REAL FRESH-WATER FLUX AS A NATURAL BOUNDARY-CONDITION FOR THE SALINITY BALANCE AND THERMOHALINE CIRCULATION FORCED BY EVAPORATION AND PRECIPITATION

Freshwater flux used as a natural boundary condition for the salinity balance is applied to a primitive equation model of the oceanic genera l circulation. Instead of the relaxation condition or the virtual salt flux boundary conditions used in many existing models, the real fresh water flux across the upper surface is specified as the vertical veloc ity boundary condition for the continuity equation, and the salinity f lux is set to identically zero at the sea surface. Numerical experimen ts show that a model with the natural boundary conditions runs smoothl y. Much important physics involving the freshwater flux emerge from th e new model. The barotropic Goldsbrough-Stommel gyres driven by the pr ecipitation and evaporation, which were excluded in the previous numer ical models, are reproduced. In addition, the model's results reveal e xtremely complex structure of the three-dimensional circulation driven by the freshwater flux. In fact, a relatively small amount of freshwa ter flux drives very strong meridional and zonal cells and baroclinic gyres, which are 100 times stronger than the driving freshwater flux. Most importantly, the model provides an accurate description of the me ridional salt fluxes and their roles in setting up the thermohaline ci rculation. It is suggested that, with or without the rigid-lid approxi mation, the real freshwater flux can be used as the upper boundary con dition in oceanic general circulation models, including the mixed-laye r models, the ice-ocean coupling models, and atmosphere-ocean coupling models.