Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.2.2.3 Water Masses ====

<div id="h3-6-siblings" class="h3-siblings"></div>

Water masses refer to connected bodies of ocean water, formed at the ocean surface with identifiable properties (temperature, salinity, density, chemical tracers) resulting from the unique formation conditions of the overlying atmosphere and/or ice, before being transferred (subducted) to the deeper ocean below the surface turbulent layer. As water masses subduct, they ventilate the subsurface ocean, transferring characteristics acquired at the ocean surface to the subsurface. By integrating surface flux changes, water masses provide higher signal-to-noise ratios for detecting and monitoring climate change than surface fluxes ( [[#Bindoff--2000|Bindoff and McDougall, 2000]] ; [[#Durack--2010|Durack and Wijffels, 2010]] ; [[#Silvy--2020|Silvy et al., 2020]] ).

Subtropical mode waters (STMW) ventilate the main thermocline of the ocean at mid- to low-latitudes and have circulation time scales away from the surface of the order of years to decades. The SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) reported that warming in the subtropical gyres penetrates deeper than in other gyres, following the density surfaces in these gyres. Consistently, we assess that STMW have deepened worldwide, with greatest deepening in the Southern Hemisphere ( ''high confidence'' ) ( [[#Häkkinen--2016|Häkkinen et al., 2016]] ; [[#Desbruyères--2017|Desbruyères et al., 2017]] ). Subsurface warming in the Northern Hemisphere STMW is larger than at the surface ( [[#Sugimoto--2017|Sugimoto et al., 2017]] ) because they are formed in winter western boundary current extensions, where surface warming is larger than the global average ( [[#9.2.1.1|Section 9.2.1.1]] ). Variability in STMW thickness or temperature has a large imprint on OHC ( [[#9.2.2.1|Section 9.2.2.1]] ; [[#Kolodziejczyk--2019|Kolodziejczyk et al., 2019]] ). STMW are observed to be freshening in the North Pacific and associated with increased salinity in the North Atlantic ( [[#Oka--2017|Oka et al., 2017]] ; [[#Silvy--2020|Silvy et al., 2020]] ), with large decadal variability ( [[#Oka--2019|Oka et al., 2019]] ; [[#Wu--2020|Wu et al., 2020]] ). Anthropogenic temperature and salinity changes in the STMW layer are projected to intensify in the future, with emergence from natural variability around 2020 to 2040 ( [[#Silvy--2020|Silvy et al., 2020]] ).

Subantarctic mode water (SAMW) and Antarctic intermediate water (AAIW) form at the Southern Ocean surface directly north of the Antarctic Circumpolar Current and ventilate the upper 1000 m of the Southern Hemisphere subtropics. The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) reported a freshening of these water masses between 1950 and 2018, and they are projected to have the largest subsurface temperature increase of the Southern Hemisphere oceans, along with a continued freshening, in the 21st century. The SROCC connected SAMW and AAIW to Southern Ocean temperature changes as the large Southern Ocean surface heat uptake is circulated and mixed along with these water masses ( ''high confidence'' ). Close to its formation region, SAMW is predominantly affected by air–sea flux changes, while further northward it is influenced by wind-forced changes ( [[#Meredith--2019|Meredith et al., 2019]] ). New evidence shows that a change in SAMW heat content over the last decade is primarily attributable to its thickening ( [[#Kolodziejczyk--2019|Kolodziejczyk et al., 2019]] ). Over the past decade, the SAMW and AAIW volumes have changed by thickening of the lighter and thinning of the denser parts of SAMW and AAIW, leading to lightening of these ventilated ocean layers overall ( [[#Hong--2020|Hong et al., 2020]] ; [[#Portela--2020|Portela et al., 2020]] ). Over the last decade, there is ''limited evidence'' of increased subduction of SAMW due to deepening mixed layers in the SAMW formation region ( [[#9.2.1.3|Section 9.2.1.3]] ; [[#Qu--2020|Qu et al., 2020]] ). Climate models from CMIP3 to CMIP5 generally simulated shallower and lighter SAMW and AAIW than is observed ( [[#Flato--2013|Flato et al., 2013]] ). New analysis of CMIP5 models suggests that the freshening of these water masses is one of the most prominent projected salinity changes in the world ocean, and that this freshening emerged from internal variability as early as the 1980s to 1990s ( [[#Silvy--2020|Silvy et al., 2020]] ).

