Mesoscale eddy modulation of winter convective mixing in the northern Arabian Sea

The formation of Arabian Sea High Salinity Water (ASHSW) during the winter monsoon in the northern Arabian Sea is driven by surface buoyancy loss, which increases surface density and triggers convective mixing. The depth of convective mixing is influenced by the interplay between surface cooling (buoyancy loss) and upper-ocean stratification. Mesoscale eddies present during winter can alter this stratification and modulate convective mixing. We investigated the impact of these eddies on convective mixing and ASHSW formation utilizing a combination of observations, data assimilative model results, and 1-D and 3-D model experiments. Our analyses consistently demonstrate that the depth of winter convective mixing is influenced by the stratification imposed by mesoscale features, resulting in distinct mixing characteristics. When subjected to identical buoyancy forcing, convective mixing associated with cyclonic eddies occurs at shallower depths compared to anticyclonic eddies. This difference is particularly pronounced during the peak period of convective mixing (January–February), exceeding 50 m, compared to the early stages (November–December). The combined effect of cyclonic and anticyclonic eddies generates spatially inhomogeneous convective mixing, which cannot be solely explained by buoyancy flux. These conclusions are supported by Argo observations and analyses of 1-D and 3-D model experiments. Overall, our study highlights the significant role of mesoscale eddies in modulating convective mixing and ASHSW formation in the northern Arabian Sea.