Sea ice in the Arctic has declined significantly in recent decades (Section 4.2), which might be expected to reduce the surface salinity and increase freshwater content as freshwater locked in multi-year sea ice is released. Generally, strong multi-decadal variability, regional variability, and the lack of historical observations have made it difficult to assess long-term trends in ocean salinity and freshwater content for the Arctic as a whole (Rawlins et al., 2010). The signal that is now emerging, including salinity observations from 2005 to 2010, indicates increased freshwater content, with medium confidence.
Over the 20th century (1920–2003) the central Arctic Ocean in the upper 150 m became fresher in the 1950s and then more saline by the early 2000s, with a net small salinification over the whole record (Polyakov et al., 2008), while at the Siberian Shelf the river discharge increased (Shiklomanov and Lammers, 2009) and the shelf waters became fresher (Polyakov et al., 2008).
Upper ocean freshening has also been observed regionally in the southern Canada basin from the period 1950–1980 to the period 1990–2000s (Proshutinsky et al., 2009; Yamamoto-Kawai et al., 2009). These are the signals reflected in the freshwater content trend from 1955 to 2010 shown in Figure 3.5a, f: salinification at the highest latitudes and a band of freshening at about 70°N to 80°N. Ice production and sustained export of freshwater from the Arctic Ocean in response to winds are suggested as key contributors to the high- latitude salinification (Polyakov et al., 2008; McPhee et al., 2009). The contrasting changes in different regions of the Arctic have been attributed to the effects of Ekman transport, sea ice formation (and melt) and a shift in the pathway of Eurasian river runoff (McPhee et al., 2009; Yamamoto- Kawai et al., 2009; Morison et al., 2012).
Between the periods 1992–1999 and 2006–2008, not only the central Arctic Ocean freshened (Rabe et al., 2011; Giles et al., 2012), but also freshening is now observed in all regions including those that were becoming more saline through the early 2000s (updated from Polyakov et al., 2008). Moreover, freshwater transport out of the Arctic has increased in that time period (McPhee et al., 2009).