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

==== 9.2.1.3 Upper-ocean Stratification and Surface Mixed Layers ====

<div id="h3-3-siblings" class="h3-siblings"></div>

The density difference from surface to deep ocean is the upper-ocean stratification. The AR5 ( [[#Rhein--2013|Rhein et al., 2013]] ) assessed that it is ''very likely'' that the thermal contribution to stratification over the fixed 0–200 m layer increased by about 1% per decade between 1971 and 2010 (based on linear trend consistently across reports). The SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) found it ''very likely'' that density stratification increased by 0.46–0.51% per decade between 60°S and 60°N from 1970 to 2017). New published estimates based on a variety of different interpolated observations show that SROCC assessed rate is too low, even using the same data and methods ( [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ). The 1960–2018 stratification increase is estimated at 1.2 ± 0.1% per decade from the IAP dataset, 1.2 ± 0.4% per decade from the Ishii product, 0.7 ± 0.5% per decade from the EN4 dataset, 0.9 ± 0.5% per decade from ORAS4, and 1.2 ±0.3% per decade from the National Centers for Environmental Information (NCEI) dataset (G. [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ). The improved methodology for computing stratification change on individual profiles before gridding yields a global annual mean increase of 0–200 m stratification change of 0.8 ± 0.2% per decade between 1960 and 2018 ( [[#Yamaguchi--2019|Yamaguchi and Suga, 2019]] ) and a global summer mean increase of 0–200 m stratification change of 1.3 ± 0.3% per decade between 1970 and 2018 ( [[#Sallée--2021|Sallée et al., 2021]] ) is of a similar magnitude to the long-term trend ( [[#Yamaguchi--2019|Yamaguchi and Suga, 2019]] ; G. [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ). In summary, there is ''limited evidence'' that focusing on changes over a fixed depth range might hide larger increases occurring at the seasonally and regionally variable pycnocline depth. There is also ''limited evidence'' that summer stratification change within the pycnocline has occurred at a rate of 8.9 ± 2.7% per decade from 1970 to 2018, and ''limited evidence'' of a winter pycnocline stratification increase ( [[#Cummins--2020|Cummins and Ross, 2020]] ; [[#Sallée--2021|Sallée et al., 2021]] ).

While AR5 and SROCC did not assess change in mixed-layer depth, the reported changes in stratification can modulate the surface mixed-layer depth, which is set by a balance between fluxes and dynamical mixing (winds, tides, waves, convection) acting against the background stratification and restratification processes (solar and dynamical). Despite the large stratification increase observed at a global scale, new evidence shows that summer mixed-layer depth deepened consistently over the globe at a rate of 2.9 ± 0.5% per decade from 1970 to 2018, with the largest deepening observed in the Southern Ocean, corresponding to overall deepening from 3–15 m per decade depending on region ( [[#Somavilla--2017|Somavilla et al., 2017]] ; [[#Sallée--2021|Sallée et al., 2021]] ). While the shorter observational record in winter (compared to summer) does not allow global winter mixed-layer trends to be reliably assessed ( [[#Sallée--2021|Sallée et al., 2021]] ), winter mixed-layer depths deepening at rates of 10 m per decade have been reported at individual long-term mid-latitude monitoring sites ( [[#Somavilla--2017|Somavilla et al., 2017]] ). Projections agree that shoaling of mixed-layer depth is expected in the 21st century, but only for strong emissions scenarios, and only in some regions (Figure 9.5). In summary, there is ''limited'' observational ''evidence'' that the mixed layer is globally deepening, while models show no emergence of a trend until later in the 21st century under strong emissions.

<div id="_idContainer016" class="Basic-Text-Frame"></div>

[[File:ebaf291d5789321709688b78e3a6575a IPCC_AR6_WGI_Figure_9_5.png]]

