153.3.4 Evidence for Change of the Hydrological Cycle from Salinity Changes

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The similarity between the geographic distribution of significant salinity and freshwater content trends (Figures 3.4, 3.5 and 3.21) and both the mean salinity pattern and the distribution of mean evaporation – precipitation (E – P; Figure 3.4) indicates, with medium confidence, that the large-scale pattern of net evaporation minus precipitation over the oceans has been enhanced. Whereas the surface salinity pattern could be enhanced by increased stratification due to surface warming, the large-scale changes in column-integrated freshwater content are very unlikely'' to result from changes in stratification in the thin surface layer. Furthermore, the large spatial scale of the observed changes in freshwater content cannot be explained by changes in ocean circulation such as shifts of gyre boundaries. The observed changes in surface and subsurface salinity require additional horizontal atmospheric water transport from regions of net evaporation to regions of net precipitation. A similar conclusion was reached in AR4 (Bindoff et al., 2007). The water vapour in the troposphere has likely increased since the 1970s, due to warming (2.4.4, 2.5.5, 2.5.6; FAQ 3.2). The inferred enhanced pattern of net E – P can be related to water vapor increase, although the linkage is complex (Emori and Brown, 2005; Held and Soden, 2006). From 1950 to 2000, the large-scale pattern of surface salinity has amplified at a rate that is larger than model simulations for the historical 20th century and 21st century projections. The observed rate of surface salinity amplification is comparable to the rate expected from a water cycle response following the Clausius–Clapeyron relationship (Durack et al., 2012).

Studies published since AR4, based on expanded data sets and new analysis approaches, have substantially decreased the level of uncertainty in the salinity and freshwater content trends (e.g., Stott et al., 2008; Hosoda et al., 2009; Roemmich and Gilson, 2009; Durack and Wijffels, 2010; Helm et al., 2010), and thus increased confidence in the inferred changes of evaporation and precipitation over the ocean.

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