZ in the BoB. Six ACEs were sampled by bio-argo floats and measured in situ hydrography and DO. All sampled eddies were formed in the eastern BoB, close to Andaman Sea, and moved toward west. Warm and DO-rich waters were observed in the core of OMZ (150-300 m), influenced by ACEs, by 0.5-1.46oC and 3.2-6.5 -M, respectively, than no eddy region in the BoB. Based on the mean lifetime of ACE, the rate of ventilation in the 100-300 m was estimated to be 0.07 ± 0.02 -mol/L/day, which is 3 to 4 times higher than bacterial respiration rates (0.02 -mol/L/day). Concerns of OMZ intensification leading to denitrification/anammox conditions in the BoB have been expressed, but with several ACEs forming in time and space in the BoB the denitrification occurrence is unlikely.