Trends in North Atlantic Deep Water (NADW) are obscured by decadal variability ( [[#Rhein--2013|Rhein et al., 2013]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ). The AR5 ( [[#Rhein--2013|Rhein et al., 2013]] ) assessed that it is ''very likely'' that the temperature, salinity, and formation rate of the Upper NADW (formed by deep convection in the Labrador and Irminger Seas) is dominated by strong decadal variability related to the North Atlantic Oscillation (NAO) and it is ''likely'' that Lower NADW (formed in the Nordic Seas and supplied to the North Atlantic by deep overflows over the sills between Scotland and Greenland) cooled from 1955 to 2005. New insights from observations have emphasized the stability of the deep overflows associated with Lower NADW ( [[#Hansen--2016|Hansen et al., 2016]] ; [[#Jochumsen--2017|Jochumsen et al., 2017]] ; [[#Østerhus--2019|Østerhus et al., 2019]] ) and even slight warming in the Faroe Bank Channel ( [[#Hansen--2016|Hansen et al., 2016]] ). As a result, the AR5 assessment that Lower NADW ''likely'' cooled between 1955 and 2005 is revised to: it is ''likely'' that any observed changes in temperature, salinity, and formation rate of the Lower NADW are dominated by decadal variability. For CMIP5 models, it was shown that AMOC variability is linked to variability in NADW formation ( [[#Heuzé--2017|Heuzé, 2017]] ) and projected AMOC decline to decreased NADW formation (both Lower NADW and Upper NADW; [[#Heuzé--2015|Heuzé et al., 2015]] ). For CMIP6 models, projected AMOC decline is also associated with a decline in NADW formation ( [[#Reintges--2017|Reintges et al., 2017]] ; [[#Weijer--2020|Weijer et al., 2020]] ). The link between AMOC and NADW formation appears insensitive to the large range in model bias in NADW water mass characteristics ( [[#Heuzé--2017|Heuzé, 2017]] ). Many models may overestimate deep water formation in the Labrador Sea, but at least one new model is consistent with recent Overturning in the Subpolar North Atlantic Program (OSNAP) observations showing very weak overturning in the western subpolar gyre, where Labrador Sea water is formed ( [[#Menary--2020a|Menary et al., 2020a]] ). The CMIP6 models show a reduced bias in NADW properties compared to CMIP5 models, but still feature varying locations of deep convection in the subpolar gyre: some convect only in the Labrador Sea (6/35 models), most in both the Labrador and Irminger Seas (26/35 models; as is observed), and some only in the Irminger Sea (3/35 models), but in general, the area where deep convection takes place has expanded relative to CMIP5, which appears unrealistic ( [[#Heuzé--2021|Heuzé, 2021]] ). Models with most deep convection in the subpolar gyre feature the smallest bias in NADW characteristics, partly associated with NADW formed in the Nordic Seas (as observed) being largely unable to leave the area ( [[#Heuzé--2021|Heuzé, 2021]] ) due to inaccurate overflows ( [[#Danabasoglu--2010|Danabasoglu et al., 2010]] ; [[#Deshayes--2014|Deshayes et al., 2014]] ; [[#Wang--2015|]] [[#Wang--2015|Wang et al., 2015]] ). Despite the wide range in model bias, it remains ''very likely'' that any long-term (multi-decadal or longer) decrease in AMOC is accompanied by a decline in NADW formation, associated with lighter densities in the northern North Atlantic and Arctic basins.

The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) assessed that the global volume of Antarctic Bottom Water (AABW) had decreased and warmed since the 1980s, most noticeably near Antarctica. The SROCC also noted freshening in the Indian and Pacific sectors of the Southern Ocean and a higher rate of freshening in the Indian Sector from the 2000s to 2010s than from the 1990s to 2000s ( ''low confidence'' ). Since SROCC, freshening of Indian Ocean AABW from 1974 to 2016 has been revealed ( [[#Aoki--2020|Aoki et al., 2020]] ). Additionally, interannual to decadal variability in AABW has been quantified to be larger than previously thought in terms of temperature, salinity and thickness, and in volume transport ( [[#Abrahamsen--2019|Abrahamsen et al., 2019]] ; [[#Purkey--2019|Purkey et al., 2019]] ; [[#Gordon--2020|Gordon et al., 2020]] ; [[#Silvano--2020|Silvano et al., 2020]] ). Multi-decadal to centennial modes of variability could have driven the observed trends of the lower cell over the past decades via the opening of a Weddell Sea Polynya (L. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ), although other studies find it contributed minimally to the observed abyssal warming ( [[#Zanowski--2015|Zanowski et al., 2015]] ; [[#Zanowski--2017|Zanowski and Hallberg, 2017]] ). Therefore, there is ''limited evidence'' and ''low agreement'' in the role of open ocean polynyas in driving past decadal observed trends of AABW. Beyond variability, all observational, theoretical, and numerical evidence supports SROCC assessment that formation and export of AABW will continue to decrease due to warming and freshening of surface source waters near the Antarctic continent. Consistent with [[#9.2.3.2|Section 9.2.3.2]] , confidence in this assessment is increased to ''medium confidence'' compared to SROCC.

