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

==== 6.3.1.1 Tropical Cyclones ====

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IPCC AR5 concluded that there was ''low confidence'' in any long-term increases in TC activity globally and in attribution of global changes to any particular cause (Bindoff et al., 2013 <sup>[[#fn:r149|149]]</sup> ; Hartmann et al., 2013 <sup>[[#fn:r150|150]]</sup> ). Based on process understanding and agreement in 21st century projections, it is ''likely'' that the global TC frequency will either decrease or remain essentially unchanged, while global mean TC maximum wind speed and precipitation rates will ''likely'' increase although there is ''low confidence'' in region-specific projections of frequency and intensity (Christensen et al., 2013 <sup>[[#fn:r151|151]]</sup> ). The AR5 concluded that circulation features have moved poleward since the 1970s, associated with a widening of the tropical belt, a poleward shift of storm tracks and jet streams, and contractions of the northern polar vortex and the Southern Ocean westerly wind belts. However it is noted that natural modes of variability on interannual to decadal time scales prevent the detection of a clear climate change signal (Hartmann et al., 2013 <sup>[[#fn:r152|152]]</sup> ).

Since the AR5 and Knutson et al. (2010), palaeoclimatic surveys of coastal overwash sediments and stalagmites have provided further evidence of historical TC variability over the past several millennia. Patterns of storm activity across TC basins show variations through time that appear to be correlated with El Niño-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and changes in atmospheric dynamics related to changes in precession of the sun (Toomey et al., 2013 <sup>[[#fn:r153|153]]</sup> ; Denommee et al., 2014 <sup>[[#fn:r154|154]]</sup> ; Denniston et al., 2015 <sup>[[#fn:r155|155]]</sup> ).

Further studies have investigated the dynamics of TCs. A modelling study investigated a series of low-frequency increases and decreases in TC activity over the North Atlantic over the 20th century (Dunstone et al., 2013 <sup>[[#fn:r156|156]]</sup> ). These variations, culminating in a recent rise in activity, are thought to be due in part to atmospheric aerosol forcing variations (aerosol forcing), which exerts a cooling effect (Booth et al., 2012 <sup>[[#fn:r157|157]]</sup> ; Dunstone et al., 2013 <sup>[[#fn:r158|158]]</sup> ). However, the relative importance of internal variability vs. radiative forcing for multidecadal variability in the Atlantic basin, including TC variability, remains uncertain (Weinkle et al., 2012 <sup>[[#fn:r159|159]]</sup> ; Zhang et al., 2013 <sup>[[#fn:r160|160]]</sup> ; Vecchi et al., 2017 <sup>[[#fn:r161|161]]</sup> ; Yan et al., 2017 <sup>[[#fn:r162|162]]</sup> ). Although the aerosol cooling effect has largely cancelled the increases in potential intensity over the observational period, according to Coupled Model Intercomparison Project Phase 5 (CMIP5) model historical runs, further anthropogenic warming in the future is expected to dominate the aerosol cooling effect leading to increasing TC intensities (Sobel et al., 2016 <sup>[[#fn:r163|163]]</sup> ).

TCs amplify wave heights along the tracks of rapidly moving cyclones (e.g., Moon et al., 2015a) and can therefore increase mixing to the surface of cooler subsurface water. Several studies found that TCs reduce the projected thermal stratification of the upper ocean in CMIP5 models under global warming, thereby slightly offsetting the simulated TC-intensity increases under climate warming conditions (Emanuel, 2015 <sup>[[#fn:r164|164]]</sup> ; Huang et al., 2015b <sup>[[#fn:r165|165]]</sup> ; Tuleya et al., 2016 <sup>[[#fn:r166|166]]</sup> ). On the other hand, freshening of the upper ocean by TC rainfall enhances density stratification by reducing near-surface salinity and this reduces the ability of TCs to cool the upper ocean, thereby having an influence opposite to the thermal stratification effect (Balaguru et al., 2015). In the late 21st century, increased salinity stratification was found to offset about 50% of the suppressive effects that TC mixing has on temperature stratification (Balaguru et al., 2015 <sup>[[#fn:r167|167]]</sup> ). Coupled ocean-atmosphere models still robustly project an increase of TC intensity with climate warming, and particularly for new TC-permitting coupled climate model simulations that compute internally consistent estimates of thermal stratification change (e.g., Kim et al., 2014a; Bhatia et al., 2018 <sup>[[#fn:r169|169]]</sup> ). Higher TC intensities in turn may further aggravate the impacts of SLR on TC-related coastal inundation extremes (Timmermans et al., 2017 <sup>[[#fn:r170|170]]</sup> ).

