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

==== 9.2.3.4 Gyres, Western Boundary Currents and Inter-basin Exchanges ====

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The AR5 ( [[#Rhein--2013|Rhein et al., 2013]] ) assessed with ''medium'' to ''high confidence'' that the North Pacific subpolar gyre, the South Pacific subtropical gyre, and the subtropical cells have intensified. They also reported that the North Pacific subtropical gyre had expanded since the 1990s, and that, overall, the changes in gyre systems were ''likely'' predominantly due to interannual-to-decadal variability. The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) complemented the AR5 assessment by reporting that the polar Beaufort Gyre in the Arctic expanded to the north-west between 2003 and 2014, contemporaneous with changes in its freshwater accumulation and alterations in wind forcing. Consistent with the reported change over the gyres, both AR5 and SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Collins--2019|Collins et al., 2019]] ) reported that western boundary currents (WBCs) have intensified (Figure 9.11), and expanded poleward, except for the Gulf Stream and the Kuroshio. [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4|Section 2.3.3.4]] provides an overall assessment of gyres and WBCs, including an assessment of change from paleoclimate archives. [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4|Section 2.3.3.4]] assesses that, while WBC strength is highly variable at multi-decadal scale ( ''high confidence'' ), WBCs and subtropical gyres have shifted poleward since 1993 ( ''medium confidence'' ), at a rate on the order of 0.04–0.1 degree per decade during 1993–2018. Figure 9.11 shows that CMIP5 and CMIP6 models agree in projecting a weaker Gulf Stream and Gulf Stream Extension, while the Kuroshio changes less ( [[#Sen%20Gupta--2016|Sen Gupta et al., 2016]] ).

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'''Figure 9.11''' '''|''' '''Simulated barotropic streamfunction, surface speed and major current transport in Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5 and CMIP6).''' '''(a)''' Mean barotropic streamfunction (unit: 10 <sup>9</sup> kg <sup></sup> s <sup>–1</sup> ; 1995–2014) and projected barotropic streamfunction change (10 <sup>9</sup> kg <sup></sup> s <sup>–1</sup> ; 2018–2100 vs 1995–2014) under '''(b)''' SSP5-8.5. '''(d)''' Mean surface (0–100 m) speed (m s <sup>–1</sup> ) and projected surface speed change (m s <sup>–1</sup> , 2081–2100) versus 1995–2014 under '''(e)''' SSP5-8.5. '''(c, f)''' Median and likely range of 1995–2014 and 2081–2100 transport of three currents with the largest transport change and four with the largest fractional change ( [[#Sen%20Gupta--2016|Sen Gupta et al., 2016]] ). '''(c)''' Deep currents: Agulhas Extension (ACx), Gulf Stream (GS), Gulf Stream Extension (GSx), Tasman Leakage (TASL), East Australia Current Extension (EACx), Indonesian Throughflow (ITF), and Brazil Current (BC). '''(f)''' Shallow currents: as for deep but with New Guinea Current (NGC), and without ACx. No overlay indicates regions with high model agreement, where ≥80% of models agree on the sign of change. Diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on the sign of change (see Cross-Chapter Box Atlas.1 for more information). Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

Although the observed wind stress curl shows systematic poleward shift in each basin as a result of anthropogenic warming ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4|Section 2.3.1.4]] ; [[#Chen--2012|Chen and Wu, 2012]] ; [[#Wu--2012|Wu et al., 2012]] ; [[#Zhai--2014|Zhai et al., 2014]] ), which has caused a systematic shift of the WBCs and subtropical gyres since 1993 ( [[#Wu--2012|Wu et al., 2012]] ; [[#Yang--2016|Yang et al., 2016]] , 2020), the response of current strength is more complex and inconsistent across regions ( [[#Sloyan--2015|Sloyan and O’Kane, 2015]] ; [[#Wang--2016|Y.-L. Wang et al., 2016]] ; [[#Elipot--2018|Elipot and Beal, 2018]] ; [[#McCarthy--2018|McCarthy et al., 2018]] ; [[#Wang--2018|Wang and Wu, 2018]] ; [[#Dong--2019|Dong et al., 2019]] ). The strength of WBCs and gyres exhibit inconsistent responses because they are dependent on wind stress forcing and because multi-scale interaction and air–sea interaction have an important role in their long-term trends and variability ( [[#Zhang--2020|Zhang et al., 2020]] ). Observed changes in gyre circulation are dominated by interannual and decadal modes of variability globally ( [[#Qiu--2012|Qiu and Chen, 2012]] ; [[#Melzer--2017|Melzer and Subrahmanyam, 2017]] ; [[#McCarthy--2018|McCarthy et al., 2018]] ; [[#Hu--2020|Hu et al., 2020]] ). The North Atlantic subpolar gyre is strongly modulated by variability associated with the NAO and AMV (Annex IV; [[#Robson--2016|Robson et al., 2016]] ). Subpolar gyre systems can change abruptly due to a positive feedback between convective mixing and salinity transport ( [[#Born--2013|Born et al., 2013]] , 2016) and air–sea interaction ( [[#Moffa-Sánchez--2014|Moffa-Sánchez et al., 2014]] ; [[#Moreno-Chamarro--2017|Moreno-Chamarro et al., 2017]] ) within the gyre. In the Arctic, both the Beaufort gyre and mesoscale eddies strengthened between 2003 and 2014 ( [[#Armitage--2017|Armitage et al., 2017]] ), which might be partly due to increased wind stress ( [[#Oldenburg--2018|Oldenburg et al., 2018]] ) or reduced sea ice thickness and changes in sea ice pack morphology ( [[#van%20der%20Linden--2019|van der Linden et al., 2019]] ). Presently, there is ''limited evidence'' in attributing causality to these changes for any of the proposed mechanisms. In the North Pacific, there has been an increasing trend in the Alaska Gyre from 1993 to 2017 ( [[#Cummins--2018|Cummins and Masson, 2018]] ), which might be attributed to Pacific Decadal Oscillation ( ''low confidence'' ) ( [[#Hristova--2019|Hristova et al., 2019]] ). In the Southern Ocean, ''limited evidence'' indicates that the subpolar gyres respond to Southern Hemisphere atmospheric modes of variability at interannual time scale ( [[#Armitage--2018|Armitage et al., 2018]] ; [[#Dotto--2018|Dotto et al., 2018]] ).

