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

== CCB.7 Southern Ocean Circulation: Drivers, Changes and Implications ==

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'''Authors''' : Michael P. Meredith (UK), Robert Hallberg (US), Alessandro Tagliabue (UK), Andrew Meijers (UK/Australia), Jamie Oliver (UK), Andrew Hogg (Australia)

'''''Horizontal circulation and movement of fronts'''''

The Southern Ocean is disproportionately important in global climate and ecological systems, being the major connection linking the Atlantic, Pacific and Indian Oceans in the global circulation. The horizontal circulation in the circumpolar Southern Ocean is comprised of an eastward-flowing mean current concentrated in a series of sinuous, braided jets exhibiting strong meandering variability and shedding small-scale transient eddies (Figure CB7.1). The mean flow circumnavigates Antarctica as the world’s largest ocean current, the Antarctic Circumpolar Current (ACC), transporting approximately 173.3 ± 10.7 ×10 6 m 3 s –1 (Donohue et al., 2016 <sup>[[#fn:r331|331]]</sup> ) of water eastward in a geostrophic balance set up by the contrasting properties of waters around Antarctica and those inside the subtropical gyres to the north of ACC. This contrast is maintained by a combination of strong westerly winds and ocean heat loss south of the ACC.

Trends in the atmospheric forcing of the Southern Ocean are dominated by a strengthening of westerly winds in recent decades (Swart et al., 2015a <sup>[[#fn:r332|332]]</sup> ), but there is no evidence that this enhanced wind stress has significantly altered the ACC transport. While the annual mean value of transport is stable in the instrumental period (Chidichimo et al., 2014 <sup>[[#fn:r333|333]]</sup> ; Koenig et al., 2014 <sup>[[#fn:r334|334]]</sup> ; Donohue et al., 2016 <sup>[[#fn:r335|335]]</sup> ) it is difficult to resolve changes in barotropic transport; overall there is ''medium confidence'' that ACC transport is only weakly sensitive to changes in winds. This is consistent with longer-term analyses that find only minimal changes in ACC transport since the last glaciation (McCave et al., 2013 <sup>[[#fn:r336|336]]</sup> ). Theoretical predictions and high-resolution ocean modelling suggest that the weak sensitivity of the ACC to changes in wind stress is a consequence of eddy saturation (Munday et al., 2013 <sup>[[#fn:r337|337]]</sup> ), whereby the time-mean state of the ocean remains close to a marginal condition for eddy instability and hence additional energy input from stronger winds cascades rapidly into the smaller-scale eddy field. Satellite measurements of eddy kinetic energy over the last two decades are consistent with this, showing a statistically significant upward trend in eddy energy in the Pacific and Indian Ocean sectors of the Southern Ocean (Hogg et al., 2015 <sup>[[#fn:r338|338]]</sup> ) ( ''medium confidence'' ). This is supported by eddy-resolving models, which also show a marked regional variability (Patara et al., 2016 <sup>[[#fn:r339|339]]</sup> ), and there is evidence that local hotspots in eddy energy, especially downstream of major topographic features including the Drake Passage, Kerguelen Plateau, Campbell Plateau and the East Pacific Rise, may dominate the regional fields (Thompson and Naveira Garabato, 2014 <sup>[[#fn:r340|340]]</sup> ).

