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

===== 3.2.1.2.1 Temperature =====

Ocean temperatures and associated heat fluxes have a primary influence on sea ice (e.g., Carmack et al., 2015; Steele and Dickinson, 2016 <sup>[[#fn:r218|218]]</sup> ). WGI AR5 (their Section 3.2.2) reported that Canada Basin surface waters warmed from 1993 to 2007, and observations over 1950–2010 show the Arctic Ocean water of Atlantic origin (i.e., the Atlantic Water Layer) warming starting in the 1970s. Warming trends have continued: August trends for 1982–2017 reveal summer mixed layer temperatures increasing at about 0.5°C per decade over large sectors of the Arctic basin that are ice-free in summer (Timmermans et al., 2017 <sup>[[#fn:r219|219]]</sup> ) (Figure 3.3). This is primarily the result of increased absorption of solar radiation accompanying sea ice loss (Perovich, 2016 <sup>[[#fn:r220|220]]</sup> ). Between 1979 and 2011, the decrease in Arctic Ocean albedo corresponded to more solar energy input to the ocean ( ''virtually certain'' ) of approximately 6.4 ± 0.9 Wm <sup>-2</sup> (Pistone et al., 2014 <sup>[[#fn:r221|221]]</sup> ), ''likely'' reducing the growth of sea ice by up to 25% in both Eurasian and Canadian basins (Timmermans, 2015 <sup>[[#fn:r222|222]]</sup> ; Ivanov et al., 2016 <sup>[[#fn:r223|223]]</sup> ) (Section 3.2.1.1).

While Atlantic Water Layer temperatures appear to show less variability since 2008, total heat content in this layer continues to increase (Polyakov et al., 2017 <sup>[[#fn:r224|224]]</sup> ). Recent changes have been dubbed the ‘Atlantification’ of the Northern Barents Sea and Eurasian Basin (Arthun et al., 2012 <sup>[[#fn:r225|225]]</sup> ; Lind et al., 2018 <sup>[[#fn:r226|226]]</sup> ), characterised by weaker stratification and enhanced Atlantic Water Layer heat fluxes further northeast ( ''medium confidence'' ). Polyakov et al. (2017) <sup>[[#fn:r227|227]]</sup> estimate 2–4 times larger heat fluxes in 2014–2015 compared with 2007–2008. In the Canadian Basin, the maximum temperature of the Pacific Water Layer increased by ~0.5°C between 2009 and 2013 (Timmermans et al., 2014 <sup>[[#fn:r228|228]]</sup> ), with a doubling in integrated heat content over 1987–2017 (Timmermans et al., 2018 <sup>[[#fn:r229|229]]</sup> ). Over 2001–2014, heat transport associated with Bering Strait inflow increased by 60%, from around 10 TW in 2001 to 16 TW in 2014, due to increases in both volume flux and temperature (Woodgate et al., 2015 <sup>[[#fn:r230|230]]</sup> ; Woodgate, 2018 <sup>[[#fn:r231|231]]</sup> ) ( ''low confidence'' ).

The Southern Ocean is important for the transfer of heat from the atmosphere to the global ocean, including heat from anthropogenic warming (Frölicher et al., 2015 <sup>[[#fn:r232|232]]</sup> ; Shi et al., 2018 <sup>[[#fn:r233|233]]</sup> ). The Southern Ocean accounted for ~75% of the global ocean uptake of excess heat during 1870–1995 (Figure SM3.2; Frölicher et al., 2015 <sup>[[#fn:r234|234]]</sup> ), of which ~43% resided in the Southern Ocean with the remainder redistributed to lower latitudes. Over 1970–2017, observations show that the upper 2000 m of the ocean south of 30°S was responsible for 35–43% of the increase in global ocean heat content (Table 3.1). Both models and observations show that, relative to its size (Table SM3.1), the Southern Ocean is disproportionately important in the increase in global upper ocean heat content ( ''high confidence'' ). Multi-decadal warming of the Southern Ocean has been attributed to anthropogenic factors, especially the role of greenhouse gases but also ozone depletion (Armour et al., 2016 <sup>[[#fn:r235|235]]</sup> ; Shi et al., 2018 <sup>[[#fn:r236|236]]</sup> ; Swart et al., 2018 <sup>[[#fn:r237|237]]</sup> ; Irving et al., 2019 <sup>[[#fn:r238|238]]</sup> ) ( ''medium confidence'' ).

