A review of salinity perspective on the intensification of the ocean water cycle

Abstract:

The ocean water cycle has intensified in response to climate warming, as expected from the nonlinear dependence of water vapor pressure (moisture holding capacity) on temperature. The cycling of freshwater between evaporation, precipitation, and runoff leaves a strong imprint on ocean salinity, rendering the salinity to act to some extent as Nature’s rain gauge. Despite the supporting evidence from salinity, the pattern and rate of the warming-induced intensification of the water cycle remain fully understood. Substantial knowledge gaps also exist regarding how to decipher the changing water cycle from salinity observations. Here we provide a synthesis review of recent progress in diagnosing, modeling, and understanding the long-term trends in the ocean water cycle and salinity. Three major results are obtained. First, while the surface freshening trends in the tropical regions are viewed as evidence supporting the wet-gets-wetter paradigm, the increase trends of surface salinity in the subtropical regions reveal the characteristics associated with the widening Hadley circulation. The expansion of the subtropical salinity maximum poleward shifted the formation location of the subtropical underwater, allowing the salinity changes at the surface to be translated into a long-term variability in the interior ocean. Second, model studies suggested the importance of including surface temperature changes in replicating the three-dimensional pattern of observed salinity change. Ocean warming acts to increase near-surface stratification, prolonging existing salinity contrasts and causing surface salinity patterns to amplify further. This process accounts for about half of the observed change in sea surface salinity with the changing water cycle impact and a minor contribution from ice mass loss explaining the rest. Lastly, significant uncertainties still remain in the current estimates of ocean-surface freshwater exchange and budget using products sourced from satellite and atmospheric reanalyses. Maintaining the space-based observing system and advancing observing capabilities for global river discharges and freshwater exchanges at polar seas are key to advancing research in this area.