Seasonal contribution to the spice generation in subtropical south Indian salinity maxima

Regions of salinity maxima (S_max) in the world oceans experience spiciness changes which in-turn subduct and advect towards equator along the shallow meridional overturning cell. Advection results in stabilizing vertical salinity gradient (salinity increasing with depth) along westward and equatorward edges of S_max region whereas eastward and poleward edges holds destabilizing vertical salinity gradient with maximum salinity at surface (salinity decreasing with depth). Based on this contrast vertical salinity gradient, subtropical south Indian Ocean salinity maxima region is divided into two boxes along 30ºS. Seasonal evolultion of spiciness with respect to atmospheric and oceanic forcings, are investigated by using high frequency (3-day), high resolution (0.25º) ECCO (Estimating the Circulation and Climate of the Ocean) estimate. It is observed that in both these boxes, spice generation mechanisms are different. During austral winter, 25-sigma isopycnal outcrops to the north of 30S (northern box), along which the Temperature/Salinity changes subduct and hence spiciness anomalies are formed below the mixed layer. However, destabilization of vertical salinity gradient to the south of 30S (southern box), concomitant with weak stratification results in convective mixing at the mixed layer base and hence the spiciness changes penetrate the main thermocline. Seasonal mixed layer heat and salt budget analysis show that the surface heat and freshwater fluxes are the main forcings controlling monthly evolution of spiciness in the northern box, whereas in the southern box entrainment and meridional advection terms are mainly contributing for the spiciness changes.