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

=== 3.5.2 Ocean Salinity ===

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While ocean assessments have primarily focused on temperature changes, improved observational salinity products since the early 2000s have supported more assessment of long-term ocean salinity change and variability from AR4 ( [[#Bindoff--2007|Bindoff et al., 2007]] ) to AR5 across both models and observations ( [[#Flato--2013|Flato et al., 2013]] ; [[#Rhein--2013|Rhein et al., 2013]] ). The AR5 assessed that it was ''very likely'' that anthropogenic forcings have made a discernible contribution to surface and subsurface ocean salinity changes since the 1960s. The SROCC augmented these insights, noting that observed high latitude freshening and warming have ''very likely'' made the surface ocean less dense with a stratification increase of between 2.18% and 2.42% from 1970 to 2017 ( [[#Bindoff--2019|Bindoff et al., 2019]] ). A recent observational analysis has expanded on these assessments, suggesting a very marked summertime density contrast enhancement across the mixed layer base of 6.2–11.6% per decade, driven by changes in temperature and salinity, which is more than six times larger than previous estimates ( [[#Sallée--2021|Sallée et al., 2021]] ). An idealized ocean modelling study suggests that the enhanced stratification can account for a third of the salinity enhancement signal since 1990 ( [[#Zika--2018|Zika et al., 2018]] ). Thus, there has been an expansion of observed global- and basin-scale salinity change assessment literature since AR5, with many new studies reproducing the key patterns of long-term salinity change reported in AR5 ( [[#Rhein--2013|Rhein et al., 2013]] ), and linking these through modelling studies to coincident changes in evaporation–precipitation patterns at the ocean surface (Sections 2.3.1.3, 3.3.2, 8.2.2.1 and 9.2.2).

Unlike SSTs, simulated sea surface salinity (SSS) does not provide a direct feedback to the atmosphere. However, some recent work has identified indirect radiative feedbacks through sea-salt aerosol interactions ( [[#Ayash--2008|Ayash et al., 2008]] ; [[#Amiri-Farahani--2019|Amiri-Farahani et al., 2019]] ; [[#Wang--2019|]] [[#Wang--2019|Z. Wang et al., 2019]] ) that can act to strengthen tropical cyclones, and increase precipitation ( [[#Balaguru--2012|Balaguru et al., 2012]] , [[#Balaguru--2016|2016]] ; [[#Grodsky--2012|Grodsky et al., 2012]] ; [[#Reul--2014|Reul et al., 2014]] ; [[#Jiang--2019|Jiang et al., 2019]] ). The absence of a direct feedback is one of the primary reasons why salinity simulation is difficult to constrain in ocean modelling systems, and why deviations from the observed near-surface salinity mean state between models and observations are often apparent ( [[#Durack--2012|Durack et al., 2012]] ; [[#Shi--2017|Shi et al., 2017]] ).

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==== 3.5.2.1 Sea Surface and Depth-profile Salinity Evaluation ====

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When compared to the routine assessment of simulated SST, simulated SSS has not received the same research attention at global- to basin-scales. For CMIP3, there was reasonable agreement between the basin-scale patterns of salinity, with a comparatively fresher Pacific when contrasted to the salty Atlantic, and basin salinity maxima features aligning well with the corresponding atmospheric evaporation minus precipitation field ( [[#Durack--2012|Durack et al., 2012]] ). Similar features are also reproduced in CMIP5 along with realistic variability in the upper layers, but less variability than observations at 300 m and deeper, especially in the poorly sampled Antarctic region ( [[#Pierce--2012|Pierce et al., 2012]] ). In a regional study, only considering the Indian Ocean, CMIP5 SSS was assessed and it was shown that model biases were primarily linked to biases in the precipitation field, with ocean circulation biases playing a secondary role ( [[#Fathrio--2017a|Fathrio et al., 2017a]] ). The sea surface salinity bias in CMIP6 models is shown in Figure 3.23b.

For the first time in AR5, alongside global zonal mean temperature, global zonal mean salinity bias with depth was assessed for the CMIP5 models. This showed a strong upper ocean (&lt;300 m) negative salinity (fresh) bias of order 0.3 PSS-78, with a tendency toward a positive salinity (salty) bias (&lt;0.25 PSS-78) in the Northern Hemisphere intermediate layers (200–3000 m) ( [[#Flato--2013|Flato et al., 2013]] ). These biases are also present in CMIP6, albeit with slightly smaller magnitudes (Figure 3.25). Here we expand the global zonal mean bias assessment to consider the three independent ocean basins individually, which allows for an assessment as to which basin biases are dominating the global zonal mean. The basin with the most pronounced biases is the Atlantic, with a strong upper ocean (&lt;300 m) fresh bias, of order 0.3 PSS-78 just like the global zonal mean, and a marked subsurface salinity bias that exceeds 0.5 PSS-78 in equatorial waters between 400–1000 m.