Circumpolar Deep Water (CDW) lies in the Southern Ocean and forms by the mixing of NADW and AABW ( [[#Talley--2013|Talley, 2013]] ). The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) assessed with ''low confidence'' that mean southward and upward CDW transport is linked to decadal wind variability ( [[#9.2.3.2|Section 9.2.3.2]] ), and that CDW has warmed south of the Antarctic Circumpolar Current (ACC) in the past decades. New evidence reinforces SROCC assessment: changes in Southern Ocean wind stress have been confirmed to drive variability and increase the large-scale southward CDW transport ( [[#Waugh--2019|Waugh et al., 2019]] ). In addition, growing evidence suggests that the upper-ocean stratification increase in the subpolar Southern Ocean since the 1970s ( [[#9.2.1.3|Section 9.2.1.3]] ) has reduced the volume of CDW that is mixed to the surface, causing subsurface CDW warming ( [[#Bronselaer--2020|Bronselaer et al., 2020]] ; [[#Haumann--2020|Haumann et al., 2020]] ; [[#Jeong--2020|Jeong et al., 2020]] ; [[#Moorman--2020|Moorman et al., 2020]] ). Large regions of the Antarctic shelves are currently isolated from warm CDW ( [[#Thompson--2018|Thompson et al., 2018]] ; [[#Jourdain--2020|Jourdain et al., 2020]] ). The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) assessed that subsurface warming extends close to Antarctica and has co-occurred with shoaling of the CDW since the 1980s, influencing the continental shelf most in the Amundsen-Bellingshausen Seas, Wilkes Land, and the Antarctic Peninsula. New evidence since SROCC reinforces confidence in the importance of the role of winds in transporting heat associated with CDW to continental shelves and ice cavities in the Amundsen-Bellingshausen Seas ( [[#Dotto--2019|Dotto et al., 2019]] ) and via variable small-scale undercurrents to the Shirase Glacier Tongue in East Antarctica ( [[#Hirano--2020|Hirano et al., 2020]] ; [[#Kusahara--2021|Kusahara et al., 2021]] ). There is ''limited evidence'' that increased greenhouse gas forcing has caused a slight mean change of the local winds from 1920 to 2018, facilitating CDW heat intrusion onto the Amundsen-Bellingshausen continental shelf and ice shelf melt ( [[#Holland--2019|Holland et al., 2019]] ). Multiple lines of observational, numerical, theoretical, and paleo evidence provide ''high confidence'' that changes in wind pattern ( [[#Spence--2014|Spence et al., 2014]] ; [[#Dotto--2019|Dotto et al., 2019]] ; [[#Holland--2019|Holland et al., 2019]] ), increased ice-shelf melt ( [[#Golledge--2019|Golledge et al., 2019]] ; [[#Moorman--2020|Moorman et al., 2020]] ), reduction in sea ice production ( [[#Timmermann--2013|Timmermann and Hellmer, 2013]] ; [[#Obase--2017|Obase et al., 2017]] ), and eddies ( [[#Stewart--2015|Stewart and Thompson, 2015]] ; [[#Thompson--2018|Thompson et al., 2018]] ) can facilitate access of CDW to the sub-ice-shelf cavities ( [[#9.4.2.1|Section 9.4.2.1]] ). However, there is ''low confidence'' in the quantitification, importance and the ability of present models, especially at coarse resolution, to project changes in each of these processes ( [[#9.4.2.2|Section 9.4.2.2]] ). Some studies have projected a possible shift from cold to warm sub-ice-shelf cavities causing a sudden flush of warm water underneath ice shelves, but there is ''low confidence'' in the driving processes and the threshold to trigger the shift (Box 9.4; [[#Hellmer--2012|Hellmer et al., 2012]] , 2017; [[#Silvano--2018|Silvano et al., 2018]] ; [[#Hazel--2020|Hazel and Stewart, 2020]] ).

<div id="9.2.3" class="h2-container"></div>

<span id="regional-ocean-circulation"></span>