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'''Table 3.1.'''
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Ocean heat content trend (0–2000 m depth) during 2005–2017 and 1970–2017 for the global ocean and Southern Ocean. Ordinary Least Square (OLS) method is used; units are 10 21 J yr -1 . Uncertainties denote the 90% confidence interval accounting for the reduction in the degrees of freedom implied by temporal correlations of residuals, as per Section 5.2. Values in curved brackets are percentages of heat gain by the Southern Ocean relative to the global ocean. Data sources are as per Table SM3.1. The mean proportion and its 5–95% confidence interval (1.65 times standard deviation of individual estimates) are in the last column.
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Surface warming during 1982–2016 was strongest along the northern flank of the ACC '','' contrasting with cooling further south (Figure 3.3). Interior warming was strongest in the upper 2000 m, peaking around 40°S–50°S (Armour et al., 2016 <sup>[[#fn:r239|239]]</sup> ) (SM3.2.1; Figures SM3.2 and SM3.3). There is ''high confidence'' that this pattern of change is driven by upper-ocean overturning circulation and mixing (Cross-Chapter Box 7 in Chapter 3), whereby heat uptake at the surface by newly upwelled waters is transmitted to the ocean interior in intermediate depth layers (Armour et al., 2016 <sup>[[#fn:r240|240]]</sup> ). Whilst temperature trends in the ACC itself are driven predominantly by air-sea flux changes (Swart et al., 2018 <sup>[[#fn:r241|241]]</sup> ), the warming on its northern side appears strongly influenced by wind-forced changes in the thickness and depth of the mode water layer (Desbruyeres et al., 2017 <sup>[[#fn:r242|242]]</sup> ; Gao et al., 2018 <sup>[[#fn:r243|243]]</sup> ) ( ''medium confidence)'' . Below the surface south of the ACC, warming extends close to Antarctica, intruding onto the continental shelf in the Amundsen-Bellingshausen Sea where temperature increases of 0.1°C–0.3°C per decade have been observed over 1983–2012 (Schmidtko et al., 2014 <sup>[[#fn:r244|244]]</sup> ) (Section 3.3.1.5). This latter warming may be driven by changes in wind forcing (Spence et al., 2014 <sup>[[#fn:r245|245]]</sup> ), and exhibits significant decadal variability (Jenkins et al., 2018 <sup>[[#fn:r246|246]]</sup> ).

After around 2005, improved upper ocean heat content estimates became available via Argo profiling floats (Section 1.8.1; Section 5.2). For 2005–2017, multiple datasets show that the heat gained by the Southern Ocean south of 30°S was 45–62% of the global ocean heat gain (Table 3.1) (equivalent figures for other indicative Southern Ocean extents are in Table SM3.2). This accords with Roemmich et al. (2015) <sup>[[#fn:r247|247]]</sup> , who found that during 2006–2013 the ocean south of 20°S accounted for 67–98% of total heat gain in the upper 2000 m of the global ocean. (The smaller proportion for 2005–2017 c.f. 2006–2013 is due to comparatively greater warming in the earlier part of the common period). The recent Southern Ocean heat gain is thus larger than its long-term trend over either the preceding several decades (1970–2004, 30–51%, Table SM3.3) or the full period 1970–2017 (35–43%; Table 3.1 and above). There is ''high confidence'' that the Southern Ocean has increased its role in global ocean heat content in recent years compared with the past several decades. Attribution of this increased role is currently lacking.

The ocean below 2000 m globally stores ~19% of the excess anthropogenic heat in the Earth system, with a large fraction (6% of global total heat excess) located in the deep Southern Ocean south of 30°S (Frölicher et al., 2015 <sup>[[#fn:r248|248]]</sup> ; Talley et al., 2016 <sup>[[#fn:r249|249]]</sup> ) ( ''medium confidence)'' . The WGI AR5-quantified warming of these waters was recently updated (Desbruyeres et al., 2017 <sup>[[#fn:r250|250]]</sup> ) to an equivalent heat uptake of 0.07 ± 0.06 W m −2 below 2000 m since the beginning of the century, resulting in an extra 34 ± 14 TW south of 30°S from 1980 to 2012 (Purkey and Johnson, 2013 <sup>[[#fn:r251|251]]</sup> ) ''.'' Antarctic Bottom Water volume is decreasing (Purkey and Johnson, 2012 <sup>[[#fn:r252|252]]</sup> ), resulting in a deepening of density surfaces and driving much of the warming on depth surfaces below 2000 m (Desbruyeres et al., 2017 <sup>[[#fn:r253|253]]</sup> ). This reduction in bottom water volume is suggestive of a decrease in its production (Purkey and Johnson, 2013 <sup>[[#fn:r254|254]]</sup> ). In the Indian and Pacific basins close to Antarctica, bottom water is freshening (Purkey and Johnson, 2013 <sup>[[#fn:r255|255]]</sup> ; Menezes et al., 2017 <sup>[[#fn:r256|256]]</sup> ) consistent with the uptake of enhanced Antarctic ice shelf and glacial melt (Purkey and Johnson, 2013 <sup>[[#fn:r257|257]]</sup> ).

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