Summary
Under climate change, both the global water cycle (evaporation minus precipitation) and mixed layer salinity exhibit a pattern amplification characterized by the "rich get richer and poor get poorer." However, the contributions of horizontal and vertical oceanic processes to these patterns remain poorly understood. Using various ocean data products (1979–2018), this study quantifies the roles of horizontal and vertical advection, diffusion, and vertical entrainment in shaping mixed layer salinity distributions through salinity budget analysis. Our findings suggest that horizontal advection mitigates local salinity buildup in evaporation-dominated subtropical regions, whereas vertical processes transfer excess freshwater into the deeper ocean in precipitation-dominated areas. These mechanisms collectively regulate the freshwater flux imprint within the mixed layer, driving distinct salinity patterns across ocean basins and salinity classes. Furthermore, we estimate global freshwater transport within and beyond the mixed layer, advancing our understanding of the relationship between the global water cycle and ocean salinity dynamics. These findings provide insights into the physical processes underpinning salinity amplification and its evolution by human induced climate change.