All climate models reproduce WBCs and gyres, but eddy-present or eddy-rich models (roughly 10–25 km and about 10 km resolution, respectively) represent these currents more realistically than eddy-parameterized models ( ''very'' ''high confidence'' ) ( [[#Small--2014|Small et al., 2014]] ; [[#Griffies--2015|Griffies et al., 2015]] ; [[#Chassignet--2017|Chassignet et al., 2017]] , 2020; [[#Hewitt--2017|Hewitt et al., 2017]] , 2020; [[#Roberts--2018|Roberts et al., 2018]] ). Compared to observations or to eddy-present and eddy-rich models, the eddy-parameterized models from CMIP5 and CMIP6 simulate weaker and wider WBCs, as well as less realistic locations of subtropical and subpolar gyre boundaries (Figure 9.11). Increased resolution admits mesoscale eddies, and also improves simulation of the strength and position of WBCs such as the Kuroshio Current, Gulf Stream, and East Australian Current ( ''very high confidence'' ) ( [[#Sasaki--2004|Sasaki et al., 2004]] ; [[#Chassignet--2008|Chassignet and Marshall, 2008]] ; [[#Delworth--2012|Delworth et al., 2012]] ; [[#Yu--2012|Yu et al., 2012]] ; [[#Small--2014|Small et al., 2014]] ; [[#Haarsma--2016|Haarsma et al., 2016]] ; [[#Chassignet--2017|Chassignet et al., 2017]] , 2020; [[#Hewitt--2020|Hewitt et al., 2020]] ). Improved boundary current location relates to improved recirculation regions ( [[#Jayne--2009|Jayne et al., 2009]] ), mean path and variability, and existence of multiple stable paths ( [[#Qiu--2005|Qiu et al., 2005]] ; [[#Delman--2015|Delman et al., 2015]] ), air–sea fluxes ( [[#Small--2014|Small et al., 2014]] ), and related coastal weather patterns ( [[#Kaspi--2011|Kaspi and Schneider, 2011]] ). The wind-current feedback, implemented by considering relative velocity of currents and wind, realistically dampens mesoscale eddies and WBCs, through mesoscale air–sea interaction ( [[#Ma--2016|Ma et al., 2016]] ; [[#Renault--2016|Renault et al., 2016]] , 2019), even though sub-mesoscale wind-current damping feedback is missing in these models ( ''medium confidence'' ) (Z. [[#Zhang--2016|]] [[#Zhang--2016|]] [[#Zhang--2016|Zhang et al., 2016]] ). As eddies potentially play a role in determining the strength of gyre circulations and their low-frequency variability ( [[#Fox-Kemper--2004|Fox-Kemper and Pedlosky, 2004]] ; [[#Berloff--2007|Berloff et al., 2007]] ), it is expected that eddy-present and eddy-rich models will differ in their decadal variability and sensitivity to changes in the wind stress of gyres from eddy-parameterized models ( ''medium confidence'' ). Nonetheless, important aspects of gyre strength depend primarily on forcing and not resolution, allowing long-term changes in gyre strength to be investigated with low-resolution climate models ( [[#Hughes--2001|Hughes and de Cuevas, 2001]] ; [[#Yeager--2015|Yeager, 2015]] ).

