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

==== 3.5.4.2 Southern Ocean Circulation ====

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The Southern Ocean circulation provides the principal connections between the world’s major ocean basins through the circulation of the Antarctic Circumpolar Current (ACC), while also largely controlling the connection between the deep and upper layers of the global ocean circulation, through its upper and lower overturning cells.

The assessment of observations presented in Sections 2.3.3.4.2 and 9.2.3.2 reports that there is no evidence of an ACC transport change, and it is ''unlikely'' that the mean meridional position of the ACC has moved southward in recent decades (Sections 2.3.3.4.2 and 9.2.3.2). This is despite observations of surface wind displaying an intensification and southward shift ( [[IPCC:Wg1:Chapter:Chapter-2#2.4.1.2|Section 2.4.1.2]] ). There is ''low confidence'' in an observed intensification of upper ocean overturning in the Southern Ocean and there is ''medium confidence'' for a slowdown of the Antarctic Bottom Water circulation and commensurate Antarctic Bottom Water volume decrease since the 1990s (Section 9.2.3.2). Section 9.2.3.2 presents new evidence, since SROCC, which assessed with ''medium confidence'' that the lower cell can episodically increase as a response to climatic anomalies, temporally counteracting the forced tendency for reduced bottom water formation.

The modelled strength of the ACC clearly improved from CMIP3, in which the models tended to underestimate the strength of the ACC, to CMIP5 ( [[#Meijers--2012|Meijers et al., 2012]] ). This improvement in the realism of ACC strength continues from CMIP5 to CMIP6, with the modelled ACC strength converging toward the magnitude of observed estimates of net flow through the Drake Passage ( [[#Beadling--2020|Beadling et al., 2020]] ). There is, however, a small number of models that still display an ACC that is much weaker than that observed, while several models also display much more pronounced ACC decadal variability than that observed ( [[#Beadling--2020|Beadling et al., 2020]] ). The increased realism of the ACC was at least partly related to noted improvements in all metrics of the Southern Ocean’s surface wind stress forcing ( [[#Beadling--2020|Beadling et al., 2020]] ). The most notable wind stress forcing improvements were found in the strength and the latitudinal position of the zonally-averaged westerly wind stress maximum ( [[#Beadling--2020|Beadling et al., 2020]] ; [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). While the two-cell structure of the overturning circulation appears to be well captured by CMIP5 models ( [[#Sallée--2013|Sallée et al., 2013]] ; [[#Russell--2018|Russell et al., 2018]] ), they tend to underestimate the intensity of the lower cell overturning, and overestimate the intensity of the upper cell overturning ( [[#Sallée--2013|Sallée et al., 2013]] ). As the lower overturning cell is closely related to Antarctic Bottom Water formation and deep convection, both fields also display substantial errors in CMIP5 models ( [[#Heuzé--2013|Heuzé et al., 2013]] , [[#Heuzé--2015|2015]] ). CMIP6 climate models show clear improvements compared to CMIP5 in their representation of Antarctic Bottom Water, which suggests an improved representation of the lower overturning cell ( [[#Heuzé--2021|Heuzé, 2021]] ).

Despite notable improvements of CMIP6 models compared to CMIP5 models, inherent limitations in the representation of important processes at play in the Southern Ocean’s horizontal and vertical circulation remain (Section 9.2.3.2). For instance, Southern Ocean mesoscale eddies are largely parameterized in the current generation of climate models and, despite their small spatial scales, they are a key element for establishing the ACC and upper overturning cell, as well as for their future evolution under changing atmospheric forcing ( [[#Kuhlbrodt--2012|Kuhlbrodt et al., 2012]] ; [[#Downes--2013|Downes and Hogg, 2013]] ; [[#Gent--2016|Gent, 2016]] ; [[#Downes--2018|Downes et al., 2018]] ; [[#Poulsen--2018|Poulsen et al., 2018]] ). The absence of ice-sheet coupling in the CMIP6 model suite is another important limitation, as basal meltwater and calving can influence the circulation, particularly the lower cell of the Southern Ocean ( [[#Bronselaer--2018|Bronselaer et al., 2018]] ; [[#Golledge--2019|Golledge et al., 2019]] ; [[#Lago--2019|Lago and England, 2019]] ; [[#Jeong--2020|Jeong et al., 2020]] ; [[#Moorman--2020|Moorman et al., 2020]] ). We note that early development of global climate models with interactive ice-shelf cavities has begun and is showing potential to be developed ( [[#Jeong--2020|Jeong et al., 2020]] ).

In summary, while there have been improvements across successive CMIP phases (from CMIP3 to CMIP6) in the representation of the Southern Ocean circulation, such that the mean zonal and overturning circulations of the Southern Ocean are now broadly reproduced, substantial observational uncertainty and climate model challenges preclude attribution of Southern Ocean circulation changes ( ''high confidence'' ).

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