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

=== 11.1.5 Effects of Large-scale Circulation on Changes in Extremes ===

<div id="h2-14-siblings" class="h2-siblings"></div>

Atmospheric large-scale circulation patterns and associated atmospheric dynamics are important determinants of the regional climate (Chapter 10). As a result, they are also important to the magnitude, frequency, and duration of extremes (Box 11.4). Aspects of changes in large-scale circulation patterns are assessed in Chapters 2, 3, 4 and 8, and representative atmospheric and oceanic modes are described in Annex IV. This subsection provides some general concepts, through a couple of examples, on why the uncertainty in the response of large-scale circulation patterns to external forcing can cascade to uncertainty in the response of extremes to external forcings. Details for specific types of extremes are covered in the relevant subsections. For example, the occurrence of the El Niño–Southern Oscillation (ENSO) influences precipitation regimes in many areas, favouring droughts in some regions and heavy rains in others (Box 11.4). The extent and strength of the Hadley circulation influences regions where tropical and extratropical cyclones occur, with important consequences for the characteristics of extreme precipitation, drought, and winds ( [[#11.7|Section 11.7]] ). Changes in circulation patterns associated with land–ocean heat contrast, which affect the monsoon circulations ( [[IPCC:Wg1:Chapter:Chapter-8#8.4.2.4|Section 8.4.2.4]] ), lead to heavy precipitation along the coastal regions in East Asia ( [[#Freychet--2015|Freychet et al., 2015]] ). As a result, changes in the spatial and/or temporal variability of the atmospheric circulation in response to warming affect characteristics of weather systems such as tropical cyclones ( [[#Sharmila--2018|Sharmila and Walsh, 2018]] ), storm tracks ( [[#Shaw--2016|Shaw et al., 2016]] ), and atmospheric rivers ( [[#11.7|Section 11.7]] ; [[#Waliser--2017|Waliser and Guan, 2017]] ). Changes in weather systems come with changes in the frequency and intensity of extreme winds, extreme temperatures, and extreme precipitation, on the backdrop of thermodynamic responses of extremes to warming (Box 11.1). Floods are also affected by large-scale circulation modes, including ENSO, the North Atlantic Oscillation (NAO), the Atlantic Multi-decadal Variability (AMV), and the Pacific Decadal Variability (PDV) ( [[#Kundzewicz--2018|Kundzewicz et al., 2018]] ; Annex IV). Aerosol forcing, through changes in patterns of sea surface temperatures (SSTs), also affects circulation patterns and tropical cyclone activities ( [[#Takahashi--2017|Takahashi et al., 2017]] ).

In general, changes in atmospheric large-scale circulation due to external forcing are uncertain, but there are some robust changes (Sections 2.3.1.4 and 8.2.2.2). Among them, there has been a ''very likely'' widening of the Hadley circulation since the 1980s and the extratropical jets and cyclone tracks have ''likely'' been shifting poleward since the 1980s ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4|Section 2.3.1.4]] ). The poleward expansion affects drought occurrence in some regions ( [[#11.6|Section 11.6]] ), and results in poleward shifts of tropical cyclones and storm tracks (Sections 11.7.1 and 11.7.2). Although it is ''very likely'' that the amplitude of ENSO variability will not robustly change over the 21st century ( [[IPCC:Wg1:Chapter:Chapter-4#4.3.3.2|Section 4.3.3.2]] ), the frequency of extreme ENSO events (Box 11.4), defined by precipitation threshold, is projected to increase with global warming (Section 6.5 of SROCC). This would have implications for projected changes in extreme events affected by ENSO, including droughts over wide areas ( [[#11.6|Section 11.6]] ; Box 11.4) and tropical cyclones ( [[#11.7.1|Section 11.7.1]] ). A case study is provided for extreme ENSO events in 2015–2016 in Box 11.4 to highlight the influence of ENSO on extremes.

In summary, large-scale atmospheric circulation patterns are important drivers for local and regional extremes. There is overall ''low confidence'' about future changes in the magnitude, frequency, and spatial distribution of these patterns, which contributes to uncertainty in projected responses of extremes, especially in the near term.

<div id="11.1.6" class="h2-container"></div>

<span id="effects-of-regional-scale-processes-and-forcings-and-feedbacks-on-changes-in-extremes"></span>