Real Freshwater Flux as a Natural Boundary Condition for the Salinity Balance and Thermohaline Circulation Forced by Evaporation and Precipitation

Abstract Freshwater flux used as a natural boundary condition for the salinity balance is applied to a primitive equation model of the oceanic general circulation. Instead of the relaxation condition or the virtual salt flux boundary conditions used in many existing models, the real freshwater flux across the upper surface is specified as the vertical velocity boundary condition for the continuity equation, and the salinity flux is set to identically zero at the sea surface. Numerical experiments show that a model with the natural boundary conditions runs smoothly. Much important physics involving the freshwater flux emerge from the new model. The barotropic Goldsbrough–Stommel gyres driven by the precipitation and evaporation, which were excluded in the previous numerical models, are reproduced. In addition, the model's results reveal extremely complex structure of the three-dimensional circulation driven by the freshwater flux. In fact, a relatively small amount of freshwater flux drives very strong mer...