'''Figure''' '''9.5 |''' '''Mixed-layer depth in (a–d) winter and (e–h) summer. (a, e)''' Observed climatological mean mixed-layer depth (based on density threshold) from the Argo Mixed Layer Depth Climatology ( [[#Holte--2017|Holte et al., 2017]] ) using observations for 2000–2019. '''(b, f)''' Bias between the observation-based estimate (2000–2019) and the 1995–2014 Coupled Model Intercomparison Project Phase 6 (CMIP6) climatological mean mixed-layer depth. '''(c, d, g, h)''' Projected mixed-layer depth (MLD) change from 1995–2014 to 2081–2100 under '''(c, g)''' SSP1-2.6 and '''(d, h)''' SSP5-8.5 scenarios. The '''(a–d)''' winter row shows December–January–February (DJF) in the Northern Hemisphere and June–July–August (JJA) in the Southern Hemisphere; The '''(e–h)''' summer row shows JJA in the Northern Hemisphere and DJF in the Southern Hemisphere. The mixed-layer depth is the depth where the potential density is 0.03 kg m <sup>–3</sup> denser than at 10 m. 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).

The SROCC assessed that upper-ocean stratification will continue to increase in the 21st century under increased radiative forcing ( ''high confidence'' ), due to increased surface temperature and high-latitude surface freshening ( [[#Bindoff--2019|Bindoff et al., 2019]] ). New climate model simulations concur with SROCC assessment of a future increase of the 0–200 m stratification under increased radiative forcing in all regions of the world ocean ( [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ). In addition, CMIP6 climate models project a shallowing of the mixed-layer in summer and winter by the end of the century under increased radiative forcing (Figure 9.5; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ), with the exception of the Arctic showing deepening of the mixed layer as a result of sea ice retreat (Figure 9.5; [[#Lique--2018|Lique et al., 2018]] ). The regions of largest shallowing are associated with the deepest climatological mixed layer, in both winter and summer, particularly affecting the North Atlantic and the Southern Ocean basins (Figure 9.5). While CMIP6 models tend to project shallowing mixed layers under a warming climate, except at high latitudes (Figure 9.5; [[#Lique--2018|Lique et al., 2018]] ; [[#Kwiatkowski--2020|Kwiatkowski et al., 2020]] ), a deepening in the summer mixed-layer depth by intensification of the surface winds and storms may explain inconsistency among models in many regions (Figure 9.5; [[#Young--2019|Young and Ribal, 2019]] ), although model mixed-layer biases are large in the summer in the Southern Ocean ( [[#Belcher--2012|Belcher et al., 2012]] ; [[#Sallée--2013a|Sallée et al., 2013a]] ; [[#Li--2016|Q. Li et al., 2016]] ; [[#Tsujino--2020|Tsujino et al., 2020]] ). Lack of observed ocean turbulence and climate model limitations do not allow for direct assessment of ocean surface turbulence change and limit confidence in past and future mixed-layer change. Understanding of turbulent processes, their representation in ocean and climate models, and their effect on mixed-layer biases have been an active and rapidly evolving topic of research since AR5 ( [[#Buckingham--2019|Buckingham et al., 2019]] ; Q. [[#Li--2019|]] [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] ). Small-scale mixed-layer processes are not resolved in climate models ( [[#D’Asaro--2014|D’Asaro, 2014]] ; [[#Buckingham--2019|Buckingham et al., 2019]] ; [[#McWilliams--2019|McWilliams, 2019]] ) and despite significant improvements in their parametrization over the last decade ( [[#Fox-Kemper--2011|Fox-Kemper et al., 2011]] ; [[#Jochum--2013|Jochum et al., 2013]] ; [[#Li--2016|Q. Li et al., 2016]] , 2019; [[#Qiao--2016|Qiao et al., 2016]] ) and significant improvement in some models ( [[#Li--2017|Li and Fox-Kemper, 2017]] ; [[#Dunne--2020|Dunne et al., 2020]] ), biases in mixed-layer representation generally persist ( [[#Heuzé--2017|Heuzé, 2017]] ; [[#Williams--2018|Williams et al., 2018]] ; [[#Cherchi--2019|Cherchi et al., 2019]] ; [[#Golaz--2019|Golaz et al., 2019]] ; [[#Voldoire--2019|Voldoire et al., 2019]] ; [[#Yukimoto--2019|Yukimoto et al., 2019]] ; [[#Boucher--2020|Boucher et al., 2020]] ; [[#Danabasoglu--2020|Danabasoglu et al., 2020]] ; [[#Dunne--2020|Dunne et al., 2020]] ; [[#Kelley--2020|Kelley et al., 2020]] ). In summary, the representation of upper-ocean stratification and mixed layers has improved in CMIP6 compared to CMIP5. While it is ''virtually certain'' that the global mean upper ocean will continue to stratify in the 21st century, there is only ''low confidence'' in the future evolution of mixed-layer depth, which is projected to mostly shoal under high emissions, except in high-latitude regions where sea ice retreats.

