Effects of Ocean Surface-Water Salinity on Osmotic Potential and Water-Vapor Emission Potential

Studies have shown that oceanic surface-water salinity varies across the globe and changes over time, while atmospheric water-vapor levels have also increased in recent decades. Evaporation from ocean and inland waters is controlled primarily by meteorological forcing, but the thermodynamic state of the water body also matters. In saline waters, dissolved solutes reduce water activity and thereby reduce the equilibrium tendency of water molecules to enter the vapor phase. In this study, the authors’ coefficient-less aqueous osmotic potential equation was used to examine the thermodynamic effect of representative oceanic salinity differences on evaporative tendency. Calculations were made for recorded surface-water salinities ranging from 31 to 38 kg·m−3 of dissolved solutes at an average temperature of 20 °C. Computed osmotic potentials ranged from −2.257 to −2.708 MPa. The corresponding semi-permeable membrane interface pressures ranged from 8.935 to 8.484 MPa, indicating an approximately 5% difference across the selected oceanic salinity range. The interface pressure calculated for solute-free water (11.192 MPa) was more than 24% higher than for the seawater cases considered. These results suggest that salinity acts as a secondary thermodynamic modifier of evaporation potential, whereas radiative, aerodynamic, humidity, and temperature controls remain dominant in determining actual evaporation fluxes. The results also indicate that freshwater bodies and changing land-based evaporative sources may contribute differently to atmospheric water vapor than saline ocean waters. The framework presented here is intended to complement, rather than replace, established evaporation formulations by clarifying how salinity-related osmotic effects can modify the water-side boundary condition.