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

==== 9.2.2.2 Ocean Salinity ====

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The AR5 ( [[#Rhein--2013|Rhein et al., 2013]] ) assessed that it was ''very likely'' that subsurface salinity changes reflect surface salinity change, and that basin-scale regions of high salinity and evaporation had trended more saline, while regions of low salinity and more precipitation had trended fresher since the 1950s. The SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) assessment was consistent with AR5. [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] strengthens evidence that subsurface salinity trends are connected to surface trends ( ''very likely'' ) , which are, in turn, linked to an intensifying hydrological cycle ( ''medium confidence'' ). Increasing evidence from updated observational records indicates that it is now ''virtually certain'' that surface salinity contrasts are increasing. At basin scale, [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] and AR5 concur that it is ''very likely'' that the Pacific and Southern Ocean have freshened, and the Atlantic has become more saline. Figures 3.25 and 3.27 compare CMIP6 models to salinity observations.

Globally the mean salinity contrast at near-surface between high- and low-salinity regions increased 0.14 [0.07 to 0.20] from 1950 to 2019 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.2|Section 2.3.3.2]] ). At regional scale, SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) assessed an Arctic liquid freshwater trend of 600 ± 300 km <sup>3</sup> yr <sup>–1</sup> (600 ± 200 Gt yr <sup>–1</sup> ) between 1992 and 2012, reflecting changes associated with continental freshwater imports that affect ocean mass (land ice, rivers) as well as changes in sea ice volume. Since AR5, regional observation-based analyses not assessed in SROCC further confirm the long-term, large-scale and regional patterns of salinity change, both at the ocean surface and in the subsurface ocean, including almost 120 years of changes in the North Atlantic ( [[#Friedman--2017|Friedman et al., 2017]] ) and 60 years of monitoring in the subpolar North Pacific ( [[#Cummins--2020|Cummins and Ross, 2020]] ). These longer time series also provide context to detect large multi-annual change from 2012 to 2016 in the subpolar North Atlantic, unprecedented over the centennial record ( [[#Holliday--2020|Holliday et al., 2020]] ). In summary, there is ''high confidence'' that salinity trends have extended for more than 60 to 100 years in the regions with long historical observation records, such as the North Pacific and the North Atlantic basin.

While there is ''low confidence'' in direct estimates of trends in surface freshwater fluxes (Sections 2.3.1.3.5, 8.3.1.1 and 9.2.1.2), as discussed in SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ), observational studies coupled with modelling studies suggest that surface flux changes drive many observed near-surface salinity changes, on top of changes specific to polar regions. Advances in salinity observations – for example, the Argo program ( [[#Riser--2016|Riser et al., 2016]] ); Soil Moisture and Ocean Salinity (SMOS), Aquarius and Soil Moisture Active Passive (SMAP; [[#Supply--2018|Supply et al., 2018]] ; [[#Vinogradova--2019|Vinogradova et al., 2019]] ) – combined with process studies (SPURS-1/2; [[#Lindstrom--2015|Lindstrom et al., 2015]] ; SPURS-2 Planning Group 2015) and methodological and numerical advances, have increased understanding of how subsurface salinity anomalies link to surface fluxes, and thus increase confidence that near-surface and subsurface salinity pattern changes since the 1950s are linked to changing surface freshwater fluxes ( [[#Zika--2018|Zika et al., 2018]] ; [[#Cheng--2020|Cheng et al., 2020]] ) with an additional contribution from changes in sea ice and land ice discharge at high latitudes ( [[#Haumann--2016|Haumann et al., 2016]] ; [[#Purich--2018|Purich et al., 2018]] ; [[#Dukhovskoy--2019|Dukhovskoy et al., 2019]] ; [[#Rye--2020|Rye et al., 2020]] ). There is therefore ''medium confidence'' in the processes linking surface fluxes to surface and subsurface salinity change.

Ocean circulation changes also affect salinity, largely on annual to decadal time scales ( [[#Du--2019|Du et al., 2019]] ; [[#Liu--2019|Liu et al., 2019]] ; [[#Holliday--2020|Holliday et al., 2020]] ). For instance, in the subpolar North Atlantic, increasing northward transport of Atlantic waters entering the subpolar gyre from the South have compensated the salinity decrease expected from increased Greenland meltwater flux since the early 1990s ( [[#Dukhovskoy--2016|Dukhovskoy et al., 2016]] , 2019; [[#Stendardo--2020|Stendardo et al., 2020]] ). After the mid-2010s the trend reversed towards a broad freshening, the largest in 120 years, in the North Atlantic ( [[#Holliday--2020|Holliday et al., 2020]] ). The long-term freshening in the Pacific Ocean has also been subject to decadal variability, such as a marked salinification since 2005 associated with increased surface fluxes (G. [[#Li--2019|]] [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] ). Local salinity anomalies forced by water cycle intensification can be weakened by rapid exchange between basins with opposing trends, such as by water mass exchange in shallow wind-driven cells between the tropics and the subtropics ( [[#Levang--2020|Levang and Schmitt, 2020]] ). Similarly, eddy exchanges between neighbouring gyres can partly counterbalance decadal time scale long-term subpolar freshening and affect deep convection ( [[#Levang--2020|Levang and Schmitt, 2020]] ). There is ''high confidence'' that, at annual to decadal time scales, regional salinity changes are driven by ocean circulation change superimposed on longer-term trends.

The CMIP5 historical simulations have patterns similar to, but with greater spatial variability than, observed estimates and correspondingly smaller amplitudes in the multi-model mean ( [[#Durack--2015|Durack, 2015]] ; [[#Cheng--2020|Cheng et al., 2020]] ; [[#Silvy--2020|Silvy et al., 2020]] ). [[IPCC:Wg1:Chapter:Chapter-3#3.5.2.1|Section 3.5.2.1]] reports, however, that the fidelity of ocean salinity simulation has improved in CMIP6, and near-surface and subsurface biases have been reduced ( ''medium confidence'' ), though the structure of the biases strongly reflects those of CMIP5. At regional scale, salinity biases are at least partially a result of inaccurate ocean dynamics ( [[#Levang--2020|Levang and Schmitt, 2020]] ). Despite the regional limitations, [[IPCC:Wg1:Chapter:Chapter-3#3.5.2.2|Section 3.5.2.2]] assesses that, at the global scale, it is ''extremely likely'' that human influence has contributed to observed surface and subsurface salinity changes since the mid-20th century (strengthened from the ''very likely'' AR5 assessment).

The SROCC ( [[#Bindoff--2019|Bindoff et al., 2019]] ) assessed that projected salinity changes in the subsurface ocean reflect changes in the rates of formation of water masses or their newly formed properties. Additional consistent newer evidence based on CMIP5 and regional climate models confirms that 21st century projections adhere to the ‘fresh gets fresher, salty gets saltier’ paradigm, through subduction of freshening high-latitude waters into the ventilated water masses in both hemispheres in the Pacific, Indian and Southern Ocean – especially the Arctic and upper Southern Ocean, and saltier subtropical and Mediterranean surface waters – lead to saltier pycnoclines and North Atlantic mode water ( [[#Metzner--2020|Metzner et al., 2020]] ; [[#Parras-Berrocal--2020|Parras-Berrocal et al., 2020]] ; [[#Silvy--2020|Silvy et al., 2020]] ; [[#Soto-Navarro--2020|Soto-Navarro et al., 2020]] ). Overall, projections confirm SROCC assessment that fresh ocean regions will continue to get fresher and salty ocean regions will continue to get saltier in the 21st century ( ''medium confidence'' ).

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