Compatible with observed changes in surface salinity, robust multi- decadal trends in subsurface salinity have been detected (Boyer et al., 2005; Boyer et al., 2007; Steele and Ermold, 2007; Böning et al., 2008; Durack and Wijffels, 2010; Helm et al., 2010; Wang et al., 2010). Global, zonally averaged multi-decadal salinity trends (1950–2008) in the upper 500 m (Figures 3.4, 3.5, 3.9 and Section 3.5) show salinity increases at the salinity maxima of the subtropical gyres, freshening of the low-salinity intermediate waters sinking in the Southern Ocean (Subantarctic Mode Water and Antarctic Intermediate Water) and North Pacific (North Pacific Intermediate Water). On average, the Pacific freshened, and the Atlantic became more saline. These trends, shown in Figures 3.5 and 3.9, are significant at a 95% confidence interval. Freshwater content in the upper 500 m very likely changed, based on the World Ocean Database 2009 (Boyer et al., 2009), analyzed by Durack and Wijffels (2010) and independently as an update to Boyer et al. (2005) for 1955–2010 (Figure 3.5a, b, e, f). Both show freshening in the North Pacific, salinification in the North Atlantic south of 50°N and salinification in the northern Indian Ocean (trends significant at 90% confidence). A significant freshening is observed in the circumpolar Southern Ocean south of 50S.
Density layers that are ventilated (connected to the sea surface) in precipitation-dominated regions have freshened, while those ventilated in evaporation-dominated regions have increased in salinity, compatible with an enhancement of the mean surface freshwater flux pattern (Helm et al., 2010). In addition, where warming has caused surface outcrops of density layers to move (poleward) into higher salinity surface waters, the subducted salinity in the density layers has increased; where outcrops have moved into fresher surface waters, the subducted salinity decreased (Durack and Wijffels, 2010). Vertical and lateral shifts of density surfaces, due to both changes in water mass renewal rates and wind-driven circulation, have also contributed to the observed subsurface salinity changes (Levitus, 1989; Bindoff and McDougall, 1994).
A change in total, globally integrated freshwater content and salinity requires an addition or removal of freshwater; the only significant source is land ice (ice sheets and glaciers). The estimate of change in globally averaged salinity and freshwater content remains smaller than its uncertainty, as was true in the AR4 assessment. For instance, the globally averaged sea surface salinity change from 1950 to 2008 is small (+0.003 [–0.056 to 0.062]) compared to its error estimate (Durack and Wijffels, 2010). Thus a global freshening due to land ice loss has not yet been discerned in global surface salinity change even if it were assumed that all added freshwater were in the ocean’s surface layer.