The Pacific Ocean shares the strongest similarity to the global bias, with a similar upper ocean (&lt;300 m) fresh bias. Lower magnitude positive salinity biases (about 0.3 PSS-78) are also present in both hemispheres between 200 and 3000 m, and deeper in the Southern Hemisphere (Figure 3.25). The Indian Ocean shows similar features to the Southern Hemisphere Pacific, with a marked upper ocean (&lt;500 m) fresh bias of order 0.3 PSS-78, and a strong near-surface positive bias of order 0.4 PSS-78 associated with the Arabian Sea (Figure 3.25).

For the Southern Ocean in CMIP5, considerable fresh biases exist through the water column, and are most pronounced in the ventilated layers representing the subtropical mode and intermediate water masses ( [[#Sallée--2013|Sallée et al., 2013]] ). A fresh bias in upper and intermediate layers of comparable magnitude is also seen in CMIP6 (Figure 3.25). The structure of the biases in the CMIP6 multi-model mean (which averages across many simulations with differing subsurface geographies and differing Southern Ocean salinity biases ( [[#Beadling--2020|Beadling et al., 2020]] )) is similar to that evident in the CMIP5 multi-model mean, but with slightly smaller magnitudes. The Arctic Ocean also on average exhibits a surface-enhanced fresh bias in the upper ocean (Figure 3.25), which is much larger than its Southern Hemisphere counterpart.

In summary, the structure of the salinity biases in the multi-model mean has not changed substantially between CMIP5 and CMIP6 ( ''medium confidence'' ), though there is ''limited evidence'' that the magnitude of subsurface biases has been reduced. Biases are sufficiently small to provide confidence in the utility of CMIP-class models for detection and attribution of ocean salinity.

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==== 3.5.2.2 Salinity Change Attribution ====

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AR5 concluded that it was ''very likely'' that anthropogenic forcings had made a discernible contribution to surface and subsurface ocean salinity changes since the 1960s ( [[#Bindoff--2013|Bindoff et al., 2013]] ; [[#Rhein--2013|Rhein et al., 2013]] ). It highlighted that the spatial patterns of salinity trends, and the mean fields of salinity and evaporation minus precipitation are all similar, with an enhancement to Atlantic Ocean salinity and freshening in the Pacific and Southern Oceans. Since AR5 all subsequent work on assessing observed and modelled salinity changes has confirmed these results.

Considerable changes to observed broad- or basin-scale ocean near-surface salinity fields have been reported (see [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] ), and these have been linked to changes in the evaporation minus precipitation patterns at the ocean surface through model simulations, typically expressing a pattern of change where climatological mean fresh regions become fresher and corresponding salty regions becoming saltier ( [[#Durack--2012|Durack et al., 2012]] , [[#Durack--2013|2013]] ; [[#Zika--2015|Zika et al., 2015]] ; [[#Lago--2016|Lago et al., 2016]] ; [[#Skliris--2016|Skliris et al., 2016]] , [[#Skliris--2018|2018]] ; [[#Cheng--2020|Cheng et al., 2020]] ), also broadly present in the CMIP6 multi-model mean (Figure 3.27). At basin-scales, the depth-integrated effect of mean salinity changes as captured in halosteric sea level for the top 0 to 2000 m has also been assessed based on observational products, and these results mirror near-surface patterns in the CMIP5 and CMIP6 models, with most areas that are becoming fresher at the surface exhibiting increases in halosteric sea level, and areas becoming saltier exhibiting decreases ( [[#Durack--2014a|Durack et al., 2014a]] ; Figure 3.28). Further investigations using observations and models together have tied the long-term patterns of surface and subsurface salinity changes to coincident changes to the evaporation minus precipitation field over the ocean ( [[#Durack--2012|Durack et al., 2012]] , [[#Durack--2013|2013]] ; [[#Durack--2015|Durack, 2015]] ; [[#Levang--2015|Levang and Schmitt, 2015]] ; [[#Zika--2015|Zika et al., 2015]] , [[#Zika--2018|2018]] ; [[#Grist--2016|Grist et al., 2016]] ; [[#Lago--2016|Lago et al., 2016]] ; [[#Cheng--2020|Cheng et al., 2020]] ), however the rate of these changes through time continues to be an active area of active research ( [[#Skliris--2014|Skliris et al., 2014]] ; [[#Zika--2015|Zika et al., 2015]] , 2018; [[#Cheng--2020|Cheng et al., 2020]] ; [[#Sallée--2021|Sallée et al., 2021]] ).

