In the tropical Pacific, surface salinity has declined by 0.1 to 0.3 over 50 years in the precipitation-dominated western equatorial regions and by up to 0.6 to 0.75 in the Intertropical Convergence Zone and the South Pacific Convergence Zone (Cravatte et al., 2009), while surface salinity has increased by up to 0.1 over the same period in the evaporation-dominated zones in the southeastern and north-central tropical Pacific (Figure 3.9). The fresh, low-density waters in the warm pool of the western equatorial Pacific expanded in area as the surface salinity front migrated eastward by 1500 to 2500 km over the period 1955–2003 (Delcroix et al., 2007; Cravatte et al., 2009). Similarly, in the Indian Ocean, the net precipitation regions in the Bay of Bengal and the warm pool contiguous with the tropical Pacific warm pool have been freshening by up to 0.1 to 0.2, while the saline Arabian Sea and south Indian Ocean have been getting saltier by up to 0.2 (Durack and Wijffels, 2010).
In the North Pacific, the subtropical thermocline has freshened by 0.1 since the early 1990s, following surface freshening that began around 1984 (Ren and Riser, 2010); the freshening extends down through the intermediate water that is formed in the northwest Pacific (Nakano et al., 2007), continuing the freshening documented by Wong et al. (1999). Warming of the surface water that subducts to supply the intermediate water is one reason for this signal, as the freshwater from the subpolar North Pacific is now entering the subtropical thermocline at lower density.
Salinity changes, together with temperature changes (Section 3.2.2), affect stratification; salinity has more impact than temperature in some regions. In the western tropical Pacific, for example, the density changes from 1970 to 2003 at a trend of –0.013 kg m–3 yr–1, about 60% of that due to salinity (Delcroix et al., 2007). The decreasing density trend mainly occurs near the surface only, which should affect stratification across the base of the mixed layer. In the Oyashio region of the western North Pacific, salinity decrease near the surface accounts for about 60% of the density decrease of –0.004 kg m–3 yr–1 from 1968 to 1998 (Ono et al., 2001).