Climatological Annual Cycle of the Salinity Budgets of the Subtropical Maxima

Six subtropical salinity maxima (S-max) exist: two each in the Pacific, Atlantic, and Indian Ocean basins. The north Indian (NI) S-max lies in the Arabian Sea while the remaining five lie in the open ocean. The annual cycle of evaporation minus precipitation (E - P) flux over the S-max is asymmetric about the equator. Over the Northern Hemisphere S-max, the semiannual harmonic is dominant (peaking in local summer and winter), while over the Southern Hemisphere S-max, the annual harmonic is dominant (peaking in local winter). Regardless, the surface layer salinity for all six S-max reaches a maximum in local fall and minimum in local spring. This study uses a multidecade integration of an eddy-resolving ocean circulation model to compute salinity budgets for each of the six S-max. The NI S-max budget is dominated by eddy advection related to the evolution of the seasonal monsoon. The five open-ocean S-max budgets reveal a common annual cycle of vertical diffusive fluxes that peak in winter. These S-max have regions on their eastward and poleward edges in which the vertical salinity gradient is destabilizing. These destabilizing gradients, in conjunction with wintertime surface cooling, generate a gradually deepening wintertime mixed layer. The vertical salinity gradient sharpens at the base of the mixed layer, making the water column susceptible to salt finger convection and enhancing vertical diffusive salinity fluxes out of the S-max into the ocean interior. This process is also observed in Argo float profiles and is related to the formation regions of subtropical mode waters.