Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.3.2 Ocean Salinity ====

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The AR5 concluded that subtropical regions of high salinity (where evaporation dominates over precipitation) had become more saline, while regions of low salinity (mostly in the tropics and high latitudes) had ''very likely'' become fresher since the 1950s, both at the near-surface, and in the ocean interior along ventilation pathways. From 1950 to 2008, the mean surface contrast between high- and low-salinity regions increased by 0.13 [0.08 to 0.17] (PSS-78, [[#UNESCO/ICES/SCOR/IAPSO--1981|UNESCO/ICES/SCOR/IAPSO, 1981]] ). Across basins, the Atlantic Ocean had become saltier and the Pacific and Southern Oceans had freshened ( ''very likely'' ) .

Prior to the instrumental record, reconstructions of near-surface salinity change are accomplished by combining isotopic and elemental proxy data from microfossil plankton shells and skeletons preserved in deep-sea sediments. These data highlight changes in the salinity contrast between the Pacific and Atlantic oceans during past glacials ( [[#Broecker--1989|Broecker, 1989]] ; [[#Keigwin--2007|Keigwin and Cook, 2007]] ; [[#Costa--2018|Costa et al., 2018]] ) and for repeated episodes of increased subtropical salinity ( [[#Schmidt--2004|Schmidt et al., 2004]] , 2006) and subpolar freshening ( [[#Cortijo--1997|Cortijo et al., 1997]] ; [[#Thornalley--2011|Thornalley et al., 2011]] ) in the North Atlantic ocean. These episodes were associated with disruptions to the large-scale deep ocean circulation ( [[#Buizert--2015|Buizert et al., 2015]] ; [[#Henry--2016|Henry et al., 2016]] ; [[#Lynch-Stieglitz--2017|Lynch-Stieglitz, 2017]] ). Further quantification of paleo salinity changes is complicated by incomplete understanding of proxy-salinity relationships and the relative influence of atmospheric and ocean processes across regions and paleo periods ( [[#Rohling--2007|Rohling, 2007]] ; [[#LeGrande--2011|LeGrande and Schmidt, 2011]] ; [[#Holloway--2016|Holloway et al., 2016]] ; [[#Conroy--2017|Conroy et al., 2017]] ).

Since AR5, new and extended multi-decadal analyses have strengthened the observational support for increased contrast between high and low near-surface salinity regions and inter-basin contrast since the mid-20th century (Section 9.2.2.2; [[#Durack--2010|Durack and Wijffels, 2010]] ; [[#Good--2013|Good et al., 2013]] ; [[#Skliris--2014|Skliris et al., 2014]] ; [[#Aretxabaleta--2017|Aretxabaleta et al., 2017]] ; [[#Cheng--2020|Cheng et al., 2020]] ). These analyses employ different statistical algorithms for interpolation, and only [[#Cheng--2020|Cheng et al. (2020)]] use CMIP5 model simulations to constrain observation-based signals in data-sparse regions.

The 1950–2019 trends reveal near-surface freshening of the northern and western Warm (and fresh) Pool of the Pacific and increased salinity maxima in the subtropical Atlantic, strengthening the inter-basin contrast (Figure 2.27a). There are indications that the subpolar freshening and subtropical salinification of the Atlantic ocean may extend back to at least 1896 ( [[#Friedman--2017|Friedman et al., 2017]] ). Over recent decades, new observations from Argo floats and ocean reanalyses provide general support that changes in the global patterns of near-surface salinity contrast are broadly associated with an intensification of the hydrological cycle (Sections 2.3.1.3.5 and 8.3.1.1). However, this assessment is complicated by changing observational techniques ( [[IPCC:Wg1:Chapter:Chapter-1#1.5.1|Section 1.5.1]] ), temporally and spatially inhomogeneous sampling and uncertainties in interpolation algorithms and the substantial influence of modes of natural variabiltity and ocean circulation processes over interannual timescales ( [[#Skliris--2014|Skliris et al., 2014]] ; [[#Durack--2015|Durack, 2015]] ; [[#Grist--2016|Grist et al., 2016]] ; [[#Aretxabaleta--2017|Aretxabaleta et al., 2017]] ; [[#Vinogradova--2017|Vinogradova and Ponte, 2017]] ; [[#Liu--2020|Liu et al., 2020]] ). Following AR5, based on the updated analysis from [[#Durack--2010|Durack and Wijffels (2010)]] which infills in situ gaps to recover large-scale patterns the mean salinity contrast between high- and low- near-surface salinity regions increased by 0.14 [0.07 to 0.20] from 1950 to 2019.

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[[File:f2b2b1d6eb71e00065f0e9c6df0f57a7 IPCC_AR6_WGI_Figure_2_27.png]]

'''Figure 2.2''' '''7 |''' '''Changes in ocean salinity.''' Estimates of salinity trends using a total least absolute differences fitting method for '''(a)''' global near-surface salinity (SSS) changes and '''(b)''' global zonal mean subsurface salinity changes. Black contours show the associated climatological mean salinity (either near-surface (a) or subsurface (b)) for the analysis period (1950–2019). Both panels represent changes in Practical Salinity Scale 1978 [PSS-78], per decade. In both panels green denotes freshening regions and orange/brown denotes regions with enhanced salinities (‘×’ marks denote non-significant changes). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Changes in the global patterns of near-surface salinity contrast are transferred to the ocean interior via ventilation pathways (Figure 2.27b). Large scale similarities in subsurface salinity changes across observational estimates point to decreasing (increasing) salinity in regions where salinity is lower (higher) than the global average, with freshening in subpolar regions and salinification in the subtropical gyres ( [[#Durack--2010|Durack and Wijffels, 2010]] ; [[#Good--2013|Good et al., 2013]] ; [[#Skliris--2014|Skliris et al., 2014]] ; [[#Durack--2015|Durack, 2015]] ; [[#Aretxabaleta--2017|Aretxabaleta et al., 2017]] ; [[#Cheng--2020|Cheng et al., 2020]] ). Regional changes in salinity are assessed in Section 9.2.2.2.

In summary, it is ''virtually certain'' that since 1950 near-surface high salinity regions have become more saline, while low salinity regions have become fresher, and it is ''very likely'' that this extends to the ocean interior along ventilation pathways. Across basins, it is ''very likely'' that the Atlantic has become saltier and the Pacific and Southern oceans have freshened. The differences between high-salinity and low-salinity regions are linked to an intensification of the hydrological cycle ( ''medium confidence'' ).

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