Working Group I (WGI) of the IPCC’s 5th Assessment Report (AR5) assessed that there was ''medium confidence'' that the mean position of the ACC had moved southwards in response to a contraction of the Southern Ocean circumpolar winds. Such movements can in principle have profound effects on marine ecosystems via, e.g., changing habitat ranges for different species (e.g., Cristofari et al., 2018; Meijers et al., 2019 <sup>[[#fn:r341|341]]</sup> ) (Section 3.2.3.2). Since AR5, however, substantial contrary evidence has emerged. While winds have strengthened over the Southern Ocean, reanalysis products show no significant shift in the annual mean latitude of zonal wind jets between 1979–2009 (Swart et al., 2015a <sup>[[#fn:r342|342]]</sup> ). Similarly, a variety of methods applied to satellite data have found no long-term trend and no statistically significant correlation of ACC position with winds (Gille, 2014 <sup>[[#fn:r343|343]]</sup> ; Chapman, 2017 <sup>[[#fn:r344|344]]</sup> ; Chambers, 2018 <sup>[[#fn:r345|345]]</sup> ). The discrepancy between these studies and those assessed in WGI AR5 appears to be caused by issues associated with using a fixed sea surface height contour as a proxy for frontal position in the presence of strongly eddying fields (Chapman, 2014 <sup>[[#fn:r346|346]]</sup> ) and large-scale increases in sea surface height consistent with mean global trends in sea level rise (Gille, 2014 <sup>[[#fn:r347|347]]</sup> ). The increase in sea surface height is ascribed largely to warming-driven steric expansion in the upper ocean, but the mechanism driving such warming is still uncertain (Gille, 2014 <sup>[[#fn:r348|348]]</sup> ). These recent findings do not preclude more local changes in frontal position, but it is now assessed as ''unlikely'' that there has been a statistically significant net southward movement of the mean ACC position over the past 20 years.

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'''Figure CB7.1'''

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'''Schematic of some of the major Southern Ocean changes assessed in this Box and in Chapters 3 and 5. Assessed changes are marked as positive (+), neutral (=), negative (–), or dominated by variability (~). The number of symbols used indicates confidence, from low (1) through medium (2) to high (3). Section numbers indicate the […]'''
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[[File:0e297bd6b37708bca25d24d2060bf4a7 IPCC-SROCC-CCB_7_1.jpg]]

Schematic of some of the major Southern Ocean changes assessed in this Box and in Chapters 3 and 5. Assessed changes are marked as positive (+), neutral (=), negative (–), or dominated by variability (~). The number of symbols used indicates confidence, from low (1) through medium (2) to high (3). Section numbers indicate the links to further information outside this box.

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'''''Overturning circulation and water mass formation'''''

The Southern Ocean is the key region globally for the upwelling of interior ocean waters to the surface, enabling waters that were last ventilated in the pre-industrial era to interact with the industrial-era atmosphere and the cryosphere. New water masses are produced that sink back into the ocean interior. Such export of both extremely cold and dense Antarctic Bottom Water and the lighter mode and intermediate waters (Figure CB7.1) represents important pathways for surface properties to be sequestered from the atmosphere for decades to millennia. This upwelling and sinking constitutes a two-limbed overturning circulation, by which much of the global deep ocean is renewed.

The Southern Ocean overturning circulation plays a strong role in mediating climate change via the transfer of heat and carbon (including that of anthropogenic origin) with the atmosphere (Sections 3.2.1.2; 5.2.2.2); it also has an impact on sea ice extent and concentration, with implications for climate via albedo (Section 3.2.1.1). It acts to oxygenate the ocean interior and sequesters nutrients that ultimately end up supporting a significant fraction of primary production in the rest of the world ocean (Section 5.2.2.2). The upwelling waters in the overturning bring heat to the Antarctic shelf seas, with consequences for ice shelves, marine-terminating glaciers and the stability of the Antarctic Ice Sheet (AIS) (Section 3.3.1). The lower limb of this overturning circulation supplies Antarctic Bottom Water that forms the abyssal layer of much of the world ocean (Section 3.2.1.2; 5.2.2.2).

It is challenging to measure the Southern Ocean overturning directly, and misinterpretation of Waugh et al. (2013) <sup>[[#fn:r349|349]]</sup> led to AR5 erroneously reporting the upper cell to have slowed (AR5 WGI, Section 3.6.4). However, additional indirect estimates since AR5 provide support for the increase in the upper ocean overturning proposed by Waugh et al. (2013). Waugh (2014) <sup>[[#fn:r350|350]]</sup> and Ting and Holzer (2017) <sup>[[#fn:r351|351]]</sup> suggest that over the 1990s–2000s water mass ages changed in a manner consistent with an increase in upwelling and overturning. However, inverse analyses suggest that such overturning experiences significant inter-decadal variability in response to wind forcing, with reductions in 2000–2010 relative to 1990–2000 (DeVries et al., 2017 <sup>[[#fn:r352|352]]</sup> ). This variability, combined with the indirect nature of observational estimates, means that there is ''low confidence'' in assessments of long-term changes in upper cell overturning.

