Diapycnal oxygen flux in the oxycline region of the core of the Arabian Sea Oxygen Minimum Zone

Abstract

This study quantified the relative contributions of double diffusion and shear-driven mixing on the diapycnal oxygen flux in the oxycline region of the core of the Arabian Sea Oxygen Minimum Zone (OMZ), a critical oceanic region regulating the local ecosystem dynamics significantly. For that purpose, we used concurrent measurements of vertical profiles of dissolved oxygen and microstructure shear collected during the scientific cruise in May 2019. It was found that the presence of a moderately strong salt finger regime in the oxycline region enhanced the downward diapycnal oxygen flux (-7.0 ± 0.5x10^-2 μmol m^-2 s^-1) by an order of magnitude higher in contrast to traditionally considered shear-driven turbulence alone (-6.4 ± 0.7 x 10^-3 μmol m^-2 s^-1). This difference is solely attributable to the small magnitude of diapycnal diffusivity due to shear-driven turbulence (2.8 ± 1.1 x 10^-6 m² s^-1) compared to salt finger (10^-5 to 10^-4 m² s^-1). For the sake of comparison, the magnitude of lateral flux of oxygen due to horizontal circulation (3x10^-4 μmol m^-2 s^-1) and oxygen consumption rates due to biological processes estimated from observation (-3x10^-3 to -40 x 10^-3 μmol m^-2 s^-1) were much smaller than salt finger induced downward diapycnal oxygen flux in the oxycline region. As a result, there was a net downward oxygen flux from the oxycline into the OMZ. These findings highlighted the need to incorporate double diffusion processes in biogeochemical models to improve the accuracy of vertical oxygen distribution simulations in the Arabian Sea OMZ.