Editing IPCC:AR6/SROCC/Chapter-3 (section)

===== 3.2.1.2.2 Salinity =====

Salinity is the dominant determinant of polar ocean density, and exerts major controls on stratification, circulation and mixing. Salinity changes are induced by freshwater runoff to the ocean (rivers and land ice), net precipitation, sea ice, and advection of mid-latitude waters, with the potential to impact water mass formation and circulation (e.g., Thornalley et al., 2018; see also Section 6.7.1).

Updating WGI AR5 (their Section 3.3.3.3), recent Arctic-wide estimates yield a freshwater increase (relative to salinity of 34.8 on the Practical Salinity Scale, used throughout this chapter) of 600 ± 300 km 3 yr –1 over 1992–2012, with about two-thirds concomitant with decreasing salinity, and the remainder with a thickening of the freshwater layer ( ''medium confidence'' ) (Rabe et al., 2014 <sup>[[#fn:r258|258]]</sup> ; Haine et al., 2015 <sup>[[#fn:r259|259]]</sup> ; Carmack et al., 2016 <sup>[[#fn:r260|260]]</sup> ). The Beaufort Gyre region has increased its freshwater by ~40% (6600 km 3 ) over 2003–2017; this, and the Gyre’s strengthening, have been attributed to dominance of clockwise wind patterns over the Canadian Basin over 1997–2016 and freshwater accumulation from sea ice-melt (Krishfield et al., 2014 <sup>[[#fn:r261|261]]</sup> ; Proshutinsky et al., 2015 <sup>[[#fn:r262|262]]</sup> ). Freshwater decreases in the East Siberian, Laptev, Chukchi and Kara seas are estimated to be ~180 km 3 over 2003–2014 (Armitage et al., 2016 <sup>[[#fn:r263|263]]</sup> ). During the 2000s, freshwater content in the upper 100 m of the northern Barents Sea declined by about 32%, from a mean of ~2.5 m (relative to a salinity of 35) in 1970–1999, to 1.7 m in 2010–2016 (Lind et al., 2018 <sup>[[#fn:r264|264]]</sup> ). An increasing trend of 30 ± 20 km 3 yr –1 in freshwater flux through Bering Strait, primarily due to increased volume flux, was measured from 1991 to 2015, with record maximum freshwater influx in 2014 of around 3500 km 3 in that year (Woodgate, 2018 <sup>[[#fn:r265|265]]</sup> ). Freshwater fluxes from rivers are also increasing (Section 3.4.1.2.2), and there have been observed increases in discharge of glacial ice from Greenland (Section 3.3.1.3).

Observed Southern Ocean freshening trends are consistent with WGI AR5; subsequent studies have increased confidence in their magnitude and sign, and also attributed them to anthropogenic influences (Swart et al., 2018 <sup>[[#fn:r266|266]]</sup> ). Changes over 1950–2010 show persistent surface water freshening over the whole Southern Ocean, with subducted mode/intermediate waters carrying trends of 0.0002–0.0008 yr –1 to below 1500 m (Skliris et al., 2014 <sup>[[#fn:r267|267]]</sup> ), whilst de Lavergne et al. (2014) observe a circumpolar freshening south of the ACC of 0.0011 ± 0.0004 yr –1 in the upper 100 m since the 1960s ( ''medium confidence'' ). This intensifies over the Antarctic continental shelves (except along the Western Antarctic Peninsula), where freshening of up to 0.01 yr –1 is observed (Schmidtko et al., 2014 <sup>[[#fn:r269|269]]</sup> ). Freshening may be driven by increases in precipitation, but while models (Pauling et al., 2016 <sup>[[#fn:r270|270]]</sup> ) and observations suggest an increase may have occurred over the last 60 years, uncertainty is presently too high to quantify its net impact (Skliris et al., 2014 <sup>[[#fn:r271|271]]</sup> ). Recently, there has been increased recognition of the importance of sea ice in driving Southern Ocean salinity changes, with Haumann et al. (2016) <sup>[[#fn:r272|272]]</sup> demonstrating that wind driven sea ice export has increased by 20 ± 10% from 1982 to 2008, and that this may have driven freshening of 0.002 ± 0.001 yr –1 in the surface and intermediate waters. Separately, the central role of sea ice in driving water mass transformations in the Southern Ocean has been highlighted (Abernathey et al., 2016 <sup>[[#fn:r273|273]]</sup> ; Pellichero et al., 2018 <sup>[[#fn:r274|274]]</sup> ; Swart et al., 2018 <sup>[[#fn:r275|275]]</sup> ), hence such changes have the potential to affect overturning circulation (Cross-Chapter Box 7 in Chapter 3). Freshwater input to the ocean from the Antarctic Ice Sheet also has the potential to affect the properties and circulation of Southern Ocean water masses; see Section 3.3.3.

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