Editing IPCC:AR6/SR15/Chapter-3 (section)

=== 3.3.6 Tropical Cyclones and Extratropical Storms ===

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Most recent studies on observed trends in the attributes of tropical cyclones have focused on the satellite era starting in 1979 (Rienecker et al., 2011) <sup>[[#fn:r221|221]]</sup> , but the study of observed trends is complicated by the heterogeneity of constantly advancing remote sensing techniques and instrumentation during this period (e.g., Landsea, 2006; Walsh et al., 2016) <sup>[[#fn:r222|222]]</sup> . Numerous studies leading up to and after AR5 have reported a decreasing trend in the global number of tropical cyclones and/or the globally accumulated cyclonic energy (Emanuel, 2005; Elsner et al., 2008; Knutson et al., 2010; Holland and Bruyère, 2014; Klotzbach and Landsea, 2015; Walsh et al., 2016) <sup>[[#fn:r223|223]]</sup> . A theoretical physical basis for such a decrease to occur under global warming was recently provided by Kang and Elsner (2015) <sup>[[#fn:r224|224]]</sup> . However, using a relatively short (20 year) and relatively homogeneous remotely sensed record, Klotzbach (2006) <sup>[[#fn:r225|225]]</sup> reported no significant trends in global cyclonic activity, consistent with more recent findings of Holland and Bruyère (2014) <sup>[[#fn:r226|226]]</sup> . Such contradictions, in combination with the fact that the almost four-decade-long period of remotely sensed observations remains relatively short to distinguish anthropogenically induced trends from decadal and multi-decadal variability, implies that there is only ''low confidence'' regarding changes in global tropical cyclone numbers under global warming over the last four decades.

Studies in the detection of trends in the occurrence of very intense tropical cyclones (category 4 and 5 hurricanes on the Saffir-Simpson scale) over recent decades have yielded contradicting results. Most studies have reported increases in these systems (Emanuel, 2005; Webster et al., 2005; Klotzbach, 2006; Elsner et al., 2008; Knutson et al., 2010; Holland and Bruyère, 2014; Walsh et al., 2016) <sup>[[#fn:r227|227]]</sup> , in particular for the North Atlantic, North Indian and South Indian Ocean basins (e.g., Singh et al., 2000; Singh, 2010; Kossin et al., 2013; Holland and Bruyère, 2014; Walsh et al., 2016) <sup>[[#fn:r228|228]]</sup> . In the North Indian Ocean over the Arabian Sea, an increase in the frequency of extremely severe cyclonic storms has been reported and attributed to anthropogenic warming (Murakami et al., 2017) <sup>[[#fn:r229|229]]</sup> . However, to the east over the Bay of Bengal, tropical cyclones and severe tropical cyclones have exhibited decreasing trends over the period 1961–2010, although the ratio between severe tropical cyclones and all tropical cyclones is increasing (Mohapatra et al., 2017) <sup>[[#fn:r230|230]]</sup> . Moreover, studies that have used more homogeneous records, but were consequently limited to rather short periods of 20 to 25 years, have reported no statistically significant trends or decreases in the global number of these systems (Kamahori et al., 2006; Klotzbach and Landsea, 2015) <sup>[[#fn:r231|231]]</sup> . Likewise, CMIP5 model simulations of the historical period have not produced anthropogenically induced trends in very intense tropical cyclones (Bender et al., 2010; Knutson et al., 2010, 2013; Camargo, 2013; Christensen et al., 2013) <sup>[[#fn:r232|232]]</sup> , consistent with the findings of Klotzbach and Landsea (2015) <sup>[[#fn:r233|233]]</sup> . There is consequently ''low confidence'' in the conclusion that the number of very intense cyclones is increasing globally.

General circulation model (GCM) projections of the changing attributes of tropical cyclones under high levels of greenhouse gas forcing (3°C to 4°C of global warming) consistently indicate decreases in the global number of tropical cyclones (Knutson et al., 2010, 2015; Sugi and Yoshimura, 2012; Christensen et al., 2013; Yoshida et al., 2017) <sup>[[#fn:r234|234]]</sup> . A smaller number of studies based on statistical downscaling methodologies contradict these findings, however, and indicate increases in the global number of tropical cyclones under climate change (Emanuel, 2017) <sup>[[#fn:r235|235]]</sup> . Most studies also indicate increases in the global number of very intense tropical cyclones under high levels of global warming (Knutson et al., 2015; Sugi et al., 2017) <sup>[[#fn:r236|236]]</sup> , consistent with dynamic theory (Kang and Elsner, 2015) <sup>[[#fn:r237|237]]</sup> , although a few studies contradict this finding (e.g., Yoshida et al., 2017) <sup>[[#fn:r238|238]]</sup> . Hence, it is assessed that under 3°C to 4°C of warming that the global number of tropical cyclones would decrease whilst the number of very intense cyclones would increase ( ''medium confidence'' ).

