Improved estimates of water cycle change from ocean salinity: the key role of ocean warming

Changes in the global water cycle critically impact environmental, agricultural, and energy systems relied upon by humanity (Jiménez Cisneros et al 2014 Climate Change 2014: Impacts, Adaptation, and Vulnerability (Cambridge: Cambridge University Press)). Understanding recent water cycle change is essential in constraining future projections. Warming-induced water cycle change is expected to amplify the pattern of sea surface salinity (Durack et al 2012 Science 336 455–8). A puzzle has, however, emerged. The surface salinity pattern has amplified by 5%–8% since the 1950s (Durack et al 2012 Science 336 455–8, Skliris et al 2014 Clim. Dyn. 43 709–36) while the water cycle is thought to have amplified at close to half that rate (Durack et al 2012 Science 336 455–8, Skliris et al 2016 Sci. Rep. 6 752). This discrepancy is also replicated in climate projections of the 21st century (Durack et al 2012 Science 336 455–8). Using targeted numerical ocean model experiments we find that, while surface water fluxes due to water cycle change and ice mass loss amplify the surface salinity pattern, ocean warming exerts a substantial influence. Warming increases near-surface stratification, inhibiting the decay of existing salinity contrasts and further amplifying surface salinity patterns. Observed ocean warming can explain approximately half of observed surface salinity pattern changes from 1957–2016 with ice mass loss playing a minor role. Water cycle change of 3.6% ± 2.1% per degree Celsius of surface air temperature change is sufficient to explain the remaining observed salinity pattern change.