5.2.3 Ocean Salinity
Ocean salinity changes are an indirect but potentially sensitive indicator for detecting changes in precipitation, evaporation, river runoff and ice melt. The patterns of salinity change can be used to infer changes in the Earth’s hydrological cycle over the oceans (Wong et al., 1999; Curry et al., 2003) and are an important complement to atmospheric measurements. Figure 5.5 shows the linear trends (based on pentadal anomaly fields) of zonally averaged salinity in the upper 500 m of the World Ocean and individual ocean basins (Boyer et al., 2005) from 1955 to 1998. A total of 2.3 million salinity profiles were used in this analysis, about one-third of the amount of data used in the ocean heat content estimates in Section 5.2.2.
Estimates of changes in the freshwater content of the global ocean have suggested that the global ocean is freshening (e.g., Antonov et al., 2002), however, sampling limitations due to data sparsity in some regions, particularly the SH, means that such estimates have an uncertainty that is not possible to quantify.
Between 15°S and 42°N in the Atlantic Ocean there is a salinity increase in the upper 500 m layer. This region includes the North Atlantic subtropical gyre. In the 42°N to 72°N region, including the Labrador, Irminger and Icelandic Seas, there is a freshening trend (discussed further in Section 5.3). The increase in salinity north of 72°N (Arctic Ocean) is highly uncertain because of the paucity of data in this region.
South of 50°S in the polar region of the Southern Ocean, there is a relatively weak freshening signal. Freshening occurs throughout most of the Pacific with the exception of the South Pacific subtropical gyre between 8°S and 32°S and above 300 m where there is an increase in salinity. The near-surface Indian Ocean is characterised mainly by increasing salinity. However, in the latitude band 5°S to 42°S (South Indian gyre) in the depth range of 200 to 1,000 m, there is a freshening of the water column.
The results shown here document that ocean salinity and hence freshwater are changing on gyre and basin scales, with the near-surface waters in the more evaporative regions increasing in salinity in almost all ocean basins. In the high-latitude regions in both hemispheres the surface waters are freshening consistent with these regions having greater precipitation, although higher runoff, ice melting, advection and changes in the MOC (Häkkinen, 2002) may also contribute. In addition to these meridional changes, the Atlantic is becoming saltier over much of the water column (Figure 5.5 and Boyer et al., 2005). Although the South Pacific subtropical region is becoming saltier, on average the whole water column in the Pacific Basin is becoming fresher (Boyer et al., 2005). The increasing difference in volume-averaged salinity between the Atlantic and Pacific Oceans suggests changes in freshwater transport between these two ocean basins.
Figure 5.5: Linear trends (1955–1998) of zonally averaged salinity (psu) in the upper 500 m of the Atlantic, Pacific, Indian and World Oceans. The contour interval is 0.01 psu per decade and dashed contours are ±0.005 psu per decade. The dark solid line is the zero contour. Red shading indicates values equal to or greater than 0.005 psu per decade and blue shading indicates values equal to or less than –0.005 psu per decade. Based on the work of Boyer et al. (2005).
We are confident that vertically coherent gyre and basin scale changes have occurred in the salinity (freshwater content) of parts of the World Ocean during the past several decades. While the available data and their analyses are insufficient to identify in detail the origin of these changes, the patterns are consistent with a change in the Earth’s hydrological cycle, in particular with changes in precipitation and inferred larger water transport in the atmosphere from low latitudes to high latitudes and from the Atlantic to the Pacific (see Section 3.3.2).