Available evidence indicates that the volume of Antarctic Bottom Water in the global ocean has decreased (Purkey and Johnson, 2013 <sup>[[#fn:r353|353]]</sup> ; Desbruyeres et al., 2017 <sup>[[#fn:r354|354]]</sup> ) ( ''medium confidence'' ), thinning at a rate of 8.1 m yr −1 since the 1950s (Azaneu et al., 2013 <sup>[[#fn:r355|355]]</sup> ); recently updated analyses confirm this trend to present day (Figure 5.4). This suggests that the production and export of this water mass has probably slowed, though direct observational evidence is difficult to obtain. The large-scale impacts of Antarctic Bottom Water changes include a potential modulation to the strength of the Atlantic Meridional Overturning Circulation (e.g., Patara and Böning, 2014; see also Section 5.2.2.2.1).

'''''Projections'''''

Projections of future trends in the Southern Ocean are dominated by the potential for a continued strengthening of the westerly winds (Bracegirdle et al., 2013 <sup>[[#fn:r356|356]]</sup> ), as well as a combination of warming and increased freshwater input from both increased net precipitation and changes in sea ice export (Downes and Hogg, 2013 <sup>[[#fn:r357|357]]</sup> ). Dynamical considerations and numerical simulations indicate that, if further increases in the westerly winds are sustained, then it is ''very likely'' that the eddy field will continue to grow in intensity (Morrison and Hogg, 2013 <sup>[[#fn:r358|358]]</sup> ; Munday et al., 2013 <sup>[[#fn:r359|359]]</sup> ), with potential consequences for the upper-ocean overturning circulation and transport of tracers (Abernathey and Ferreira, 2015 <sup>[[#fn:r360|360]]</sup> ) (including heat, carbon, oxygen and nutrients), and ''likely'' that the mean position and strength of the ACC will remain only weakly sensitive to winds.

The considerable Coupled Model Intercomparison Project Phase 5 (CMIP5) inter-model variations in Southern Ocean time-mean circulation projections reported in WGI AR5 (Meijers et al., 2012 <sup>[[#fn:r361|361]]</sup> ; Downes and Hogg, 2013 <sup>[[#fn:r362|362]]</sup> ) remain largely unchanged. Some of the differences in projected changes have been found to be correlated with biases in the various models’ ability to simulate the historical state of the Southern Ocean, such as mixed layer depth (Sallée et al., 2013a <sup>[[#fn:r363|363]]</sup> ) and westerly wind jet latitude (Bracegirdle et al., 2013 <sup>[[#fn:r364|364]]</sup> ). This suggests that bias reduction against observed historical metrics (Russell et al., 2018 <sup>[[#fn:r365|365]]</sup> ) in future generations of coupled models (e.g., Coupled Model Intercomparison Project Phase 6 (CMIP6)) should lead to improved confidence in aspects of projected Southern Ocean changes. CMIP5 models suggest that the subduction of mode and intermediate water will increase (Sallée et al., 2013b <sup>[[#fn:r366|366]]</sup> ), which will affect oxygen and nutrient transports, and the overall transport of the Southern Ocean upper overturning cell will increase by up to 20% (Downes and Hogg, 2013 <sup>[[#fn:r367|367]]</sup> ), but model performance is limited by the inability to explicitly resolve eddy processes (Gent, 2016 <sup>[[#fn:r368|368]]</sup> ; Downes et al., 2018 <sup>[[#fn:r369|369]]</sup> ). The formation and export of Antarctic Bottom Water is predicted to continue decreasing (Heuzé et al., 2015 <sup>[[#fn:r370|370]]</sup> ) due to warming and freshening of surface source waters near the continent. These are, however, some of the most poorly represented processes in global models. Further uncertainty derives from increased meltwater from the AIS not being considered in the CMIP5 climate models, despite its potential for significant impact on Southern Ocean dynamics and the global climate, and its potential for positive feedbacks (Bronselaer et al., 2018 <sup>[[#fn:r371|371]]</sup> ). Due to these uncertainties, ''low confidence'' is therefore ascribed to the CMIP5-based model projections of future Southern Ocean circulation and water masses.