Under future scenarios RCP4.5 and RCP8.5, AR5 ( [[#Collins--2013|Collins et al., 2013]] ) assessed an intensification and poleward extension of the southern Hemisphere subtropical gyres in the 21st century. New evidence since AR5 further reinforces their conclusions, which are now extended to all subtropical gyre systems in the Northern and Southern hemispheres ( [[#Yang--2016|Yang et al., 2016]] , 2020). CMIP6 models project changes in WBCs that are consistent with projected changes in the surface winds. Under strong radiative forcing, in scenario SSP5-8.5, CMIP6 models project that the East Australian Current Extension, Agulhas Current Extension and Brazil Current will intensify in the 21st century, while the Gulf Stream will weaken (Figure 9.11). Although CMIP5/CMIP6 are limited in resolution, ''medium confidence'' is given to changes in WBCs due to consistency across generations of climate models, including CMIP6, despite changes in model structure, resolution and parametrizations.

The SROCC ( [[#Collins--2019|Collins et al., 2019]] ) concluded with ''high confidence'' that Indonesian Throughflow (ITF) transport from the Pacific Ocean to the Indian Ocean has increased in the past two decades as a result ( ''medium confidence'' ) of an unprecedented intensification of the equatorial Pacific trade wind system. [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4|Section 2.3.3.4]] assesses that there is ''high confidence'' that the increase in the ITF over the past two decades is linked to multi-decadal scale variability rather than a longer-term trend. Consistently, in the future, as winds change under increased radiative forcing, most models project a decline of the ITF on the centennial time scale (Figure 9.11). One of the clearest changes of ocean current transport simulated by climate models is a weakening of the Indonesian Throughflow, projected in CMIP5 simulations under RCP4.5 and RCP8.5 scenarios ( [[#Sen%20Gupta--2016|Sen Gupta et al., 2016]] ; [[#Stellema--2019|Stellema et al., 2019]] ), and in CMIP6 simulations under the SSP5-8.5 scenario ( ''high confidence'' , Figure 9.11).

The SROCC reports with ''high confidence'' that the Agulhas leakage from the Indian to the Atlantic Ocean has increased in the past two decades ( [[#Collins--2019|Collins et al., 2019]] ), and there is no additional evidence since then allowing this assessment to be revisited ( [[#Biastoch--2015|Biastoch et al., 2015]] ; [[#Loveday--2015|Loveday et al., 2015]] ; [[#Lübbecke--2015|Lübbecke et al., 2015]] ). There is ''low confidence'' in future projections of Agulhas leakage because most CMIP models cannot directly simulate it, due to coarse resolution. However, there is ''medium evidence'' that the strength of the Southern Hemisphere westerlies controls Agulhas leakage ( [[#Durgadoo--2013|Durgadoo et al., 2013]] ; [[#Biastoch--2015|Biastoch et al., 2015]] ; [[#Loveday--2015|Loveday et al., 2015]] ), and ''high confidence'' that the strength of the Southern Hemisphere westerlies will increase under increased radiative forcing, except in lower warming scenarios (SSP1-1.9, SSP1.2-6; [[IPCC:Wg1:Chapter:Chapter-4#4.3.3.1|Section 4.3.3.1]] ; [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). There is also evidence that increasing Agulhas leakage is consistent with observed change of the temperature and salinity structure in the Atlantic ocean, and with variability of the AMOC ( [[#9.2.3.1|Section 9.2.3.1]] ; [[#Biastoch--2015|Biastoch et al., 2015]] ). This range of indirect evidence provides ''medium confidence'' that the Agulhas leakage will increase in the 21st century, except for the strongest mitigation scenario (Figure 9.11).

The SROCC assessed that the annual Bering Strait volume transport from the Pacific to the Arctic Ocean increased from 2001–2014, consistent with an estimated increased northward heat transport of about 60% from 2001–2014, and an increased freshwater transport of 30 ± 20 km <sup>3</sup> yr <sup>–1</sup> from 1991 to 2015 ( [[#Meredith--2019|Meredith et al., 2019]] ). [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4|Section 2.3.3.4]] assesses that volume transport from the Pacific to the Arctic has increased since the 1990s from 0.8 Sv to 1.0 Sv over 1990–2015. Realistic representation of the Bering Strait transport in the current generation of climate models is challenging because the strait is narrow compared to the resolution of climate models ( [[#Clement%20Kinney--2014|Clement Kinney et al., 2014]] ; [[#Aksenov--2016|Aksenov et al., 2016]] ). For the Atlantic to Arctic transport, [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4|Section 2.3.3.4]] reports that the major branches of Atlantic Water inflow across the Greenland–Scotland Ridge have remained stable, with only the smaller pathway of Atlantic Water north of Iceland showing a strengthening trend during 1993–2018. [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.4|Section 2.3.3.4]] also assesses that the Arctic outflow remained stable from the mid-1990s to the mid-2010s. Future changes in these currents have not yet been studied in CMIP6 models.

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