The derivation of surface boundary conditions for salt and freshwater budgets, in the presence of an imbalance of evaporation and precipitation, is revisited in order to: (i) clarify the physical concepts involved and (ii) derive consistent approximations. Proper surface boundary conditions can be obtained by assuming that the air-sea interface is a material surface for the salt continuum. This requires all the mass exchange at the air-sea interface to be fulfilled by the freshwater continuum. The need of diffusive fluxes is crucial for this assumption to hold. If the air-sea interface varies in time (either by seawater advection, an imbalance of evaporation minus precipitation, or a combination of both) there must be a corresponding salt flux, in order for the transport across such moving surface to be nil. Here it is essential to distinguish the flux vector of a property from its transport across a (possibly moving) surface. Only in a motionless ocean surface approximation, which includes the classical rigid lid approximation, the advective and diffusive salt fluxes perpendicular to the surface cancel out exactly.
- Boundary condition
ASJC Scopus subject areas
- Computer Science (miscellaneous)
- Geotechnical Engineering and Engineering Geology
- Atmospheric Science