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Australian Hot and Dry Extremes Induced by Weakenings of the Stratospheric Polar Vortex

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Australian Hot and Dry Extremes Induced by Weakenings of the Stratospheric Polar Vortex ARTICLES https://doi.org/10.1038/s41561-019-0456-x Australian hot and dry extremes induced by weakenings of the stratospheric polar vortex Eun-Pa Lim 1*, Harry H. Hendon 1, Ghyslaine Boschat 2,3, Debra Hudson 1, David W. J. Thompson4, Andrew J. Dowdy1 and Julie M. Arblaster 2,3,5 The occurrence of extreme hot and dry conditions in warm seasons can have large impacts on human health, energy and water supplies, agriculture and wildfires. Australian hot and dry extremes have been known to be associated with the occurrence of El Niño and other variations of tropospheric circulation. Here we identify an additional driver: variability of the stratospheric Antarctic polar vortex. On the basis of statistical analyses using observational data covering the past 40 yr, we show that weakenings and warmings of the stratospheric polar vortex, which episodically occur during austral spring, substantially increase the chances of hot and dry extremes and of associated fire-conducive weather across subtropical eastern Australia from austral spring to early summer. The promotion of these Australian climate extremes results from the downward coupling of the weakened polar vortex to tropospheric levels, where it is linked to the low-index polarity of the Southern Annular Mode, an equatorward shift of the mid-latitude westerly jet stream and subsidence and warming in the subtropics. Because of the long timescale of the polar vortex variations, the enhanced likelihood of early-summertime hot and dry extremes and wildfire risks across eastern Australia may be predictable a season in advance during years of vortex weakenings. nderstanding, predicting and anticipating extreme high the westerly vortex. The polar vortex in the Northern Hemisphere temperature and low rainfall in the warm seasons is cru- (NH) is mainly perturbed during boreal winter to early spring11,12, cially important in Australia because of the risks and and the Southern Hemisphere (SH) vortex is perturbed during U 13,14 impacts on human health, agriculture, wildfires, utilities, infra- austral winter to spring . Variability of the stratospheric polar structure and water management1. In particular, the impact of the vortex produced by variability of upward-propagating planetary- reduced productivity of dairy and meat under extreme hot and scale waves can couple downward to the troposphere and promote dry conditions can be felt beyond Australia as Australia is one of low-frequency variations in tropospheric circulation and tempera- the world’s major exporters of these products2. The warm phase of ture, thus serving as a source of extended-range predictability of the El Niño–Southern Oscillation (ENSO)3, the positive phase of the surface weather and climate15–18. For example, downward coupling Indian Ocean Dipole (IOD)4 and the negative index polarity of the from the stratosphere to the troposphere in association with sud- Southern Annular Mode (SAM)5 are important drivers of extreme den stratospheric warmings in the NH, which commonly occurs in hot and dry conditions over various regions of Australia, mainly in boreal winter and early spring, often results in sustained impacts spring but to some extent in summer as well6–8. Here we highlight on surface climate via promoting the negative polarity of the North that a weaker than normal springtime stratospheric polar vortex Atlantic Oscillation/Northern Annular Mode15,19,20. over Antarctica is a key driver and an important source of predict- Although generally not as spectacular as sudden stratospheric ability of extreme hot and dry climate conditions over subtropical warming events in the NH, stratospheric polar vortex variations eastern Australia during its warm seasons. Anomalous weakening episodically occur in the SH in austral spring to early summer21,22. of the Southern Hemisphere polar stratospheric vortex in spring is Anomalous SH stratospheric polar vortex weakening or intensi- shown to subsequently promote the negative polarity of the SAM, fication during spring, which can be viewed as an earlier or later which then promotes Australian high daytime temperature and low shift in the seasonal evolution of the vortex, respectively17,23, can rainfall extremes and associated dangerous atmospheric conditions lead to sustained occurrence of the negative or positive polarity of for wildfire from mid-spring to early summer. the tropospheric SAM (in this study, SAM denotes tropospheric The stratospheric polar vortex, which occurs in both hemi- SAM unless otherwise stated), respectively, during spring–sum- spheres, is characterized by maximum westerly winds in the mer15,17,18,24,25. The SAM is well recognized to drive variations in upper stratosphere–lower mesosphere that circle the hemisphere. surface climate throughout the SH, including changes in precipita- It is strongest in the winter half