<div id="box-9.2" class="h2-container box-container"></div>

'''Box 9.2 | Marine Heatwaves'''

<div id="h2-11-siblings" class="h2-siblings"></div>

Marine heatwaves (MHW) are periods of extreme high sea temperature relative to the long-term mean seasonal cycle ( [[#Hobday--2016|Hobday et al., 2016]] ). Studies since the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#Collins--2019|Collins et al., 2019]] ) confirm the assessment that MHW can lead to severe and persistent impacts on marine ecosystems – from mass mortality of benthic communities, including coral bleaching, changes in phytoplankton blooms, shifts in species composition and geographical distribution, and toxic algal blooms, to decline in fisheries catch and mariculture ( [[#Smale--2019|Smale et al., 2019]] ; [[#Cheung--2020|Cheung and Frölicher, 2020]] ; [[#Hayashida--2020|Hayashida et al., 2020]] ; [[#Piatt--2020|Piatt et al., 2020]] ). Unlike synoptic atmospheric heatwaves [[IPCC:Wg1:Chapter:Chapter-11#11.3|Section 11.3]] ), MHWs can extend for millions of square kilometres, persist for weeks to months, and occur at subsurface ( [[#Bond--2015|Bond et al., 2015]] ; [[#Schaeffer--2017|Schaeffer and Roughan, 2017]] ; [[#Perkins-Kirkpatrick--2019|Perkins-Kirkpatrick et al., 2019]] ; [[#Laufkötter--2020|Laufkötter et al., 2020]] ).

The SROCC established that MHWs have occurred in all basins over the last decades. Additional evidence documenting widespread occurrence of marine heat waves in all basins and marginal seas continues to accumulate (Y. [[#Li--2019|]] [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] ; [[#Yao--2020|Yao et al., 2020]] ). The SROCC highlighted the role of large-scale climate modes of variability in amplifying or suppressing MHW occurrences, which has since been further corroborated, increasing confidence in climate modes as important drivers of MHWs ( [[#Holbrook--2019|Holbrook et al., 2019]] ; [[#Sen%20Gupta--2020|Sen Gupta et al., 2020]] ). More generally, understanding of processes leading to MHWs has increased since SROCC, including air–sea heat flux [[#9.2.1.2|Section 9.2.1.2]] ), increased horizontal heat advection, shoaling of the mixed-layer and suppressed mixing processes [[#9.2.1.3|Section 9.2.1.3]] ), reduced coastal upwelling and Ekman pumping [[#9.2.3.5|Section 9.2.3.5]] ), changes in eddy activities and planetary waves, and the re-emergence of warm subsurface anomalies ( [[#Holbrook--2020|Holbrook et al., 2020]] ; [[#Sen%20Gupta--2020|Sen Gupta et al., 2020]] ).

The SROCC reported with ''high confidence'' that MHWs – defined as days exceeding the 99th percentile in sea surface temperature (SST) from 1982 to 2016 – have ''very likely'' doubled in frequency between 1982 and 2016. Additional observation-based evidence and acquisition of longer observation time series since SROCC have confirmed and expanded on this assessment: since the 1980s MHWs have also become more intense and longer ( [[#Frölicher--2018|Frölicher and Laufkötter, 2018]] ; [[#Smale--2019|Smale et al., 2019]] ; [[#Laufkötter--2020|Laufkötter et al., 2020]] ). Satellite observations and reanalyses of SST show an increase in intensity of 0.04°C per decade from 1982 to 2016, an increase in spatial extent of 19% per decade from 1982 to 2016, and an increase in annual MHW days of 54% between the 1987–2016 period compared to 1925–1954 ( [[#Frölicher--2018|Frölicher et al., 2018]] ; [[#Oliver--2019|Oliver, 2019]] ). The SROCC assessed that 84–90% of all MHWs that occurred between 2006 and 2015 are ''very likely'' caused by anthropogenic warming. There is new evidence since SROCC that the frequency of the most impactful marine heatwaves over the last few decades has increased more than 20-fold because of anthropogenic global warming ( [[#Laufkötter--2020|Laufkötter et al., 2020]] ). In summary, there is ''high confidence'' that MHWs have increased in frequency over the 20th century, with an approximate doubling from 1982 to 2016, and ''medium confidence'' that they have become more intense and longer since the 1980s.