Kossin et al. (2014) <sup>[[#fn:r171|171]]</sup> identified a poleward expansion of the latitudes of maximum TC intensity in recent decades, which has been linked to an anthropogenically-forced tropical expansion (Sharmila and Walsh, 2018 <sup>[[#fn:r172|172]]</sup> ) and a continued poleward shift of cyclones projected over the western North Pacific in a warmer climate (Kossin et al., 2016 <sup>[[#fn:r173|173]]</sup> ). A 10% slowdown in translation speed of TCs over the 1949–2016 period has been linked to the weakening of the tropical summertime circulation associated with tropical expansion and a more pronounced slowdown in the range 16–22% was found over land areas affected by TCs in the western North Pacific, North Atlantic and Australian regions (Kossin, 2018 <sup>[[#fn:r174|174]]</sup> ). Slow-moving TCs together with higher moisture carrying capacity can cause significantly greater flood hazards (Emanuel, 2017 <sup>[[#fn:r175|175]]</sup> ; Risser and Wehner, 2017 <sup>[[#fn:r176|176]]</sup> ; van Oldenborgh et al., 2017 <sup>[[#fn:r177|177]]</sup> ; see also Table 6.2 and Box 6.1).

Trends in TCs over decades to a century or more have been investigated in several new studies. Key findings include: i) decreasing frequency of severe TCs that make landfall in eastern Australia since the late 1800s (Callaghan and Power, 2011 <sup>[[#fn:r178|178]]</sup> ); ii) increase in frequency of moderately large US storm surge events since 1923 (Grinsted et al., 2012 <sup>[[#fn:r179|179]]</sup> ); iii) recent increase of extremely severe cyclonic storms over the Arabian Sea in the post-monsoon season (Murakami et al., 2017 <sup>[[#fn:r180|180]]</sup> ); iv) intense TCs that make landfall in East and Southeast Asia in recent decades (Mei and Xie, 2016 <sup>[[#fn:r181|181]]</sup> ; Li et al., 2017 <sup>[[#fn:r182|182]]</sup> ); and v) an increase in annual global proportion of hurricanes reaching Category 4 or 5 intensity in recent decades (Holland and Bruyère, 2014 <sup>[[#fn:r183|183]]</sup> ).

Rapid intensification of tropical cyclones (RITCs) poses forecast challenges and increased risks for coastal communities (Emanuel, 2017 <sup>[[#fn:r184|184]]</sup> ). Warming of the upper ocean in the central and eastern tropical Atlantic associated with the positive phase of the Atlantic Multidecadal Oscillation (AMO) (Balaguru et al., 2018 <sup>[[#fn:r185|185]]</sup> ) and in the western North Pacific in recent decades due to a La Niña-like pattern (Zhao et al., 2018 <sup>[[#fn:r186|186]]</sup> ) has favoured RITCs in these regions. One new modelling study suggests there has been a detectable increase in RITC occurrence in the Atlantic basin in recent decades, with a positive contribution from anthropogenic forcing (Bhatia et al., 2019 <sup>[[#fn:r187|187]]</sup> ). Nonetheless, the background conditions that favour RITCs across the Atlantic basin as a whole tend to be associated with less favourable conditions for TC occurrence along the US east coast (Kossin, 2017 <sup>[[#fn:r188|188]]</sup> ).

New studies have used event attribution to explore attribution of certain individual TC events or anomalous seasonal cyclone activity events to anthropogenic forcing (Lackmann, 2015 <sup>[[#fn:r189|189]]</sup> ; Murakami et al., 2015 <sup>[[#fn:r190|190]]</sup> ; Takayabu et al., 2015 <sup>[[#fn:r191|191]]</sup> ; Zhang et al., 2016 <sup>[[#fn:r192|192]]</sup> ; Emanuel, 2017 <sup>[[#fn:r193|193]]</sup> ; see also Table 6.2 and Box 6.1). Risser and Wehner (2017) and van Oldenborgh et al. (2017) concluded that for the Hurricane Harvey event, there is a detectable human influence on extreme precipitation in the Houston area, although their detection analysis is for extreme precipitation in general and not specifically for TC-related precipitation.

