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

==== 9.2.3.5 Eastern Boundary Upwelling Systems ====

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Eastern boundary upwelling systems (EBUS) exist where trade winds draw cold and generally low-pH/low-oxygen waters upward. Coastal upwelling plays a key role in supplying the food chain with nutrients, hence the richness and productivity of EBUS ( [[#Bindoff--2019|Bindoff et al., 2019]] ). The SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) assessed with ''high confidence'' that three out of the four major EBUS have experienced large-scale wind intensification in the past 60 years (only the trend for the Canary Current is considered uncertain). However, it also emphasized that various processes can also modulate, or even reverse, wind trends locally ( [[#Bindoff--2019|Bindoff et al., 2019]] ). Here we revisit SROCC assessment ( [[#Bindoff--2019|Bindoff et al., 2019]] ) based on evidence showing ''low agreement'' between studies that have investigated trends over past decadess of upwelling-favourable winds ( [[#Varela--2015|Varela et al., 2015]] ). This ''low agreement'' has been related to differences in wind products, season of interest, and length of the considered time series ( [[#Varela--2015|Varela et al., 2015]] ). Based on this, we assess that only the California Current system has experienced large-scale upwelling-favorable wind intensification over the period 1982–2010, albeit with regional differences ( [[#García-Reyes--2010|García-Reyes and Largier, 2010]] ; [[#Seo--2012|Seo et al., 2012]] ). In the Benguela, Canary, and Humboldt systems, large-scale, upwelling-favourable wind trends are ambiguous, owing to ''low confidence'' in long-term in situ marine wind data ( [[#Cardone--1990|Cardone et al., 1990]] ; [[#Bakun--2010|Bakun et al., 2010]] ) and ''low agreement'' among available studies ( [[#Narayan--2010|Narayan et al., 2010]] ; [[#Sydeman--2014|Sydeman et al., 2014]] ; [[#Varela--2015|Varela et al., 2015]] ). Our assessment confirms SROCC assessment ( [[#Bindoff--2019|Bindoff et al., 2019]] ) in that high natural variability of EBUS and their inadequate representation by most climate models gives ''low confidence'' in attribution of observed changes, while anthropogenic changes are projected to emerge primarily in the second half of the 21st century ( ''limited evidence'' : one model and one study) ( [[#Brady--2017|Brady et al., 2017]] ).

Under increased radiative forcing, SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) assessed that climate models project, in the 21st century, a reduction of wind and upwelling intensity in EBUS at low latitudes, and enhancement at high latitudes, under scenario RCP8.5, with an overall reduction in either upwelling intensity or extension. It also highlighted that coastal warming and wind intensification may lead to variable countervailing responses to upwelling intensification at local scales. Despite differences among EBUS (D. [[#Wang--2015|]] [[#Wang--2015|Wang et al., 2015]] ), there is growing evidence since SROCC in this pattern of change. While it has long been hypothesized that, for upwelling winds, change is linked to air temperature contrast between ocean and land ( [[#Bakun--1990|Bakun, 1990]] ), this hypothesis has increasingly been challenged. Changes in sea level pressure and wind fields in EBUS appear to be primarily tied to those affecting subtropical highs ( [[#García-Reyes--2013|García-Reyes et al., 2013]] ). Poleward expansion of the Hadley cell ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.1|Section 2.3.1.4.1]] ; [[#Staten--2018|Staten et al., 2018]] ) and the related poleward migration of subtropical highs ( [[#He--2017|He et al., 2017]] ; [[#Cherchi--2018|Cherchi et al., 2018]] ), produce robust patterns of changes of reduced upwelling at low latitude and enhanced upwelling at high latitude ( [[#Echevin--2012|Echevin et al., 2012]] ; [[#Belmadani--2014|Belmadani et al., 2014]] ; [[#Bettencourt--2015|Bettencourt et al., 2015]] ; [[#Rykaczewski--2015|Rykaczewski et al., 2015]] ; [[#Sousa--2017|Sousa et al., 2017]] ; [[#Lamont--2018|Lamont et al., 2018]] ; [[#Sylla--2019|Sylla et al., 2019]] ). These patterns are most apparent in summer in both hemispheres. Synoptic variability of upwelling winds, important to the functioning of upwelling ecosystems ( [[#García-Reyes--2014|García-]] [[#Reyes--2014|Reyes et al., 2014]] ), may also be affected by climate change ( [[#Aguirre--2019|Aguirre et al., 2019]] ). However, coarse resolution model projections of winds in upwelling regions may be more consistent than higher-resolution projections, as these regions are highly sensitive to resolution ( [[#Small--2015|Small et al., 2015]] ).

Projected future annual cumulative upwelling wind changes at most locations, and seasons remain within ±10–20% of present-day values in the 21st century, even in the context of high-end emissions scenarios (4×CO <sub>2</sub> or RCP8.5) ( ''medium confidence'' ). Changes due to wind stress curl and alongshore pressure gradients tend to agree with alongshore wind changes ( [[#Oerder--2015|Oerder et al., 2015]] ; [[#Sylla--2019|Sylla et al., 2019]] ). Direct estimation of oceanic upward transport ( [[#Oyarzún--2019|Oyarzún and Brierley, 2019]] ; [[#Sylla--2019|Sylla et al., 2019]] ) and nutrient flux into the euphotic layer ( [[#Jacox--2018|Jacox et al., 2018]] ) provide a meaningful estimator of upwelling, integrating all relevant processes, including changes in wind stress curl. However, there is ''limited evidence'' from vertical velocity of climate models and missing processes in coarse-resolution climate models that presently limit this approach. Change in upper-ocean stratification ( [[#9.2.1.3|Section 9.2.1.3]] ) is projected to increase confinement of upwelling vertical velocities to near the ocean surface ( ''high confidence'' ) ( [[#Oerder--2015|Oerder et al., 2015]] ; [[#Oyarzún--2019|Oyarzún and Brierley, 2019]] ).

In summary, SROCC and this Report conclude that the California Current system has experienced some upwelling-favourable wind intensification since the 1980s ( ''high confidence'' ), while ''low agreement'' among reported wind changes in the Benguela, Canary, and Humboldt systems prevent a similar assessment. As in SROCC, there is ''low confidence'' in attribution of observed changes to anthropogenic or natural causes. New evidence reinforces our confidence in SROCC assessment that, under increased radiative forcing, EBUS winds will change with a dipole spatial pattern within each EBUS of reduction (weaker and/or shorter) at low latitude, and enhancement (stronger and/or longer) at high latitude ( ''high confidence'' ). There is ''medium confidence'' that, across all scenarios, upwelling wind changes in EBUS will remain moderate in the 21st century, within ±10–20% from present-day values.

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