Consistent with SROCC, future MHWs are defined with reference to the historical climate conditions. The SROCC assessed that MHWs will ''very likely'' further increase in frequency, duration, spatial extent and intensity under future global warming in the 21st century. The CMIP6 projections allow us to confirm this assessment and quantify future change based on global mean probability ratio change (Box 9.2, Figure 1): they project MHWs will become four times (5–95% range: 2–9 times] more frequent in 2081–2100 compared to 1995–2014 under SSP1-2.6, or eight times (3–15 times) more frequent under SSP5-8.5. The SROCC highlighted that future change of MHWs will not be globally uniform, with the largest changes in the frequency of marine heatwaves being projected to occur in the western tropical Pacific and the Arctic Ocean ( ''medium confidence'' ). New evidence from the latest generation of climate models confirms and complements SROCC assessment (Box 9.2, Figure 1). Moderate increases are projected for mid-latitudes, and only small increases are projected for the Southern Ocean ( ''medium confidence'' ) ( [[#Hayashida--2020|Hayashida et al., 2020]] ). While under the SSP5-8.5 scenario, permanent MHWs (more than 360 days per year) are projected to occur in the 21st century in parts of the tropical ocean, the Arctic Ocean and around 45°S, the occurrence of such permanent MHWs can largely be avoided under the SSP1-2.6 scenario ( [[#Frölicher--2018|Frölicher et al., 2018]] ; [[#Oliver--2019|Oliver et al., 2019]] ; [[#Plecha--2020|Plecha and Soares, 2020]] ). The resolution of current climate models (CMIP5 and CMIP6) capture the broad features of MHWs, but they may have a bias towards weaker and longer MHWs in the historical period ( ''medium confidence'' ) ( [[#Frölicher--2018|Frölicher et al., 2018]] ; [[#Pilo--2019|Pilo et al., 2019]] ; [[#Plecha--2020|Plecha and Soares, 2020]] ) and greater intensification in western boundary current regions ( [[#Hayashida--2020|Hayashida et al., 2020]] ).

<div id="_idContainer018" class="Basic-Text-Frame"></div>

Box 9.2

[[File:4de4e488565e4e63a395c016fcb1136e IPCC_AR6_WGI_Box_9_2_Figure_1.png]]

'''Box 9.2, Figur'''

'''e 1 |'''

'''Observed and simulated regional probability ratio of marine heatwaves (MHWs) for the 198''' '''5–2''' '''014 period and for the end of the 21st century under two different greenhouse gas emissions scenarios.''' The probability ratio is the proportion by which the number of MHW days per year has increased relative to pre-industrial times. An MHW is defined as a deviation beyond the daily 99th percentile (11-day window) in the deseasonalized sea surface temperature. '''(a)''' The MHW probability ratio from satellite observations (NOAA OISST V2.1; Huang et al. 2020) during 1985–2014. The mean warming pattern (difference in ERSST5 (Huang et al. 2017) sea surface temperature between the 1985–2014 and 1854–1900 periods) has been added to the satellite observations to calculate the probability ratio. '''(b–d)''' Coupled Model Intercomparison Project Phase 6 (CMIP6) simulated multi-model mean probability ratio of the '''(b)''' 1985–2014 period, and 2081–2100 period in the '''(c)''' SSP1 2.6 and '''(d)''' SSP5 8.5 scenarios. The areas with grey diagonal lines in (d) indicate permanent MHWs (&gt;360 heatwave days per year). These 14 CMIP6 models are included in the analysis: ACCESS-CM2, CESM2, CESM2-WACCM, CMCCCM2-SR5, CNRM-CM6-1, CNRM-ESM2-1, CanESM5, EC-Earth3, IPSL-CM6A-LR, MIROC6, MRI-ESM2-0, NESM3, NorESM2-LM, NorESM2-MM. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

<div id="9.2.2" class="h2-container"></div>

<span id="changes-in-heat-and-salinity"></span>