Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.3.4.2 Western boundary currents and inter-basin exchanges =====

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Both AR5 and SROCC reported that western boundary currents (WBCs) have undergone an intensification, warming and poleward expansion, except for the Gulf Stream and the Kuroshio, but did not provide confidence statements. The AR5 reported with ''medium'' to ''high confidence'' intensification of the North Pacific subpolar gyre, the South Pacific subtropical gyre, and the subtropical cells, along with an expansion of the North Pacific subtropical gyre since the 1990s. It was pointed out that these changes are ''likely'' predominantly due to interannual-to-decadal variability, and in the case of the subtropical cells represent a reversal of earlier multi-decadal changes. SROCC concluded that it was ''unlikely'' that there has been a statistically significant net southward movement of the mean Antarctic Circumpolar Current (ACC) position over the past 20 years, in contrast to AR5, where this change had been assessed with ''medium confidenc'' e.

The intensity of the Kuroshio current system in the north-west Pacific varied in conjunction with the glaciation cycles over the last 1 Myr, with some limited glacial-interglacial variability in position ( [[#Jian--2000|Jian et al., 2000]] ; [[#Gallagher--2015|Gallagher et al., 2015]] ). The Agulhas current has strengthened substantially during the warming associated with deglaciations of the past 1 Myr ( [[#Peeters--2004|Peeters et al., 2004]] ; [[#Bard--2009|Bard and Rickaby, 2009]] ; [[#Martínez-Méndez--2010|Martínez-Méndez et al., 2010]] ; [[#Marino--2013|Marino et al., 2013]] ; [[#Ballalai--2019|Ballalai et al., 2019]] ). According to sediment core analyses, the Agulhas leakage varied by about 10 Sv during major climatic transitions over the past 640 kyr ( [[#Caley--2014|Caley et al., 2014]] ). Available data suggests that there was relatively little change in the net flow of the ACC during the LGM, with no consensus on the sign of changes ( [[#McCave--2013|McCave et al., 2013]] ; [[#Lamy--2015|Lamy et al., 2015]] ; [[#Lynch-Stieglitz--2016|Lynch-Stieglitz et al., 2016]] ), except at one location at the northern edge of the Drake Passage where a 40% decrease of transport had been reported ( [[#Lamy--2015|Lamy et al., 2015]] ). Longer time series from the northern entrance to Drake Passage suggest a consistent transport variability of 6–16% through glacial climate cycles, with higher current speeds during interglacial times and reduced current speeds during glacial intervals ( [[#Toyos--2020|Toyos et al., 2020]] ). Inferred variability in the size and strength of the North Atlantic subpolar gyre was substantial, and included rapid changes on millennial time scales during both interglacial and glacial intervals over the last 150 kyr ( [[#Born--2010|Born and Levermann, 2010]] ; [[#Mokeddem--2014|Mokeddem et al., 2014]] ; [[#Irvalı--2016|Irvalı et al., 2016]] ; [[#Mokeddem--2016|Mokeddem and McManus, 2016]] ). North Atlantic – Arctic exchange has also varied in the past, with indications of an increasing inflow of Atlantic waters into the Arctic during the late Holocene ( [[#Ślubowska--2005|Ślubowska et al., 2005]] ) with an acceleration to the recent inflow that is now the largest of the past 2 kyr ( [[#Spielhagen--2011|Spielhagen et al., 2011]] ).

