Salinity effects in a tropical ocean model

A reduced gravity, primitive equation, ocean general circulation model (GCM) with a variable depth mixed layer and a natural boundary condition for freshwater fluxes is employed to investigate the role of salinity in tropical ocean dynamics and thermodynamics. Surface heat fluxes are computed without any feedback to observations by an advective atmospheric mixed layer (AML) model which is coupled to the ocean GCM. We analyze the differences in the tropical Atlantic, Pacific, and the Indo-Pacific basins between control runs (simulations with complete hydrology) and simulations where (1) precipitation (P) is neglected, (2) salinity effects are neglected, or (3) salinity is held constant in each layer. Salinity contributes to pressure gradient forces, mixed layer processes, and vertical stability/mixing. Setting P = 0 in the tropical Atlantic produces larger sea surface temperature (SST) changes than previously estimated due to the realistic oceanic mixed layer model and surface flux formulation. Neglecting salinity effects leads to a different choice of mixing parameters, which feeds back into model dynamics and thermodynamics. Salinity anomalies produce an asymmetric response across the equator in the Atlantic due to differences in the air-sea interactions. Including salinity effects in the tropical Pacific leads to an improved cold tongue simulation. The result is a reduced SST gradient at the equator which will have significant feedback in a coupled system. The same experiment with a restoring surface heat flux leads to an increased SST gradient, indicating that the surface flux parameterization is crucial for interpreting the role of salinity. The Indonesian throughflow (ITF) is reduced when salinity is neglected or held constant. The NINO3 and NINO4 SST indices are almost identical for the control run and the simulations when climatological P is used. However, associated subsurface temperature differences are larger, and they may play a role on decadal timescales. It is thus shown with a comprehensive set of experiments that even in the tropics, salinity plays an important role in the model dynamics and thermodynamics.