Subsurface mixing and ventilation of oxygen minimum zone waters in the southern Bay of Bengal during the summer monsoon

Abstract

During the summer monsoon, the local wind forcings around Sri Lanka causes the formation of a cold dome called the Sri Lanka Dome (SLD), which upwells subsurface waters. To the east of SLD, the summer monsoon current (SMC) flows into the Bay of Bengal (BoB), transporting high-salinity water from the Arabian Sea. We show that the SMC and the upwelled waters of the SLD are ventilated episodically during summer monsoon in the southern BoB, leading to a net exchange of low oxygen subsurface waters with saturated mixed layers. We observed presence of hypoxic boundary < 63 µmol kg^− 1 very close to the surface. Within the SLD, it shoaled between 35 to 40 m, with oxygen values reaching as low as 6 µmol kg^− 1 at the bottom of the thermocline. Negative fluxes showing the ingassing rates ranged between − 0.33 and − 9.43 µmol m^− 2 sec^− 1 within the SLD and SMC. We propose that the episodic ventilation seen during this investigation may lead to disequilibrium between mixed layer and below thereby contributing to mid-depth oxygen enrichment. This study possibly illustrates a pathway through which the oxygen minimum zone in BoB may be gaining oxygen, thereby preventing from becoming denitrifying.