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=== 3.2.2 Projected Changes in Sea Ice and Ocean ===

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==== 3.2.2.1 Sea Ice ====

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The multi-model ensemble of historical simulations from CMIP5 models identify declines in total Arctic sea ice extent and thickness (Sections 3.2.1.1.1; 3.2.1.1.2; Figure 3.3) which agree with observations (Massonnet et al., 2012 <sup>[[#fn:r372|372]]</sup> ; Stroeve et al., 2012a <sup>[[#fn:r373|373]]</sup> ; Stroeve et al., 2014a <sup>[[#fn:r374|374]]</sup> ; Stroeve and Notz, 2015 <sup>[[#fn:r375|375]]</sup> ). There is a range in the ability of individual models to simulate observed sea ice thickness spatial patterns and sea ice drift rates (Jahn et al., 2012 <sup>[[#fn:r376|376]]</sup> ; Stroeve et al., 2014a <sup>[[#fn:r377|377]]</sup> ; Tandon et al., 2018 <sup>[[#fn:r378|378]]</sup> ). Reductions in Arctic sea ice extent scale linearly with both global temperatures and cumulative CO 2 emissions in simulations and observations (Notz and Stroeve, 2016 <sup>[[#fn:r379|379]]</sup> ), although aerosols influenced historical sea ice trends (Gagné et al., 2017 <sup>[[#fn:r380|380]]</sup> ). The uncertainty in sea ice sensitivity (ice extent loss per unit of warming) is quite large (Niederdrenk and Notz, 2018 <sup>[[#fn:r381|381]]</sup> ) and the model sensitivity is too low in most CMIP5 models (Rosenblum and Eisenman, 2017 <sup>[[#fn:r382|382]]</sup> ). Emerging evidence suggests, however, that internal variability, including links between the Arctic and lower latitude, strongly influences the ability of models to simulate observed reductions in Arctic sea ice extent (Swart et al., 2015b <sup>[[#fn:r383|383]]</sup> ; Ding et al., 2018 <sup>[[#fn:r384|384]]</sup> ).

CMIP5 models project continued declines in Arctic sea ice through the end of the century (Figure 3.3) (Notz and Stroeve, 2016 <sup>[[#fn:r385|385]]</sup> ) ( ''high confidence'' ). There is a large spread in the timing of when the Arctic may become ice free in the summer, and for how long during the season (Massonnet et al., 2012 <sup>[[#fn:r386|386]]</sup> ; Stroeve et al., 2012a <sup>[[#fn:r387|387]]</sup> ; Overland and Wang, 2013 <sup>[[#fn:r388|388]]</sup> ) as a result of natural climate variability (Notz, 2015 <sup>[[#fn:r389|389]]</sup> ; Swart et al., 2015b <sup>[[#fn:r390|390]]</sup> ; Screen and Deser, 2019 <sup>[[#fn:r391|391]]</sup> ), scenario uncertainty (Stroeve et al., 2012a <sup>[[#fn:r392|392]]</sup> ; Liu et al., 2013 <sup>[[#fn:r393|393]]</sup> ), and model uncertainties related to sea ice dynamics (Rampal et al., 2011 <sup>[[#fn:r394|394]]</sup> ; Tandon et al., 2018 <sup>[[#fn:r395|395]]</sup> ) and thermodynamics (Massonnet et al., 2018 <sup>[[#fn:r396|396]]</sup> ). Internal climate variability results in an uncertainty of approximately 20 years in the timing of seasonally ice-free conditions (Notz, 2015 <sup>[[#fn:r397|397]]</sup> ; Jahn, 2018 <sup>[[#fn:r398|398]]</sup> ), but the clear link between summer sea ice extent and cumulative CO 2 emissions provides a basis for when consistent ice-free conditions may be expected ( ''high confidence'' ). For stabilised global warming of 1.5°C, sea ice in September is ''likely'' to be present at end of century with an approximately 1% chance of individual ice-free years (Notz and Stroeve, 2016 <sup>[[#fn:r399|399]]</sup> ; Sanderson et al., 2017 <sup>[[#fn:r400|400]]</sup> ; Jahn, 2018 <sup>[[#fn:r401|401]]</sup> ; Sigmond et al., 2018 <sup>[[#fn:r402|402]]</sup> ); after 10 years of stabilised warming at a 2°C increase, more frequent occurrence of an ice-free summer Arctic is expected (around 10-35%) (Mahlstein and Knutti, 2012 <sup>[[#fn:r403|403]]</sup> ; Jahn et al., 2016 <sup>[[#fn:r404|404]]</sup> ; Notz and Stroeve, 2016 <sup>[[#fn:r405|405]]</sup> ). Model simulations show that a temporary temperature overshoot of a warming target has no lasting impact on ice cover (Armour et al., 2011 <sup>[[#fn:r406|406]]</sup> ; Ridley et al., 2012 <sup>[[#fn:r407|407]]</sup> ; Li et al., 2013 <sup>[[#fn:r408|408]]</sup> ).