A latitudinal shift of subtropical/subpolar gyres on the order of 0.1 ± 0.04° per decade is derived by an indirect method using remote sensing data during 1993–2018 ( [[#Yang--2020|Yang et al., 2020]] ). Direct observations show a systematic poleward migration of WBCs ( [[#Wu--2012|Wu et al., 2012]] ; [[#Yang--2016|Yang et al., 2016]] , 2020; [[#Bisagni--2017|Bisagni et al., 2017]] ). However, they do not support an intensification of WBCs, with a weakening, broadening, or little change reported for the Kuroshio (Y.-L. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ; [[#Wang--2018|Wang and Wu, 2018]] ; [[#Collins--2019|Collins et al., 2019]] ), Gulf Stream ( [[#McCarthy--2018|McCarthy et al., 2018]] ; [[#Collins--2019|Collins et al., 2019]] ; [[#Dong--2019|Dong et al., 2019]] ; [[#Andres--2020|Andres et al., 2020]] ), Agulhas ( [[#Beal--2016|Beal and Elipot, 2016]] ; [[#Elipot--2018|Elipot and Beal, 2018]] ) and East Australian ( [[#Sloyan--2015|Sloyan and O’Kane, 2015]] ) currents. The Gulf Stream has recently reversed a long-term poleward migration ( [[#Bisagni--2017|Bisagni et al., 2017]] ). Multidecadal variability of the strength and position of WBCs ( [[#Hsin--2015|Hsin, 2015]] ; [[#Bisagni--2017|Bisagni et al., 2017]] ; [[#McCarthy--2018|McCarthy et al., 2018]] ) and short records from direct observations obscure the detection of any long-term trends ( [[#Yang--2020|Yang et al., 2020]] ).

The Pacific to Arctic exchange at the Bering Strait plays a minor role in the total Arctic exchange with the global ocean, which has increased from 0.8 Sv to 1.0 Sv over 1990–2015 ( [[#Woodgate--2018|Woodgate, 2018]] ). For Atlantic-Arctic exchange, major branches of Atlantic Water inflow from the North Atlantic into the Arctic across the Greenland–Scotland Ridge have remained stable since the mid-1990s ( [[#Berx--2013|Berx et al., 2013]] ; [[#Hansen--2015|Hansen et al., 2015]] ; [[#Jochumsen--2017|Jochumsen et al., 2017]] ; [[#Østerhus--2019|Østerhus et al., 2019]] ), with only the smaller pathway of Atlantic Water north of Iceland showing a strengthening trend during 1993–2018 ( [[#Casanova-Masjoan--2020|Casanova-Masjoan et al., 2020]] ), but with associated heat transport strengthening through the 1990s ( [[#Rossby--2020|Rossby et al., 2020]] ; [[#Tsubouchi--2021|Tsubouchi et al., 2021]] ). The Arctic outflow remained broadly stable from the mid-1990s to the mid 2010s ( [[#Østerhus--2019|Østerhus et al., 2019]] ).

The heat and mass transport of the Indonesian throughflow (ITF) shows substantial variability at seasonal to decadal time scales ( [[#Zhuang--2013|Zhuang et al., 2013]] ; Q.-Y. [[#Liu--2015|]] [[#Liu--2015|Liu et al., 2015]] ; [[#Susanto--2015|Susanto and Song, 2015]] ; [[#Feng--2017|Feng et al., 2017]] , 2018; M. [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ; [[#Sprintall--2019|Sprintall et al., 2019]] ; [[#Xie--2019|Xie et al., 2019]] ). Q.-Y. [[#Liu--2015|]] [[#Liu--2015|Liu et al. (2015)]] reported an increasing trend in the ITF geostrophic transport of 1 Sv per decade over 1984–2013, consistent with direct estimates ( [[#Sprintall--2014|Sprintall et al., 2014]] ), and results from reanalyses (M. [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ), and this appears to be linked to multi-decadal scale variability rather than a long-term trend ( [[#Kosaka--2013|Kosaka and Xie, 2013]] ; [[#England--2014|England et al., 2014]] ; [[#Lee--2015|Lee et al., 2015]] ).

Southern Ocean circulation changes are assessed in SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ), and are confirmed and synthesized in Section 9.2.3.2 which shows that there is no indication of ACC transport change, and that it is ''unlikely'' that the mean meridional position of the ACC has moved southward in recent decades.

In summary, over the past 3–4 decades, the WBC strength is highly variable ( ''high confidence'' ), and WBCs and subtropical gyres have shifted poleward since 1993 ( ''medium confidence'' ). Net Arctic Ocean volume exchanges with the other ocean basins remained stable over the mid-1990s to the mid-2010s ( ''high confidence'' ). There is ''high confidence'' that the ITF shows strong multi-decadal scale variability since the 1980s.

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