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

==== 3.2.2.3 Changes in Ocean Circulation, Stratification and Coastal Upwelling ====

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Ocean circulation and its variations are key to the evolution of the physical, chemical and biological properties of the ocean. Vertical mixing and upwelling are critical factors affecting the supply of nutrients to the sunlit ocean and hence the magnitude of primary productivity. Ocean currents not only transport heat, salt, carbon and nutrients, but they also control the dispersion of many organisms and the connectivity between distant populations.

Ocean stratification is an important factor controlling biogeochemical cycles and affecting marine ecosystems. WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.3]] ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ) assessed that it is ''virtually certain'' that stratification in the upper 200 m of the ocean has been increasing since 1970. Recent evidence has strengthened estimates of the rate of change ( [[#Yamaguchi--2019|Yamaguchi and Suga, 2019]] ; [[#Li--2020a|Li et al., 2020a]] ; [[#Sallée--2021|Sallée et al., 2021]] ), with an estimated increase of 1.0 ± 0.3% ( ''very likely'' range) per decade over the period 1970–2018 ( ''high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.3]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), higher than assessed in SROCC. It is ''very likely'' that stratification in the upper few hundred metres of the ocean will increase substantially in the 21st century in all ocean basins, driven by intensified surface warming and near-surface freshening at high latitudes (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.3]] ; [[#Capotondi--2012|Capotondi et al., 2012]] ; [[#Fu--2016|Fu et al., 2016]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

Contrasting changes among the major eastern boundary coastal upwelling systems (EBUS) were identified in AR5 ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ). While SROCC assessed with ''high confidence'' that three (Benguela, Peru-Humboldt, California) out of the four major EBUS have experienced upwelling-favourable wind intensification in the past 60 years ( [[#Sydeman--2014|Sydeman et al., 2014]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ), WGI AR6 revisited this assessment based on evidence showing ''low agreement'' between studies that have investigated trends over past decades ( [[#Varela--2015|Varela et al., 2015]] ). WGI AR6 assessed that only the California Current system has undergone large-scale upwelling-favourable wind intensification since the 1980s ( ''medium confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.1.5]] ; [[#García-Reyes--2010|García-Reyes and Largier, 2010]] ; [[#Seo--2012|Seo et al., 2012]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

While no consistent pattern of contemporary changes in upwelling-favourable winds emerges from observation-based studies, numerical and theoretical work projects that summertime winds near poleward boundaries of upwelling zones will intensify, while winds near equatorward boundaries will weaken ( ''high confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.3.5]] ; [[#García-Reyes--2015|García-Reyes et al., 2015]] ; [[#Rykaczewski--2015|Rykaczewski et al., 2015]] ; [[#Wang--2015|Wang et al., 2015]] ; [[#Aguirre--2019|Aguirre et al., 2019]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Nevertheless, projected future annual cumulative upwelling wind changes at most locations and seasons remain within ±10–20% of present-day values ( ''medium confidence'' ) (WGI AR6 [[IPCC:Wg2:Chapter:Chapter-9#9.2|Section 9.2.3.5]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ).

Continuous observation of the Atlantic meridional overturning circulation (AMOC) has improved the understanding of its variability ( [[#Frajka-Williams--2019|Frajka-Williams et al., 2019]] ), but there is ''low confidence'' in the quantification of AMOC changes in the 20th century because of ''low agreement'' in quantitative reconstructed and simulated trends (WGI AR6 Sections 2.3.3, 9.2.3.1; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). Direct observational records since the mid-2000s remain too short to determine the relative contributions of internal variability, natural forcing and anthropogenic forcing to AMOC change ( ''high confidence'' ) (WGI AR6 Sections 2.3.3, 9.2.3.1; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). Over the 21st century, AMOC will ''very likely'' decline for all SSP scenarios but will not involve an abrupt collapse before 2100 (WGI AR6 Sections 4.3.2, 9.2.3.1; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Lee--2021|Lee et al., 2021]] ).

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