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

==== 9.2.3.2 Southern Ocean ====

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The changing Southern Ocean circulation system exerts a strong influence on the global climate by modulating: (i) global OHC ( [[#9.2.2.1|Section 9.2.2.1]] ); (ii) global ocean anthropogenic carbon uptake (Cross-chapter Box 5.3); global ocean overturning circulation ( [[#9.2.3.1|Section 9.2.3.1]] ); (iii) climate sensitivity ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4|Section 7.4.4]] and Cross-chapter Box 5.3); (iv) sea level through basal melt of ice shelves (9.4.2); and (v) Southern Hemisphere sea ice cover ( [[#9.3.2|Section 9.3.2]] ).

The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) had ''low confidence'' in all CMIP5-based model projections due to their inability to explicitly resolve eddy processes, and their inability to properly consider future meltwater change from the Antarctic Ice Sheet. These limitations of climate models to represent the Southern Ocean persist due to most CMIP6 models still using parameterized mesoscale eddy processes, which are limited in projecting the future response of the horizontal and vertical circulation under climate warming, and also because of the continued absence of active ice-shelf and ice-sheet coupling in the CMIP6 model suite, therefore ignoring basal meltwater and calving feedback on the circulation ( [[#Meredith--2019|Meredith et al., 2019]] ). In addition, two important limitations of CMIP6 models of the Southern Ocean involve processes that were not assessed in SROCC. First, the poor representation of dense overflows causes most of the Antarctic Bottom Water (AABW) to be formed by spurious open ocean convection rather than by dense overflows from the Antarctic continental shelves that feed the lower overturning cell ( [[#Snow--2015|Snow et al., 2015]] ; [[#Dufour--2017|Dufour et al., 2017]] ; [[#Heuzé--2021|Heuzé, 2021]] ). Second, Antarctic continental shelf waters are poorly simulated because potentially important controlling mechanisms tend to be too small and transient to observe and resolve in CMIP ocean models. These small processes include: the heterogeneity of observed sub-ice-shelf melt with warm water driving narrow basal channels that cut underneath the ice ( [[#Drews--2015|Drews, 2015]] ; [[#Alley--2016|Alley et al., 2016]] ; [[#Marsh--2016|Marsh et al., 2016]] ; [[#Milillo--2019|Milillo et al., 2019]] ); eddies and tides ( [[#Stewart--2018|Stewart et al., 2018]] ; [[#Jourdain--2019|Jourdain et al., 2019]] ; [[#Hausmann--2020|Hausmann et al., 2020]] ), which can drive Circumpolar Deep Water (CDW) onto the continental shelves or dynamically increase melting ( [[#9.2.3.6|Section 9.2.3.6]] ); and feedback mechanisms between ocean, atmosphere and cryosphere that can weaken or amplify initial perturbations ( [[#Donat-Magnin--2017|Donat-Magnin et al., 2017]] ; [[#Spence--2017|Spence et al., 2017]] ; [[#Turner--2017|Turner et al., 2017]] ; [[#Silvano--2018|Silvano et al., 2018]] ; [[#Webber--2019|Webber et al., 2019]] ; [[#Hazel--2020|Hazel and Stewart, 2020]] ). In addition, the Southern Ocean in CMIP5 and CMIP6 models exhibit surface temperature biases ( [[#9.2.1.1|Section 9.2.1.1]] ), which have been linked in CMIP5 models to errors in atmospheric model cloud-related shortwave radiation ( [[#Hyder--2018|Hyder et al., 2018]] ) and are somewhat improved in High Resolution Model Intercomparison Project (HighResMIP) models (Figure 9.3). In summary, there is ''high confidence'' that future change in the subpolar Southern Ocean region, including sea ice cover and ocean temperature change on Antarctic continental shelves, depends on feedback mechanisms involving the ocean, atmosphere and cryosphere that are poorly understood and not represented in the current generation of climate models. This results in large uncertainty and ''low confidence'' in the future sea ice cover ( [[#9.3.2|Section 9.3.2]] ) and in ocean temperature change on the Antarctic continental shelf ( [[#9.4.2.3|Section 9.4.2.3]] ).

Despite these challenges, the CMIP6 ensemble does represent the main Southern Ocean circulation characteristics: the simulated Antarctic Circumpolar Current (ACC) transport is generally lower than observation-based values but consistent when considering ensemble spread, and the inter-model spread in ACC transport has greatly reduced from previous generations of climate models from CMIP3 to CMIP6 ( [[#Beadling--2019|Beadling et al., 2019]] , 2020). The structure (but not the magnitude) of the two-cell zonally averaged overturning is captured by most CMIP6 models ( [[#Russell--2018|Russell et al., 2018]] ; [[#Beadling--2019|Beadling et al., 2019]] ). In addition, while issues remain, CMIP6 climate models show clear improvements in their representation of AABW compared to CMIP5: several models correctly represent or parameterize Antarctic shelf processes, fewer models exhibit Southern Ocean deep convection, bottom density biases are reduced, and abyssal overturning is more realistic ( [[#Heuzé--2021|Heuzé, 2021]] ). In terms of atmospheric wind forcing, CMIP6 models show an improvement compared to CMIP5 models, with an overall reduction in the equatorward bias of the annual mean westerly jet from 1.9° in CMIP5 to 0.4° in CMIP6, but in contrast, they show no such overall improvements for their representation of the Amundsen Sea Low ( [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ; [[#Lyu--2020a|Lyu et al., 2020a]] ), which can be critical in driving variability of water masses on the Antarctic continental shelf in west Antarctica, the Weddell Sea or the Ross Sea ( [[#Holland--2019|Holland et al., 2019]] ; [[#Silvano--2020|Silvano et al., 2020]] ).

