The Salinity Budget of the Subtropical Salinity Maxima

We compute the salinity budgets from output of an eddy-resolving ocean circulation model for the six oceanic subtropical salinity maxima (STSM) using two control volumes: a volume vertically integrated from the surface to the base of the mixed layer and a volume bound by the isohaline surfaces surrounding each salinity maximum. The annual cycle of the salinity budgets of the open-ocean STSM (those not including the North Indian) differ in magnitude but are qualitatively similar. While the surface layer salinity budgets typically focus on the advection of salinity within the surface layer, we examine role of both mode water formation and the seasonal cycle of the density ratio near the base of the mixed layer to demonstrate the influence of Type II (Eastern Subtropical) mode water formation and salt finger convection at the base of the mixed layer on the budget. The model predicts that significant amounts of salt fingers exist seasonally within the mixed layer in regions exhibiting low density ratio and enhance the vertical diffusive salt flux through the base of the mixed layer. We also compute transformation within the isohaline volumes surrounding each STSM. It demonstrates that the dominant mixing process is vertical diffusion for all of the STSM, with the notable exception of the South Indian, where vertical diffusion transforms 6 Sv and eddy induced mixing transforms 14 Sv.