Variability of dissolved oxygen in the Arabian Sea Oxygen Minimum Zone and its driving mechanisms

The Arabian Sea hosts one of the most intense, perennial Oxygen Minimum Zones (OMZ) in the world ocean. Observations along a meridional transect at 68 degrees E extending from 8 to 21 degrees N showed large seasonal as well as interannual changes in the dissolved oxygen and nitrite concentrations. Unlike previous studies that used observations from the periphery of the OMZ, our observations are from its core and also allow us demarcating the southern extent of the OMZ. The southern boundary of the OMZ extended up to similar to 9 degrees N during the southwest monsoon but was restricted to similar to 11 degrees N during the northeast monsoon. While previous studies report ventilation from the oxygenated Indian Central Water (ICW) at intermediate depths of the OMZ, none have so far discussed how this ventilation takes place or its variability. Our observations reveal intrusion of the ICW at intermediate depths (200-500 m) into the OMZ up to 16 degrees N, in contrast to 11 degrees N reported in earlier studies. The oxygen variations at the southern boundary of the OMZ are coherent with those in the sea level, associated with the westward propagating Rossby waves radiated off the west coast of India. Besides modulating the oxygen distribution in the surface layer and the upper boundary of the OMZ through upwelling or downwelling, these Rossby waves can potentially influence the oxygen at intermediate depths by controlling the isopycnal mixing of the ICW within the OMZ, leading to variations in oxygen and nitrogen concentrations. Further investigations are needed to better understand the potential influence of these Rossby waves on the OMZ variability and the associated nitrogen cycling.