The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) established that, while trends in the atmospheric forcing of the Southern Ocean have been dominated by a strengthening of the Southern Hemisphere westerly winds in recent decades, there is ''medium confidence'' that ACC transport is weakly sensitive to changes in winds. It also reported that, instead of increasing the mean ACC transport, additional energy input associated with increased wind stress cascades into the eddy field ( ''medium confidence'' ). In contrast with the AR5 assessment ( [[#Rhein--2013|Rhein et al., 2013]] ), SROCC evaluated that it was ''unlikely'' that there has been a net southward migration of the mean ACC position over the past 20 years. There is no additional evidence to revisit SROCC assessment on wind sensitivity. However, new evidence does suggest that air–sea buoyancy forcing associated with idealized 4×CO <sub>2</sub> forcing leads to an increase in ACC transport ( ''limited evidence'' ) ( [[#Shi--2020|Shi et al., 2020]] ). The SROCC noted that, if the general strengthening in westerly winds is sustained, then it is ''very likely'' that the eddy field will continue to increase in intensity, and it is ''likely'' that the mean position and strength of the ACC will remain only weakly sensitive to winds. In the future, the strength of the Southern Hemisphere westerly wind jet results from a competition between decrease due to ozone hole recovery and increase due to increased radiative forcing ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.3.1|Section 4.3.3.1]] ). This competition results in an increased atmospheric jet by 2100 compared to present day under SSP2-4.5, SSP3-7.0, and SSP5-8.5, but a decreased jet by 2100 under SSP1-2.6 ( [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). There is little inter-model spread in the CMIP6 future response of the atmospheric westerly jet, providing ''high confidence'' in this assessment (in contrast, CMIP6 models show no consistency in their future projection of easterly wind change along the Antarctic continental shelf break; [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). Paleo-oceanographic evidence suggests that ACC flow through Drake Passage was consistently stronger during warm intervals of the past (both during interstadials and interglacials), but with relatively little change and no consensus on the sign of change in other regions ( [[#Lamy--2015|Lamy et al., 2015]] ; [[#Toyos--2020|Toyos et al., 2020]] ). In summary, additional evidence since SROCC confirms that there is ''medium confidence'' that the ACC has been weakly sensitive to Southern Hemisphere atmospheric jet increase in the past decades. New evidence since SROCC suggests that there is ''high confidence'' that the Southern Hemisphere atmospheric jet will increase in the 21st century for all scenarios (except for SSP1-1.9 and SSP1-2.6; [[IPCC:Wg1:Chapter:Chapter-4#4.3.3.1|Section 4.3.3.1]] ) with a greater increase for larger radiative forcing. An increase in westerly winds will ''very likely'' force an increase of the eddy field in the ACC, and while there is ''medium confidence'' that the ACC is weakly sensitive to wind change, new advances since SROCC provide ''limited evidence'' that the ACC transport will nevertheless increase in response to wind and buoyancy fluxes.

For the upper cell overturning circulation, SROCC concluded that: its transport has experienced significant inter-decadal variability in response to wind forcing since the 1990s; and there is ''low confidence'' in the assessments of a long-term increase in upper-ocean overturning. Consistent with SROCC, the importance of eddy processes and winds in driving long-term change and variability have been reinforced, with a potential fast wind response partially counteracted by a slower eddy response ( [[#Doddridge--2019|Doddridge et al., 2019]] ; [[#Waugh--2019|Waugh et al., 2019]] ; [[#Stewart--2020|Stewart et al., 2020]] ). Eddy parametrizations affect the strength of overturning, its sensitivity to winds and the ACC transport ( [[#Mak--2017|Mak et al., 2017]] ). Even in eddy-resolving simulations, sub-gridscale dissipation affects the overturning and ACC ( [[#Pearson--2017|Pearson et al., 2017]] ). In addition, there has been progress in understanding the importance of Antarctic Ice Shelf meltwater and sea ice, in driving the observed changes in the near surface and in the upper overturning cell over the past decades, on top of changes induced by winds and eddies ( [[#Bronselaer--2020|Bronselaer et al., 2020]] ; [[#Haumann--2020|Haumann et al., 2020]] ; [[#Jeong--2020|Jeong et al., 2020]] ; [[#Rye--2020|Rye et al., 2020]] ). In particular, increased stratification caused by increased freshwater flux to the surface ocean ( [[#9.2.1.3|Section 9.2.1.3]] ) can cause a shoaling and warming of the CDW layer, and create a positive feedback, enhancing basal melt of the Antarctic Ice Sheet ( [[#9.4.2.1|Section 9.4.2.1]] ; [[#Bronselaer--2018|Bronselaer et al., 2018]] ; [[#Golledge--2019|Golledge et al., 2019]] ; [[#Schloesser--2019|Schloesser et al., 2019]] ; [[#Sadai--2020|Sadai et al., 2020]] ). There is ''medium confidence'' in the existence of this feedback mechanism but ''low agreement'' on the magnitude of the feedback. The SROCC reported that CMIP5 models project that the overall transport of upper-ocean overturning cell will increase by up to 20% in the 21st century, and no new studies alter that assessment.

