A note on boundary conditions for salt and freshwater balances

F. J. Beron-Vera, J. Ochoa, P. Ripa

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)111-118
Number of pages8
JournalOcean Modelling
Issue number2-4
StatePublished - Oct 1 1999
Externally publishedYes


  • Balance
  • Boundary condition
  • Flux
  • Freshwater
  • Salt

ASJC Scopus subject areas

  • Computer Science (miscellaneous)
  • Oceanography
  • Geotechnical Engineering and Engineering Geology
  • Atmospheric Science


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