Editing IPCC:AR6/SROCC/Chapter-6 (section)

==== 6.4.1.1 Recent Documented MHWs and Key Driving Mechanisms ====

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MHWs have been observed and documented in all ocean basins over the last two decades (Figure 6.3a, Figure 6.2, Table 6.2). Prominent examples include the Northeast Pacific 2013–2015 MHW (often called ‘The Blob’; Bond et al. 2015 <sup>[[#fn:r315|315]]</sup> ), the Yellow Sea/East China Sea 2016 MHW (KMA, 2016; KMA, 2017; KMA, 2018), the Western Australia 2011 MHW (Pearce and Feng, 2013 <sup>[[#fn:r316|316]]</sup> ; Kataoka et al. 2014 <sup>[[#fn:r317|317]]</sup> ), and the Northwest Atlantic 2012 MHW (Mills et al. 2013 <sup>[[#fn:r318|318]]</sup> ).

The dominant ocean and/or atmospheric processes leading to the buildup, persistence and decay of MHWs vary greatly among the individual MHWs and depend on the location and time of occurrence. One of the most important global driver of MHWs are El Niño events (Oliver et al., 2018a <sup>[[#fn:r319|319]]</sup> ). During El Niño events, the SST, in particular of the central and eastern equatorial Pacific and the Indian Ocean, are anomalously warm (see Section 6.5). MHWs may also be associated with other large-scale modes of climate variability, such as the Pacific Decadal Oscillation (PDO), AMO, Indian Ocean Dipole (IOD), North Pacific Oscillation and NAO, which modulate ocean temperatures at the regional scale (Benthuysen et al., 2014 <sup>[[#fn:r320|320]]</sup> ; Bond et al., 2015 <sup>[[#fn:r321|321]]</sup> ; Chen et al., 2015b <sup>[[#fn:r322|322]]</sup> ; Di Lorenzo and Mantua, 2016). These modes can change the strength, direction and location of ocean currents that build up areas of extreme warm waters, or they can change the air-sea heat flux, leading to a warming of the ocean surface from the atmosphere. For example, predominant La Niña conditions in 2010 and 2011 strengthened and shifted the Leeuwin Current southward along the west coast of Australia leading to the Western Australia 2011 MHW (Pearce and Feng, 2013 <sup>[[#fn:r323|323]]</sup> ; Kataoka et al., 2014 <sup>[[#fn:r324|324]]</sup> ). Another example is The Blob, which emerged in 2013 in response to teleconnections between the North Pacific and the weak El Niño that drove strong positive sea level pressure anomalies across the northeast Pacific inducing a smaller heat loss from the ocean (Bond et al., 2015 <sup>[[#fn:r325|325]]</sup> ; Di Lorenzo and Mantua, 2016). Low sea ice concentrations in the Arctic, however, may have also played a role (Lee et al., 2015a <sup>[[#fn:r326|326]]</sup> ).

The buildup and decay of extreme warm SSTs may also be caused by small-scale atmospheric and oceanic processes, such as ocean mesoscale eddies or local atmospheric weather patterns (Carrigan and Puotinen, 2014 <sup>[[#fn:r327|327]]</sup> ; Schlegel et al., 2017a <sup>[[#fn:r328|328]]</sup> ; Schlegel et al., 2017b <sup>[[#fn:r329|329]]</sup> ). For example, the Tasman Sea 2015–2016 MHW was caused by enhanced southward transport in the East Australian current driven by increased wind stress curl across the mid-latitude South Pacific (Oliver and Holbrook, 2014 <sup>[[#fn:r330|330]]</sup> ; Oliver et al., 2017 <sup>[[#fn:r331|331]]</sup> ) with local downwelling-favourable winds also having played a role in the subsurface intensification of the MHW (Schaeffer and Roughan, 2017 <sup>[[#fn:r332|332]]</sup> ). In addition, the 2016 MHW in the southern part of the Great Barrier Reef was mitigated by the ETC Winston that passed over Fiji on February 20th. The cyclone caused strong winds, cloud cover and rain, which lowered SST and prevented corals from bleaching (Hughes et al., 2017b <sup>[[#fn:r333|333]]</sup> ).

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