Editing IPCC:AR6/WGI/Chapter-11 (section)

=== 11.7.4 Extreme Winds ===

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Extreme winds are defined here in terms of the strongest near-surface wind speeds that are generally associated with extreme storms, such as TCs, ETCs, and severe convective storms. In previous IPCC reports, near-surface wind speed (including extremes), has not been assessed as a variable in its own right, but rather in the context of other extreme atmospheric or oceanic phenomena. The exception was the SREX report ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ), which specifically examined past changes and projections of mean and extreme near-surface wind speeds. A strong decline in extreme winds compared to mean winds was reported for the continental northern mid-latitudes. Due to the small number of studies and uncertainties in terrestrial-based surface wind measurements, the findings were assigned ''low confidence'' in SREX. The AR5 reported a weakening of mean and maximum winds from the 1960s or 1970s to the early 2000s in the tropics and mid-latitudes, and increases in high latitudes, but with ''low confidence'' in changes in the observed surface winds over land ( [[#Hartmann--2013|Hartmann et al., 2013]] ). Observed trends in mean wind speed over land and the ocean are assessed in [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.4|Section 2.3.1.4.4]] . Aspects of climate impact-drivers for winds are addressed in Sections 12.3.3 and 12.5.2, and their regional changes are assessed in [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] .

Observationally, although not specifically addressing extreme wind speed changes, negative surface wind speed trends (stilling) were found in the tropics and mid-latitudes of both hemispheres of –0.014 m s <sup>–1</sup> yr <sup>–1</sup> , while positive trends were reported at high latitudes poleward of 70 degrees, based on a review of 148 studies ( [[#McVicar--2012b|McVicar et al., 2012b]] ). An earlier study attributed the stilling to both changes in atmospheric circulation and an increase in surface roughness due to an overall increase in vegetation cover ( [[#Vautard--2010|Vautard et al., 2010]] ). Since then, a number of studies have mostly confirmed these general negative mean-wind trends based on anemometer data for Spain ( [[#Azorin-Molina--2017|Azorin-Molina et al., 2017]] ), Turkey, ( [[#Dadaser-Celik--2014|Dadaser-Celik and Cengiz, 2014]] ), the Netherlands, ( [[#Wever--2012|Wever, 2012]] ), Saudi Arabia, ( [[#Rehman--2013|Rehman, 2013]] ), Romania, ( [[#Marin--2014|Marin et al., 2014]] ), and China ( [[#Chen--2013|Chen et al., 2013]] ). [[#Lin--2013|Lin et al. (2013)]] note that wind speed variability over China is greater at high-elevation locations compared to those closer to mean sea level. [[#Hande--2012|Hande et al. (2012)]] , using radiosonde data, found an increase in surface wind speed on Macquarie Island of Australia.

A number of new studies have examined surface wind speeds over the ocean using ship-based measurements, satellite altimeters, and Special Sensor Microwave/Imagers ( [[#Tokinaga--2011|Tokinaga and Xie, 2011]] ; [[#Zieger--2014|Zieger et al., 2014]] ). It has been noted that wind speed trends tend to be stronger in altimeter measurements, although the spatial patterns of change are qualitatively similar in both instruments ( [[#Zieger--2014|Zieger et al., 2014]] ). Q. [[#Liu--2016|]] [[#Liu--2016|Liu et al. (2016)]] found positive trends in surface wind speeds over the Arctic Ocean in 20 years of satellite observations. Small positive trends in mean wind speed were found in 33 years of satellite data, together with larger trends in the 90th percentile values over global oceans ( [[#Ribal--2019|Ribal and Young, 2019]] ). These results were consistent with an earlier study that found a positive trend in 1-in-100-year wind speeds ( [[#Young--2012|Young et al., 2012]] ). A positive change in mean wind speeds was found for the Arabian Sea and the Bay of Bengal ( [[#Shanas--2015|Shanas and Kumar, 2015]] ) and [[#Zheng--2017|Zheng et al. (2017)]] found that positive wind speed trends over the ocean were larger during winter seasons than summer seasons.

Changes in extreme winds are associated with changes in the characteristics (locations, frequencies, and intensities) of extreme storms, including TCs, ETCs, and severe convective storms. For TCs, as assessed in [[#11.7.1.5|Section 11.7.1.5]] , it is projected that the average peak TC wind speeds will increase globally with warming, while the global frequency of TCs over all categories will decrease or remain unchanged; the average location where TCs reach their peak wind intensity will migrate poleward in the western North Pacific Ocean as the tropics expand with warming. Frequency, intensities, and geographical distributions of extreme wind events associated with TCs will change according to these TC changes. For ETCs, by the end of the century, CMIP5 models show that the number of ETCs associated with extreme winds will significantly decrease in the mid- and high latitudes of the Northern Hemisphere in winter, with the projected decrease being larger over the Atlantic ( [[#Kar-Man%20Chang--2018|Kar-Man Chang, 2018]] ), while it will significantly increase irrespective of the season in the Southern Hemisphere ( [[#11.7.2.4|Section 11.7.2.4]] ; [[#Chang--2017|Chang, 2017]] ). Over the ocean in the subtropics, a large ensemble of 60-km global model simulations indicated that extreme winds associated with storm surges will intensify over 15–35°N in the Northern Hemisphere ( [[#Mori--2019|Mori et al., 2019]] ). However, extreme surface wind speeds will mostly decrease due to decreases in the number and intensity of TCs over most tropical areas of the Southern Hemisphere ( [[#Mori--2019|Mori et al., 2019]] ). The projected changes in the frequency of extreme winds are associated with the future changes in TCs and ETCs.

Extreme cyclonic windstorms that share some characteristics with both TCs and ETCs occur regularly over the Mediterranean Sea and are often referred to as ‘medicanes’ (Ragone et al., 2018; [[#Miglietta--2019|Miglietta and Rotunno, 2019]] ; [[#Zhang--2021|Zhang et al., 2021]] ). Medicanes pose substantial threats to regional islands and coastal zones. A growing body of literature consistently found that the frequency of medicanes decreases under warming, while the strongest medicanes become stronger (Gaertner et al., 2007; Romero and [[#Emanuel--2013|Emanuel, 2013]] , 2017; [[#Cavicchia--2014|Cavicchia et al., 2014]] ; [[#Tous--2016|Tous et al., 2016]] ; [[#Romera--2017|Romera et al., 2017]] ; [[#González-Alemán--2019|González-Alemán et al., 2019]] ). This is also consistent with expected global changes in TCs under warming ( [[#11.7.1|Section 11.7.1]] ). Based on the consistency of these studies, it is ''likely'' that medicanes will decrease in frequency, while the strongest medicanes become stronger under warming scenario projections ( ''medi'' ''um confidence'' ).

In summary, the observed intensity of extreme winds is becoming less severe in the low to mid-latitudes, while becoming more severe in high latitudes poleward of 60 degrees ( ''low confidence'' ). Projected changes in the frequency and intensity of extreme winds are associated with projected changes in the frequency and intensity of TCs and ETCs ( ''medi'' ''um confidence'' ).

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