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

==== 6.5.1.2 Indian Ocean Basin-wide Warming and Changes in Indian Ocean Dipole (IOD) Events ====

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The Indian Ocean has experienced consistent warming from the surface to 2,000 m during 1960–2015, with most of the warming occurring in the upper 300 m (Cheng et al., 2015 <sup>[[#fn:r521|521]]</sup> ; Nieves et al., 2015 <sup>[[#fn:r522|522]]</sup> ; Cheng et al., 2017 <sup>[[#fn:r523|523]]</sup> ; Gnanaseelan et al., 2017 <sup>[[#fn:r524|524]]</sup> ). New historical ocean heat content (OHC) estimates show an abrupt increase in the Indian Ocean upper 700 m OHC after 1998, contributing to more than 28% of the global ocean heat gain, despite representing only about 12% of the global ocean area (Cheng et al., 2017 <sup>[[#fn:r525|525]]</sup> ; Makarim et al., 2019 <sup>[[#fn:r526|526]]</sup> ). The tropical Indian Ocean SST has warmed by 1.04°C during 1950–2015, while the tropical SST warming is 0.83°C and the global SST warning is 0.65°C. More than 90% of the surface warming in the Indian Ocean has been attributed to changes in GHG emissions (Dong et al., 2014 <sup>[[#fn:r527|527]]</sup> ), with the heat redistributed in the basin via local ocean and atmospheric dynamics (Liu et al., 2015b <sup>[[#fn:r528|528]]</sup> ), the ITF (Section 6.6.1; Susanto et al., 2012 <sup>[[#fn:r529|529]]</sup> ; Sprintall and Revelard, 2014 <sup>[[#fn:r530|530]]</sup> ; Lee et al., 2015b <sup>[[#fn:r531|531]]</sup> ; Susanto and Song, 2015 <sup>[[#fn:r532|532]]</sup> ; Zhang et al., 2018 <sup>[[#fn:r533|533]]</sup> ) and the Walker circulation (Roxy et al., 2014 <sup>[[#fn:r534|534]]</sup> ; Abish et al., 2018 <sup>[[#fn:r535|535]]</sup> ).

The dynamic processes related to the projected changes in IOD under global warming have a large inter-model spread (Cai et al., 2013 <sup>[[#fn:r536|536]]</sup> ). The frequency of extreme positive IOD events are projected to increase by almost a factor of three, from a one-in-seventeen-year event in the 20th century to a one-in-six-year event in the 21st century ( ''low confidence'' ). The bias in the CMIP5 models and internal variability could enlarge the projected increase in the extreme positive IOD events (Li et al., 2016a <sup>[[#fn:r537|537]]</sup> ; Hui and Zheng, 2018 <sup>[[#fn:r538|538]]</sup> ). The increase in IOD events is not linked to the change in the frequency of El Niño events but instead to mean state change—with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean (Cai et al., 2014b <sup>[[#fn:r539|539]]</sup> ). A combination of extreme ENSO and IOD events has led to a northward shift in the Intertropical Convergence Zone (ITCZ) during 1979–2015, which is expected to increase further in the future (Freitas et al., 2017 <sup>[[#fn:r540|540]]</sup> ).

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