To date, only two studies have directly explored the changing tropical cyclone attributes under 1.5°C versus 2°C of global warming. Using a high resolution global atmospheric model, Wehner et al. (2018a) <sup>[[#fn:r239|239]]</sup> concluded that the differences in tropical cyclone statistics under 1.5°C versus 2°C stabilization scenarios, as defined by the HAPPI protocols (Mitchell et al., 2017) <sup>[[#fn:r240|240]]</sup> are small. Consistent with the majority of studies performed for higher degrees of global warming, the total number of tropical cyclones is projected to decrease under global warming, whilst the most intense (categories 4 and 5) cyclones are projected to occur more frequently. These very intense storms are projected to be associated with higher peak wind speeds and lower central pressures under 2°C versus 1.5°C of global warming. The accumulated cyclonic energy is projected to decrease globally from 1.5°C to 2°C, in association with a decrease in the global number of tropical cyclones under progressively higher levels of global warming. It is also noted that heavy rainfall associated with tropical cyclones was assessed in the IPCC SREX as ''likely'' to increase under increasing global warming (Seneviratne et al., 2012) <sup>[[#fn:r241|241]]</sup> . Two recent articles suggest that there is ''high confidence'' that the current level of global warming (i.e., about 1°C, see Section 3.3.1) increased the heavy precipitation associated with the 2017 Hurricane Harvey by about 15% or more (Risser and Wehner, 2017; van Oldenborgh et al., 2017) <sup>[[#fn:r242|242]]</sup> . Hence, it can be inferred, under the assumption of linear dynamics, that further increases in heavy precipitation would occur under 1.5°C, 2°C and higher levels of global warming ( ''medium confidence'' ). Using a high resolution regional climate model explored the effects of different degrees of global warming on tropical cyclones over the southwest Indian Ocean, using transient simulations that downscaled a number of RCP8.5 GCM projections. Decreases in tropical cyclone frequencies are projected under both 1.5°C and 2°C of global warming. The decreases in cyclone frequencies under 2°C of global warming are somewhat larger than under 1.5°C, but no further decreases are projected under 3°C. This suggests that 2°C of warming, at least in these downscaling simulations, represents a type of stabilization level in terms of tropical cyclone formation over the southwest Indian Ocean and landfall over southern Africa (Muthige et al., 2018) <sup>[[#fn:r244|244]]</sup> . There is thus ''limited evidence'' that the global number of tropical cyclones will be lower under 2°C compared to 1.5°C of global warming, but with an increase in the number of very intense cyclones ( ''low confidence'' ).

The global response of the mid-latitude atmospheric circulation to 1.5°C and 2°C of warming was investigated using the HAPPI ensemble with a focus on the winter season (Li et al., 2018) <sup>[[#fn:r245|245]]</sup> . Under 1.5°C of global warming a weakening of storm activity over North America, an equatorward shift of the North Pacific jet exit and an equatorward intensification of the South Pacific jet are projected. Under an additional 0.5°C of warming a poleward shift of the North Atlantic jet exit and an intensification on the flanks of the Southern Hemisphere storm track are projected to become more pronounced. The weakening of the Mediterranean storm track that is projected under low mitigation emerges in the 2°C warmer world (Li et al., 2018) <sup>[[#fn:r246|246]]</sup> . AR5 assessed that under high greenhouse gas forcing (3°C or 4°C of global warming) there is ''low confidence'' in projections of poleward shifts of the Northern Hemisphere storm tracks, while there is ''high confidence'' that there would be a small poleward shift of the Southern Hemisphere storm tracks (Stocker et al., 2013) <sup>[[#fn:r247|247]]</sup> . In the context of this report, the assessment is that there is ''limited evidence'' and ''low confidence'' in whether any projected signal for higher levels of warming would be clearly manifested under 2°C of global warming.

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