The net evaporative North Atlantic has become saltier as a whole over the past 50 years (Figure 3.9; Boyer et al., 2007). The largest increase in the upper 700 m occurred in the Gulf Stream region (0.006 per decade between 1955–1959 and 2002–2006) (Wang et al., 2010). Salinity increase is also evident following the circulation pathway of Mediterranean Outflow Water (Figure 3.9; Fusco et al., 2008). This increase can be traced back to the western basin of the Mediterranean, where salinity of the deep water increased during the period from 1943 to the mid-2000s (Smith et al., 2008; Vargas-Yáñez et al., 2010).
During the time period between 1955–1959 and 2002–2006 (using salinities averaged over the indicated 5-year ranges), the upper 700 m of the subpolar North Atlantic freshened by up to 0.002 per decade (Wang et al., 2010), while an increase in surface salinity was found between the average taken over 1960–1989 and the 5-year average over 2003–2007 (Hosoda et al., 2009). Decadal and multi-decadal variability in the subpolar gyre and Nordic Seas is vigorous and has been related to various climate modes such as the NAO, the Atlantic multi-decadal oscillation (AMO, Box 2.5), and even El Niño-Southern Oscillation (ENSO; Polyakov et al., 2005; Yashayaev and Loder, 2009), obscuring long-term trends. The 1970s to 1990s freshening of the northern North Atlantic and Nordic Seas (Dickson et al., 2002; Curry et al., 2003; Curry and Mauritzen, 2005) reversed to salinification (0 to 2000 m depth) starting in the late 1990s (Boyer et al., 2007; Holliday et al., 2008), and the propagation of this signal could be followed along the eastern boundary from south of 60°N in the Northeast Atlantic to Fram Strait at 79°N (Holliday et al., 2008). Advection has also played a role in moving higher salinity subtropical waters to the subpolar gyre (Hatun et al., 2005; Bersch et al., 2007; Lozier and Stewart, 2008; Valdimarsson et al., 2012). The variability of the cross equatorial transport contribution to this budget is highly uncertain. Reversals of North Atlantic surface salinity of similar amplitude and duration to those observed in the last 50 years are apparent in the early 20th century (Reverdin et al., 2002; Reverdin, 2010). The evaporation-dominated subtropical South Atlantic has become saltier by 0.1 to 0.3 during the period from 1950 to 2008 (Hosoda et al., 2009; Durack and Wijffels, 2010; Figure 3.4).