There have been more TC dynamical or statistical/dynamical downscaling studies and higher resolution General Circulation Model (GCM) experiments (e.g., Emanuel, 2013; Manganello et al., 2014 <sup>[[#fn:r196|196]]</sup> ; Knutson et al., 2015 <sup>[[#fn:r197|197]]</sup> ; Murakami et al., 2015 <sup>[[#fn:r198|198]]</sup> ; Roberts et al., 2015 <sup>[[#fn:r199|199]]</sup> ; Wehner et al., 2015 <sup>[[#fn:r200|200]]</sup> ; Yamada et al., 2017 <sup>[[#fn:r201|201]]</sup> ). The findings of these studies generally support the AR5 projections of a general increase in intensity of the most intense TCs and a decline in TC frequency overall. However, the projected increase in global TC frequency by Emanuel (2013) <sup>[[#fn:r202|202]]</sup> and Bhatia et al. (2018) <sup>[[#fn:r203|203]]</sup> differed from most other TC frequency projections and previous assessments. For studies into future track changes of TCs under climate warming scenarios (Li et al., 2010 <sup>[[#fn:r204|204]]</sup> ; Kim and Cai, 2014 <sup>[[#fn:r205|205]]</sup> ; Manganello et al., 2014 <sup>[[#fn:r206|206]]</sup> ; Knutson et al., 2015 <sup>[[#fn:r207|207]]</sup> ; Murakami et al., 2015 <sup>[[#fn:r208|208]]</sup> ; Roberts et al., 2015 <sup>[[#fn:r209|209]]</sup> ; Wehner et al., 2015 <sup>[[#fn:r210|210]]</sup> ; Nakamura et al., 2017 <sup>[[#fn:r211|211]]</sup> ; Park et al., 2017 <sup>[[#fn:r212|212]]</sup> ; Sugi et al., 2017 <sup>[[#fn:r213|213]]</sup> ; Yamada et al., 2017 <sup>[[#fn:r214|214]]</sup> ; Yoshida et al., 2017 <sup>[[#fn:r215|215]]</sup> ; Zhang et al., 2017a <sup>[[#fn:r216|216]]</sup> ), it is difficult to identify a robust consensus of projected change in TC tracks, although several of the studies found either poleward or eastward expansion of TC occurrence over the North Pacific region resulting in greater storm occurrence in the central North Pacific. There have been new studies on storm size (Kim et al., 2014a <sup>[[#fn:r217|217]]</sup> ; Knutson et al., 2015 <sup>[[#fn:r218|218]]</sup> ; Yamada et al., 2017 <sup>[[#fn:r219|219]]</sup> ) under climate warming scenarios. These project TC size changes of up to ±10% between basins and studies and provide preliminary findings on this issue that future studies will continue to investigate. Several studies of TC storm surge (e.g., Lin et al., 2012; Garner et al., 2017 <sup>[[#fn:r220|220]]</sup> ) suggest that SLR will dominate the increased height of storm surge due to TCs under climate change.

Taking the above into account, the following is a summary assessment of TC detection and attribution. The observed poleward migration of the latitude of maximum TC intensity in the western North Pacific appears to be unusual compared to expected natural variability and therefore there is ''low to medium confidence'' that this change represents a detectable climate change, though with only ''low confidence'' that the observed shift has a discernible positive contribution from anthropogenic forcing. Anthropogenic forcing is believed to be producing some poleward expansion of the tropical circulation with climate warming. Additional studies of observed long-term TC changes such as: an increase in annual global proportion of Category 4 or 5 TCs in recent decades, severe TCs occurring in the Arabian Sea and making landfall in East and Southeast Asia, the increasing frequency of moderately large US storm surge events since 1923 and the decreasing frequency of severe TCs that make landfall in eastern Australia since the late 1800s, may each represent emerging anthropogenic signals, but still with ''low confidence (limited evidence)'' . The lack of confident climate change detection for most TC metrics continues to limit confidence in both future projections and in the attribution of past changes and TC events, since TC event attribution in most published studies is generally being inferred without support from a confident climate change detection of a long-term trend in TC activity.

TCs projections for the late 21st century are summarised as follows: 1) there is ''medium confidence'' that the proportion of TCs that reach Category 4–5 levels will increase, that the average intensity of TCs will increase (by roughly 1–10%, assuming a 2 ° C global temperature rise), and that average TCs precipitation rates (for a given storm) will increase by at least 7% per degree Celsius SST warming, owing to higher atmospheric water vapour content, 2) there is ''low confidence (low agreement, medium evidence)'' in how global TC frequency will change, although most modelling studies project some decrease in global TC frequency and 3) SLR will lead to higher storm surge levels for the TCs that do occur, assuming all other factors are unchanged ( ''very'' ''high confidence'' ).

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