Salinity Distribution at the Ocean Surface

Although these two key physico-chemical variables of the ocean states are intimately related, the spatial distribution of sea surface salinity in the ocean (first plot below)  is significantly different from the sea surface temperature one (second plot below). Global salinity patterns are linked to rainfall and evaporation. Salinity affects seawater density, which in turn governs ocean circulation and climate.The higher salinity of the Atlantic sustains the oceanic deep overturning circulation. Salinity variations are driven by precipitation, evaporation, runoff and ice freezing and melting 

Figure 1: Annual mean of the sea surface salinity distribution (World Ocean Atlas, 2005) and sea surface temperature

 This is mainly because the surface sources of variability for temperature are different than for salinity:  the ocean is indeed heated up in the tropics and cooled at high latitudes while salinity is dominantly modified by concentration-dilution related to the evaporation-precitation- river runoff flux (E-P-R) .
The SSS spatial pattern reflects the climate belts associated with general atmospheric circulation. Comparison of SSS to net sea-air freshwater fluxes reveals remarkably similar patterns (Schmitt et al., 1989; Baumgartner and Reichel, 1975). The great subtropical deserts at the poleward edges of the atmosphere’s Hadley cells are apparent over the ocean as a SSS maximum in the 15° to 30° latitude band. Tropical rain lowers the SSS along the Intertropical Convergence Zone. From the midlatitudes to the polar regions, excess precipitation lowers SSS. The marine hydrological cycle varies in longitude, too. Net evaporation leads to a relatively salty Atlantic; net input of freshwater generates low salinity, characteristic of the Pacific Ocean.


             Map of Precipitation                                          Map of evaporation
Figure 2: Example of precipitation (top left) and evaporation (top right) distribution. Bottom: relashionships between surface salinity and Evaporation minus Precipitation (E-P) patterns.
If it rains more than evaporated (E-P<0), for example in area of strong atmospheric convection (e.g., equatorial) or at moderate latitudes, the salinity diminishes at the ocean surface. In subtropical zones, evaporation dominates over precipitation (E-P>0) and salinity increases.