Ocean salinity across space-time scales: from water cycle indicator to dynamical driver

Abstract. Ocean salinity plays a complementary role in the climate system: it integrates changes in the global water cycle while also helping drive ocean circulation through its control on seawater density. Salinity has long been viewed as the ocean's “rain gauge”, a largely passive recorder of surface evaporation, precipitation, and runoff. Yet salinity also shapes the currents and mixing that redistribute heat and freshwater, raising a central question: when does salinity mainly record climate forcing, and when does it actively influence climate dynamics? This review synthesizes two decades of satellite and in situ observations within a regime-dependent framework in which salinity's function is set by the competition among freshwater forcing, advection, and mixing timescales. At basin scales (> 1000 km) over decades, salinity tracks water-cycle change through pattern amplification, with fresh regions freshening and salty regions becoming saltier. At regional to mesoscale (∼ 10–500 km) and seasonal-to-interannual timescales, salinity traces circulation pathways; subsurface anomalies often reflect subduction and ventilation histories from years earlier. At submesoscales (O(10 km)) and synoptic timescales (hours to days), salinity becomes dynamically active, sharpening density fronts, modulating stratification, and altering mixing in ways that feed back on its own transport and air–sea exchange. Understanding ocean climate response requires resolving regime boundaries where these balances shift. The critical observational gap is global sea-surface salinity at O(10 km), where salinity transitions from passive tracer to active driver yet current satellite products cannot resolve this scale. Observations at regime boundaries would show how water-cycle intensification and ocean circulation changes interact, improving projections of climate change, ocean heat storage and distribution, and ecosystem dynamics at regional and global scales.