For the lower cell overturning circulation, SROCC assessed that a slowdown of its transport is consistent with the observed decrease in volume ( ''medium confidence'' ) of AABW in the global ocean ( [[#9.2.2.3|Section 9.2.2.3]] ). Additional evidence since SROCC strengthens confidence that increased glacial meltwater flux will reduce the density of bottom waters during the 21st century. It will eventually reach a point where deep convection will be curtailed, and shelf water will become too buoyant to sink to the ocean interior, thereby slowing the lower cell overturning circulation ( [[#Bronselaer--2018|Bronselaer et al., 2018]] ; [[#Golledge--2019|Golledge et al., 2019]] ; [[#Lago--2019|Lago and England, 2019]] ; [[#Moorman--2020|Moorman et al., 2020]] ). While such changes are consistent with the observed freshening and decreased volume of the AABW layer reported in SROCC (as discussed in [[#9.2.2.3|Section 9.2.2.3]] ), new observation-based studies have highlighted how the lower cell overturning can episodically increase as a response to climate anomalies, temporally counteracting the tendency for melt to reduce AABW formation ( [[#Abrahamsen--2019|Abrahamsen et al., 2019]] ; [[#Castagno--2019|Castagno et al., 2019]] ; [[#Gordon--2020|Gordon et al., 2020]] ; [[#Silvano--2020|Silvano et al., 2020]] ). In addition, while the opening of open ocean polynyas can affect the lower cell on decadal to centennial time scales, there is ''limited evidence'' and ''low agreement'' in the role of open ocean polynyas in driving observed trends of the lower cell in the last decade ( [[#9.2.2.3|Section 9.2.2.3]] ). Based on CMIP5 models, SROCC reported with ''low confidence'' that formation and export of AABW associated with the lower overturning cell will decrease in the 21st century, and there is no new evidence to revisit that assessment from climate models. However, additional paleo evidence from marine sediments suggests that AABW formation/ventilation was vulnerable to freshwater fluxes during past interglacials ( [[#Hayes--2014|Hayes et al., 2014]] ; [[#Huang--2020|Huang et al., 2020]] ; [[#Turney--2020|Turney et al., 2020]] ) and that AABW formation was strongly reduced ( [[#Skinner--2010|Skinner et al., 2010]] ; [[#Gottschalk--2016|Gottschalk et al., 2016]] ; [[#Jaccard--2016|Jaccard et al., 2016]] ) or possibly totally curtailed ( [[#Huang--2020|Huang et al., 2020]] ) during the Last Glacial Maximum (LGM) and transient cold intervals of marine isotope stages 2 and 3 (MIS2 and MIS3). Specifically, sedimentary reconstructions show a transient reduction in AABW ventilation in the Atlantic sector of the Southern Ocean during MIS5e, which is assessed to have been warmer than modern climate ( [[#Thomas--2020|Thomas et al., 2020]] ). However, long multi-centennial or millennial model runs under higher-than-pre-industrial CO <sub>2</sub> concentrations show that, after 500–1000 years, ventilation in the Southern Ocean resumes, and possibly overshoots with enhanced convection in the Weddell and Ross seas, leading to enhanced bottom water ventilation globally ( [[#Yamamoto--2015|Yamamoto et al., 2015]] ; [[#Frölicher--2020|Frölicher et al., 2020]] ). AABW ventilation increased at the onset of the last deglacial transition, promoting the release of previously sequestered CO <sub>2</sub> to the atmosphere on centennial to millennial time scales ( [[#Bauska--2016|Bauska et al., 2016]] ; [[#Jaccard--2016|Jaccard et al., 2016]] ; [[#Rae--2018|Rae et al., 2018]] ), concomitant with a southward shift of the Southern Hemisphere westerly wind belt ( [[#Denton--2010|Denton et al., 2010]] ; [[#Jaccard--2016|Jaccard et al., 2016]] ) and reduced sea ice cover ( [[#Ferrari--2014|Ferrari et al., 2014]] ; [[#Stein--2020|Stein et al., 2020]] ). In summary, the combination of observational, numerical and paleoclimate evidence provides us with ''medium confidence'' that the lower cell will continue decreasing in the 21st century as a result of increased basal melt from the Antarctic Ice Sheet.

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