Virtual Salt Flux

Virtual salt flux is a modeling device that represents the dilution and concentration effects of freshwater fluxes as an equivalent salt flux applied to the salinity equation, rather than adding or removing water. It arose from the rigid-lid approximation, which fixed ocean volume to suppress fast external gravity waves, so evaporation minus precipitation and runoff could not change cell thickness. The surface boundary condition becomes virtual salt flux = (E - P - R) times sea-surface salinity. Modern free-surface models replace it with real freshwater flux, since the virtual approach biases regional salinity and sea level under strong forcing.