CMIP5 models show a wide range of mean states and trends in Antarctic sea ice (Turner et al., 2013 <sup>[[#fn:r409|409]]</sup> ; Shu et al., 2015 <sup>[[#fn:r410|410]]</sup> ). The ensemble mean across multiple models show a decrease in total Antarctic sea ice extent during the satellite era, in contrast to the lack of any observed trend (Figure 3.3; Section 3.2.1.1.1). Interannual sea ice variability in the models is larger than observations (Zunz et al., 2013 <sup>[[#fn:r411|411]]</sup> ), which may mask disparity between models and observations. Internal variability (Polvani and Smith, 2013 <sup>[[#fn:r412|412]]</sup> ; Zunz et al., 2013 <sup>[[#fn:r413|413]]</sup> ), and model sensitivity to warming (Rosenblum and Eisenman, 2017 <sup>[[#fn:r414|414]]</sup> ) are also important sources of uncertainty. During the historical period, regional trends of Antarctic sea ice are not captured by the models, particularly the decrease in the Bellingshausen Sea and the expansion in the Ross Sea (Hobbs et al., 2015 <sup>[[#fn:r415|415]]</sup> ). There is a very wide spread of model responses in the Weddell Sea (Hobbs et al., 2015 <sup>[[#fn:r416|416]]</sup> ; Ivanova et al., 2016 <sup>[[#fn:r417|417]]</sup> ), a region with complex ocean-sea ice interactions that many models do not replicate (de Lavergne et al., 2014).

There is ''low confidence'' in projections of Antarctic sea ice because there are multiple anthropogenic forcings (ozone and greenhouse gases) and complicated processes involving the ocean, atmosphere, and adjacent ice sheet (Section 3.2.1.1.). Model deficiencies are related to stratification (Sallée et al., 2013a <sup>[[#fn:r418|418]]</sup> ), freshening by ice shelf melt water (Bintanja et al., 2015 <sup>[[#fn:r419|419]]</sup> ), atmospheric processes including clouds (Schneider and Reusch, 2015 <sup>[[#fn:r420|420]]</sup> ; Hyder et al., 2018 <sup>[[#fn:r421|421]]</sup> ), and wind and ocean driven processes (Purich et al., 2016a <sup>[[#fn:r422|422]]</sup> ; Purich et al., 2016b <sup>[[#fn:r423|423]]</sup> ; Schroeter et al., 2017 <sup>[[#fn:r424|424]]</sup> ; Purich et al., 2018 <sup>[[#fn:r425|425]]</sup> ; Zhang et al., 2018a <sup>[[#fn:r426|426]]</sup> ). Uncertainty in sea ice projections reduces confidence in projections of Antarctic Ice Sheet surface mass balance because sea ice affects Antarctic temperature and precipitation trends (Bracegirdle et al., 2015 <sup>[[#fn:r427|427]]</sup> ), and impacts projected changes in the Southern Hemisphere westerly jet (Bracegirdle et al., 2018 <sup>[[#fn:r428|428]]</sup> ; England et al., 2018 <sup>[[#fn:r429|429]]</sup> ) with implications for the Southern Ocean overturning circulation (Cross-Chapter Box 7 in Chapter 3).

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