On the Instantaneous Salt Mixing due to Freshwater Boundary Fluxes

Abstract In this paper, we detail the instantaneous transformation between freshwater and seawater while crossing the ocean boundary. From the kinematic boundary conditions for the seawater continuum, we derive the associated flux of squared salinity and link it to an instantaneous mixing of salt (salinity variance decay) at the boundary. Within the water mass transformation framework, this mixing occurs across virtual isohalines in salinity space. Our derivations complement the analytical description of seawater and clarify the demixing during freshwater removal. Significance Statement The purpose of this work is to analytically describe the mixing of salt associated with freshwater fluxes across the ocean boundary. The key finding is that the governing equations for seawater imply the instantaneous mixing of freshwater with seawater while crossing the ocean boundary. Evaporation is associated with instantaneous demixing .