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[[File:d595a2eb3fab5e2e6b766e28c0048d42 IPCC_AR6_WGI_Figure_3_27.png]]

Figure 3.27 | '''Maps of multi-decadal salinity trends for the near-surface''' '''ocean.''' Units are Practical Salinity Scale 1978 [PSS-78] per decade. '''(Top)''' The best estimate ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] ) observed trend (1950 – 2019, [[#Durack--2010|Durack and Wijffels, 2010]] ). '''(Bottom)''' Simulated trend from the CMIP6 historical experiment multi-model mean (1950–2014). Black contours show the climatological mean salinity in increments of 0.5 PSS-78 (thick lines 1 PSS-78). Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

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[[File:802053c0a35a550a20c5b3cc04ca9b87 IPCC_AR6_WGI_Figure_3_28.png]]

Figure 3.28 | '''Long-term trends in halosteric and thermosteric sea level in CMIP6 models and observations.''' Units are mm yr <sup>–1</sup> . In The '''right-hand column''' , three observed maps of 0 to 2000 m halosteric sea level trends are shown: '''top (D&amp;W)''' from [[#Durack--2010|Durack and Wijffels (2010)]] , 1950–2019, updated; '''upper-middle (EN4)''' from [[#Good--2013|Good et al. (2013)]] , 1950–2019, updated; and '''lower-middle (Ishii)''' from [[#Ishii--2017|Ishii et al. (2017)]] , 1955–2019, updated. '''Bottom-right:''' the CMIP6 historical multi-model mean (1950–2014). Red and orange colours show a halosteric contraction (enhanced salinity) and blue and green a halosteric expansion (reduced salinity). In The '''left-hand column''' , basin-integrated halosteric '''(top)''' and thermosteric '''(bottom)''' trends for the Atlantic and Pacific, the two largest ocean basins, are shown, where Pacific anomalies are presented on the x-axis and Atlantic on the y-axis. Observational estimates are presented in black, CMIP6 historical (all forcings) simulations are shown in orange squares, with the multi-model mean shown as a dark orange diamond with a black bounding box. CMIP6 hist-nat (historical natural forcings only) simulations are shown in green squares with the multi-model mean as a dark green diamond with a black bounding box. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

Climate change detection and attribution studies have considered salinity, with the first of these assessed in AR5 ( [[#Bindoff--2013|Bindoff et al., 2013]] ). Since that time, the positive detection conclusions ( [[#Stott--2008|Stott et al., 2008]] ; [[#Pierce--2012|Pierce et al., 2012]] ; [[#Terray--2012|Terray et al., 2012]] ) have been supported by a number of more recent and independent assessments which have reproduced the multi-decadal basin-scale patterns of change in observations and models (Figures 3.27 and 3.28; [[#Durack--2014a|Durack et al., 2014a]] ; [[#Durack--2015|Durack, 2015]] ; [[#Levang--2015|Levang and Schmitt, 2015]] ; [[#Skliris--2016|Skliris et al., 2016]] ). Observed depth-integrated basin responses, contrasting the Pacific and Atlantic basins (freshening Pacific and enhanced salinity Atlantic) were also shown to be replicated in most historical (natural and anthropogenically forced) simulations, with this basin contrast absent in CMIP5 and CMIP6 natural-only simulations that exclude anthropogenic forcing ( [[#Durack--2014a|Durack et al., 2014a]] ; Figure 3.28).

While observational sparsity considerably limits quantification of regional changes, a recent study by [[#Friedman--2017|Friedman et al. (2017)]] assessed salinity changes in the Atlantic Ocean from 1896 to 2013 and confirmed the pattern of mid-to-low latitude enhanced salinity and high latitude North Atlantic freshening over this period exists even after accounting for the effects of the NAO and AMO.

Considering the bulk of evidence, it is ''extremely likely'' that human influence has contributed to observed near-surface and subsurface salinity changes across the globe since the mid-20th century. All available multi-decadal assessments have confirmed that the associated pattern of change corresponds to fresh regions becoming fresher and salty regions becoming saltier ( ''high confidence'' ). CMIP5 and CMIP6 models are only able to reproduce these patterns in simulations that include greenhouse gas increases ( ''medium confidence'' ). Changes to the coincident atmospheric water cycle and ocean-atmosphere fluxes (evaporation and precipitation) are the primary drivers of the basin-scale observed salinity changes ( ''high confidence'' ). This result is supported by all available observational assessments, along with a growing number of climate modelling studies targeted at assessing ocean and water cycle changes. The basin-scale changes are consistent across models and intensify on centennial scales from the historical period through to the projections of future climate ( ''high confidence'' ).

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