Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.1.4.5 Stratospheric polar vortex and sudden warming events =====

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The AR5 assessed changes in the polar vortices and reported a ''likely'' decrease in the lower-stratospheric geopotential heights over Antarctica in spring and summer at least since 1979.

Multiple definitions for the polar vortex strength and sudden stratospheric warming (SSW) events have been proposed and compared ([[#Butler--2015|Butler et al., 2015]]; [[#Palmeiro--2015|Palmeiro et al., 2015]]; [[#Waugh--2017|Waugh et al., 2017]]; [[#Butler--2018|Butler and Gerber, 2018]]), and new techniques identifying daily vortex patterns and SSWs have been developed (D.M. [[#Mitchell--2013|]] [[#Mitchell--2013|Mitchell et al., 2013]]; [[#Kretschmer--2018|Kretschmer et al., 2018]]). Errors in reanalysis stratospheric winds were assessed and discrepancies in stratospheric atmospheric circulation and temperatures between reanalyses, satellites and radiosondes have been reported (D.M. [[#Mitchell--2013|]] [[#Mitchell--2013|Mitchell et al., 2013]]; [[#Duruisseau--2017|Duruisseau et al., 2017]]).

The northern stratospheric polar vortex has varied intra-seasonally and with altitude during recent decades. Multiple reanalysis and radiosonde datasets show that the midwinter lower stratospheric geopotential height (150 hPa) over the polar region north of 60°N has increased significantly since the early 1980s ([[#Bohlinger--2014|Bohlinger et al., 2014]]; [[#Garfinkel--2017|Garfinkel et al., 2017]]). This signal extends to the middle and upper stratosphere. In January-February zonal winds north of 60°N at 10 hPa have been weakening ([[#Kim--2014|Kim et al., 2014]]; [[#Kretschmer--2018|Kretschmer et al., 2018]]). Daily atmospheric circulation patterns over the northern polar stratosphere exhibit a decreasing frequency of strong vortex events and commensurate increase in more-persistent weak events, which largely explains the observed significant weakening of the vortex during 1979–2015 ([[#Kretschmer--2018|Kretschmer et al., 2018]]). The northern polar vortex has weakened in early winter but strengthened during late winter ([[#Bohlinger--2014|Bohlinger et al., 2014]]; [[#Garfinkel--2015a|Garfinkel et al., 2015a]], 2017; [[#Ivy--2016|Ivy et al., 2016]]; [[#Seviour--2017|Seviour, 2017]]; [[#Kretschmer--2018|Kretschmer et al., 2018]]). In the middle and upper stratosphere, a strengthening trend of the northern polar vortex during DJF has occurred since 1998, contrasting the weakening trend beforehand (D. [[#Hu--2018|]] [[#Hu--2018|Hu et al., 2018]]). The position of the polar vortex also has long-term variations, exhibiting a persistent shift toward Northern Siberia and away from North America in February over the period 1979–2015 ([[#Zhang--2016|Zhang et al., 2016]]; J. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]]). Multiple measures show similar location changes ([[#Seviour--2017|Seviour, 2017]]).

Sudden stratospheric warming (SSW), a phenomenon of rapid stratospheric air temperature increases (sometimes by more than 50°C in 1–2 days), is tightly associated with the reversal of upper stratospheric zonal winds, and a resulting collapse or substantial weakening of the stratospheric polar vortex ([[#Butler--2015|Butler et al., 2015]]; [[#Butler--2018|Butler and Gerber, 2018]]) and on average occurs approximately 6 times per decade in the NH winter ([[#Charlton--2007|Charlton et al., 2007]]; [[#Butler--2015|Butler et al., 2015]]). The SSW record from all modern reanalyses is very consistent. There is a higher occurrence of major midwinter SSWs in the 1980s and 2000s with no SSW events during 1990–1997 ([[#Reichler--2012|Reichler et al., 2012]]; [[#Butler--2015|Butler et al., 2015]]). An assessment of multi-decadal variability and change in SSW events is sensitive to both chosen metric and methods ([[#Palmeiro--2015|Palmeiro et al., 2015]]). Due to the lack of assimilation of upper air data, the centennial-scale reanalyses do not capture SSW events, even for the most recent decades ([[#Butler--2015|Butler et al., 2015]], 2017) and hence cannot inform on earlier behaviour. There has been considerably less study of trends in the SH stratosphere polar vortex strength despite the interest in the ozone hole and the potential impact of the SH stratosphere polar vortex strength on it. The occurrence of SSW events in the SH is not as frequent as in the NH, with only 3 documented events in the last 40 years ([[#Shen--2020|Shen et al., 2020]]).

In summary, it is ''likely'' that the northern lower stratospheric polar vortex has weakened since the 1980s in midwinter, and its location has shifted more frequently toward the Eurasian continent. The short record and substantial decadal variability yields ''low confidence'' in any trends in the occurrence of SSW events in the NH winter and such events in the SH are rare.

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