of the year and disappears during tion and surface temperatures6,26–28, via the concomitant latitudinal summer, making its seasonal progression from the mid-latitudes shifts in the extratropical storm track and edge of the Hadley towards the pole from early winter to spring9–11. This seasonal march circulation29,30. Impacts of the SAM on Australian climate are par- also involves a downward progression of the maximum westerlies ticularly pronounced, causing changes in extreme maximum and from the upper stratosphere in winter to the lower troposphere in minimum temperatures and rainfall across much of the south- spring which is caused by vigorous interactions with planetary-scale ern and eastern parts of the continent, especially during spring waves travelling upward from the troposphere that act to decelerate and summer6,7,27,31. 1Bureau of Meteorology, Melbourne, Victoria, Australia. 2ARC Centre of Excellence for Climate Extremes, Monash University, Clayton, Victoria, Australia. 3School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria, Australia. 4Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA. 5National Center for Atmospheric Research, Boulder, CO, USA. *e-mail: [email protected] NatURE GEOSCIENCE | www.nature.com/naturegeoscience ARTICLES NATURE GEOSCIENCE a 3.0 2004, 2005, 2012, 2013 and 2016 have been identified as significant 2.0 vortex weakening years, most of which are also marked by signifi- cantly earlier final breakdown dates of the SH stratospheric polar 1.0 vortex (Supplementary Fig. 1). We then examine the subsequent 0 variation of October–January mean Australian daily maximum –1.0 temperature (Tmax), daily minimum temperature (Tmin), rainfall and –2.0 wildfire potential as measured by the McArthur Forest Fire Danger Standardized amplitude 32 –3.0 Index (FFDI) . We also monitor variations in the SAM, mean sea- 1980 1984 1988 1992 1996 2000 2004 2008 2012 2016 level pressure (MSLP), geopotential height and vertical velocity at Year the 500 hPa level (GPH500 and ω500, respectively) and total cloud U b 55–65° S over wind averaged 1 10 cover. We compare the means of these variables in the 9 signifi- 42 8 cant vortex weakening years to the control group of the other 29 yr 5 6 when the S–T coupled mode index had weak positive or negative 35 Height (km) 4 10 (m amplitudes (that is, when the S–T coupled mode index <0.8σ). We 28 2 s 0 –1 include the years of SH stratospheric vortex strengthening in the 50 21 –2 ) control group because their impact on Australian climate is not 100 14 –4 much different from the normal conditions (Methods). To focus on 200 –6 Pressure level (hPa) Pressure level 300 the impact of the SH polar vortex variation that is independent of 500 7 –8 1,000 –10 ENSO, we removed the influence of ENSO from the variables exam- ined here using linear regression (Methods). We also detrended all Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Calendar month data for the period 1979–2016 to concentrate on the impacts of year-to-year variations of the polar vortex. c 0.30 Impact of vortex weakening on Australian seasonal climate The composite difference of Tmax, Tmin and rainfall anomalies aver- 0 aged over October–January for the significant nine polar vortex weakening years (vortex weakening years) compared with the con- –0.30 trol group of all the other years (vortex non-weakening years) is dis- played in Fig. 2. During the vortex weakening years, the strongest Correlation coefficient –0.60 positive Tmax anomalies occurred over subtropical eastern Australia— Queensland, New South Wales and northeastern South Australia (130–156° E, 10–30° S; see Supplementary Fig. 2 for the locations of Jul Apr May Jun Aug Sep Oct Nov Dec Jan Feb Mar Australian states and territory)—where the 4-month mean anomaly Calendar month is significantly higher, reaching up to 2 °C. Coincident rainfall reduc- tion is found over most of eastern Australia but especially over cen- Fig. 1 | The S–T coupled mode and its relationship with the SAM. tral Queensland. The significant increases of Tmax and decreases of a,b, The S–T coupled mode index (a) and its eigenvector (b), which rainfall during the vortex weakening years are statistically robust at 33 represent the interannual variability of the SH stratospheric polar vortex the 5% level, assessed by a permutation resampling test (two-sided) and its downward coupling18. The positive phase of the S–T coupled mode using 4,000 random resamples (Methods). However, Tmin appears to represents weakening of the SH spring polar vortex. In b, the S–T coupled be relatively less sensitive to polar vortex weakening; therefore, we mode eigenvector (colour shading) is overlaid by the climatological zonal- exclude Tmin in the subsequent analysis and discussion. mean zonal wind (U wind) averaged over 55–65° S (contours). Negative Occurrences of extreme seasonal mean Tmax and rainfall are contours indicating easterlies are dashed, and the zero contour is thickened. especially affected by significant vortex weakening. We compute the The contour interval is 10 m s−1.
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