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

===== 3.3.1.5.1 Ocean drivers =====

The reduction of ice shelf buttressing that has dominated AIS mass loss (Section 3.3.1.2) has been driven primarily by increases in sub-ice shelf melting (Khazendar et al., 2013 <sup>[[#fn:r1071|1071]]</sup> ; Pollard et al., 2015 <sup>[[#fn:r1072|1072]]</sup> ; Cook et al., 2016 <sup>[[#fn:r1073|1073]]</sup> ; Rintoul et al., 2016 <sup>[[#fn:r1074|1074]]</sup> ; Walker and Gardner, 2017 <sup>[[#fn:r1075|1075]]</sup> ; Adusumilli et al., 2018 <sup>[[#fn:r1076|1076]]</sup> ; Dow et al., 2018a <sup>[[#fn:r1077|1077]]</sup> ; Minchew et al., 2018 <sup>[[#fn:r1078|1078]]</sup> ) ( ''high confidence'' ). Shoaling of relatively warm Circumpolar Deep Water has controlled recent variability in melting in the Amundsen and Bellingshausen seas, Wilkes Land (Roberts et al., 2018 <sup>[[#fn:r1079|1079]]</sup> ) and the AP ( ''medium confidence'' ) (Jacobs et al., 2011 <sup>[[#fn:r1080|1080]]</sup> ; Pritchard et al., 2012 <sup>[[#fn:r1081|1081]]</sup> ; Depoorter et al., 2013 <sup>[[#fn:r1082|1082]]</sup> ; Rignot et al., 2013 <sup>[[#fn:r1083|1083]]</sup> ; Dutrieux et al., 2014 <sup>[[#fn:r1084|1084]]</sup> ; Paolo et al., 2015 <sup>[[#fn:r1085|1085]]</sup> ; Wouters et al., 2015 <sup>[[#fn:r1086|1086]]</sup> ; Christianson et al., 2016 <sup>[[#fn:r1087|1087]]</sup> ; Cook et al., 2016 <sup>[[#fn:r1088|1088]]</sup> ; Jenkins et al., 2018 <sup>[[#fn:r1089|1089]]</sup> ; Roberts et al., 2018 <sup>[[#fn:r1090|1090]]</sup> ). Changes in winds have driven this shoaling by affecting continental shelf edge undercurrents (Walker et al., 2013 <sup>[[#fn:r1091|1091]]</sup> ; Dutrieux et al., 2014 <sup>[[#fn:r1092|1092]]</sup> ; Kimura et al., 2017 <sup>[[#fn:r1093|1093]]</sup> ) and overturning in coastal polynyas (St ‐ Laurent et al., 2015 <sup>[[#fn:r1094|1094]]</sup> ; Webber et al., 2017 <sup>[[#fn:r1095|1095]]</sup> ) ( ''medium confidence'' ). Winds over the Amundsen Sea are highly variable, however, with complex interactions between SAM, El Niño/Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation, and the Amundsen Sea Low (Uotila et al., 2013 <sup>[[#fn:r1095|1095]]</sup> ; Li et al., 2014 <sup>[[#fn:r1096|1096]]</sup> ; Turner et al., 2016 <sup>[[#fn:r1097|1097]]</sup> ) (SM3.1.3).

Through their effects on Antarctic coastal ocean circulation, ENSO or other tropical-ocean variability may have triggered changes to Pine Island Glacier in the 1940s (Smith et al., 2017c <sup>[[#fn:r1098|1098]]</sup> ) and again in the 1970s and 1990s (Jenkins et al., 2018 <sup>[[#fn:r1099|1099]]</sup> ), and recent ENSO variability is correlated with recent changes in ice shelf thickness (Paolo et al., 2018 <sup>[[#fn:r1100|1100]]</sup> ) ( ''medium confidence'' ). Such coupling between wind variability, ocean upwelling, ice shelf melt, buttressing and glacier flow rate has also been observed in EAIS, at Totten Glacier, Wilkes Land (Greene et al., 2017 <sup>[[#fn:r1101|1101]]</sup> ).

Around Greenland, an anomalous inflow of subtropical water driven by wind changes, multi-decadal natural ocean variability (Andresen et al., 2012 <sup>[[#fn:r1102|1102]]</sup> ), and a long-term increase in the North Atlantic’s upper ocean heat content since the 1950s (Cheng et al., 2017 <sup>[[#fn:r1103|1103]]</sup> ), all contributed to a warming of the subpolar North Atlantic (Häkkinen et al., 2013 <sup>[[#fn:r1104|1104]]</sup> ) ( ''medium confidence'' ). Water temperatures near the grounding zone of GIS outlet glaciers are critically important to their calving rate (O’Leary and Christoffersen, 2013) ( ''medium confidence'' ), and warm waters have been observed interacting with major GIS outlet glaciers ( ''high confidence'' ) (e.g., Holland et al., 2008; Straneo et al., 2017 <sup>[[#fn:r1105|1105]]</sup> ).

The processes behind warm-water incursions in coastal Greenland that force glacier retreat remain unclear, however (Straneo et al., 2013 <sup>[[#fn:r1106|1106]]</sup> ; Xu et al., 2013b <sup>[[#fn:r1107|1107]]</sup> ; Bendtsen et al., 2015 <sup>[[#fn:r1108|1108]]</sup> ; Murray et al., 2015 <sup>[[#fn:r1109|1109]]</sup> ; Cowton et al., 2016 <sup>[[#fn:r1110|1110]]</sup> ; Miles et al., 2016 <sup>[[#fn:r1111|1111]]</sup> ), and there is ''low confidence'' in understanding coastal GIS glacier response to ocean forcing because submarine melt rates, calving rates (Rignot et al., 2010 <sup>[[#fn:r1112|1112]]</sup> ; Todd and Christoffersen, 2014 <sup>[[#fn:r1113|1113]]</sup> ; Benn et al., 2017 <sup>[[#fn:r1114|1114]]</sup> ), bed and fjord geometry and the roles of ice melange and subglacial discharge (Enderlin et al., 2013 <sup>[[#fn:r1115|1115]]</sup> ; Gladish et al., 2015 <sup>[[#fn:r1116|1116]]</sup> ; Slater et al., 2015 <sup>[[#fn:r1117|1117]]</sup> ; Morlighem et al., 2016 <sup>[[#fn:r1118|1118]]</sup> ; Rathmann et al., 2017 <sup>[[#fn:r1119|1119]]</sup> ) are poorly understood, and extrapolation from a small sample of glaciers is impractical (Moon et al., 2012 <sup>[[#fn:r1120|1120]]</sup> ; Carr et al., 2013 <sup>[[#fn:r1121|1121]]</sup> ; Straneo et al., 2016 <sup>[[#fn:r1122|1122]]</sup> ; Cowton et al., 2018 <sup>[[#fn:r1123|1123]]</sup> ).

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