Salinity changes in the World Ocean since 1950 in relation to changing surface freshwater fluxes

Global hydrographic and air-sea freshwater flux datasets are used to investigate ocean salinity changes over 1950-2010 in relation to surface freshwater flux. On multi-decadal timescales, surface salinity increases (decreases) in evaporation (precipitation) dominated regions, the Atlantic-Pacific salinity contrast increases, and the upper thermocline salinity maximum increases while the salinity minimum of intermediate waters decreases. Potential trends in E-P are examined for 1950-2010 (using two reanalyses) and 1979-2010 (using four reanalyses and two blended products). Large differences in the 1950-2010 E-P trend patterns are evident in several regions, particularly the North Atlantic. For 1979-2010 some coherency in the spatial change patterns is evident but there is still a large spread in trend magnitude and sign between the six E-P products. However, a robust pattern of increased E-P in the southern hemisphere subtropical gyres is seen in all products. There is also some evidence in the tropical Pacific for a link between the spatial change patterns of salinity and E-P associated with ENSO. The water cycle amplification rate over specific regions is subsequently inferred from the observed 3-D salinity change field using a salt conservation equation in variable isopycnal volumes, implicitly accounting for the migration of isopycnal surfaces. Inferred global changes of E-P over 1950-2010 amount to an increase of 1 +/- A 0.6 % in net evaporation across the subtropics and an increase of 4.2 +/- A 2 % in net precipitation across subpolar latitudes. Amplification rates are approximately doubled over 1979-2010, consistent with accelerated broad-scale warming but also coincident with much improved salinity sampling over the latter period.