Rx. Huang, REAL FRESH-WATER FLUX AS A NATURAL BOUNDARY-CONDITION FOR THE SALINITY BALANCE AND THERMOHALINE CIRCULATION FORCED BY EVAPORATION AND PRECIPITATION, Journal of physical oceanography, 23(11), 1993, pp. 2428-2446
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.