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153.3.5 Conclusions

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Both positive and negative trends in ocean salinity and freshwater content have been observed throughout much of the ocean, both at the sea surface and in the ocean interior. While similar conclusions were reached in AR4, the recent studies summarized here, based on expanded data sets and new analysis approaches, provide high confidence in the assessment of trends in ocean salinity. It is virtually certain that the salinity contrast between regions of high and low surface salinity has increased since the 1950s. It is very likely that since the 1950s, the mean regional pattern of upper ocean salinity has been enhanced: saline surface waters in the evaporation-dominated mid-latitudes have become more saline, while the relatively fresh surface waters in rainfall- dominated tropical and polar regions have become fresher. Similarly, it is very likely that the interbasin contrast between saline Atlantic and fresh Pacific surface waters has increased, and it is very likely that freshwater content in the Southern Ocean has increased. There is medium confidence that these patterns in salinity trends are caused by increased horizontal moisture transport in the atmosphere, suggesting changes in evaporation and precipitation over the ocean as the lower atmosphere has warmed.

Trends in salinity have been observed in the ocean interior as well. It is likely that the subduction of surface water mass anomalies and the movement of density surfaces have contributed to the observed salinity changes on depth levels. Changes in freshwater flux and the migration of surface density outcrops caused by surface warming (e.g., to regions of lower or higher surface salinity) have likely both contributed to the formation of salinity anomalies on density surfaces.

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