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

==== 3.3.3.2 Physical Oceanography ====

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The major large-scale impacts of freshwater release from Greenland on ocean circulation relate to the potential modulation/inhibition of the formation of water masses that represent the headwaters of the Atlantic Meridional Overturning Circulation. The timescales and likelihood of such effects are assessed separately in Chapter 6 (Section 6.7). Freshwater release also affects local circulation within fjords through two principle mechanisms; subglacial release from tidewater glaciers enhances buoyancy driven circulation, whereas runoff from land-terminating glaciers contributes to surface layer freshening and estuarine circulation (Straneo and Cenedese, 2015 <sup>[[#fn:r1229|1229]]</sup> ). There is ''limited evidence'' that freshening occurred between 2003 and 2015 in North East Greenland fjords and coastal waters (Sejr et al., 2017 <sup>[[#fn:r1230|1230]]</sup> ).

For Antarctica, freshwater input to the ocean from the ice sheet is divided approximately equally between melting of calved icebergs and of ice shelves ''in situ'' (Depoorter et al., 2013 <sup>[[#fn:r1231|1231]]</sup> ; Rignot et al., 2014 <sup>[[#fn:r1232|1232]]</sup> ). There is ''high confidence'' that the input of ice shelf meltwater has increased in the Amundsen and Bellingshausen Seas since the 1990s, but ''low confidence'' in trends in other sectors (Paolo et al., 2015 <sup>[[#fn:r1233|1233]]</sup> ).

Freshwater injected from the AIS affect water mass circulation and transformation, though sea ice dominates upper ocean properties away from the Antarctic ice shelves (Abernathey et al., 2016 <sup>[[#fn:r1234|1234]]</sup> ; Haumann et al., 2016 <sup>[[#fn:r1235|1235]]</sup> ). Over the ice shelf regions, where dense waters sink and flood the global ocean abyss, the role of glacial freshwater input is clearer. From 1980 to 2012, the salinity of Antarctic Bottom Water reduced by an amount equivalent to 73 ± 26 Gt y –1 of freshwater added, around half the estimated increase in freshwater input by Antarctic glacial discharge up to that time (Purkey and Johnson, 2013 <sup>[[#fn:r1236|1236]]</sup> ). In some places, notably the Indian-Australian sector, Antarctic Bottom Water freshening may be accelerating (Menezes et al., 2017 <sup>[[#fn:r1237|1237]]</sup> ). There is ''medium confidence'' in an overall freshening trend and ''low confidence'' that this is accelerating, given the sparsity of information and significant interannual variability in Antarctic Bottom Water properties at other export locations (Meijers et al., 2016 <sup>[[#fn:r1238|1238]]</sup> ).

For the Southern Ocean, there is ''limited evidence'' for stratification changes in the post-AR5 period, and ''low confidence'' in how stratification changes are affecting sea ice and basal ice shelf melt. An increase in stratification caused by release of freshwater from the AIS was invoked as a mechanism to suppress vertical heat flux and permit an increase in sea ice extent (Bintanja et al., 2013 <sup>[[#fn:r1239|1239]]</sup> ; Bronselaer et al., 2018 <sup>[[#fn:r1240|1240]]</sup> ; Purich et al., 2018 <sup>[[#fn:r1241|1241]]</sup> ), though some studies conclude that glacial freshwater input is insufficient to cause a significant sea ice expansion (Swart and Fyfe, 2013 <sup>[[#fn:r1242|1242]]</sup> ; Pauling et al., 2017 <sup>[[#fn:r1243|1243]]</sup> ) (Section 3.2.1.1). In contrast, where warm water intrusions drive melting within ice shelf cavities, a significant entrained heat flux to the surface can exist and increase stratification and potentially reduce sea ice extent (Jourdain et al., 2017 <sup>[[#fn:r1244|1244]]</sup> ; Merino et al., 2018 <sup>[[#fn:r1245|1245]]</sup> ). It has been argued that freshening from glacial melt can enhance basal melting of ice shelves by reducing dense water production and modulating oceanic heat flow into ice shelf cavities (Silvano et al., 2018 <sup>[[#fn:r1246|1246]]</sup> ).

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