Australian hot and dry extremes induced by weakenings of the stratospheric polar vortex

• Published in: Nature geoscience Vol. 12; no. 11; pp. 896 - 901
• Main Authors: Lim, Eun-Pa, Hendon, Harry H., Boschat, Ghyslaine, Hudson, Debra, Thompson, David W. J., Dowdy, Andrew J., Arblaster, Julie M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-11-2019; Nature Publishing Group
• Subjects: 704/106; 704/172; 704/4111; Agriculture; Antarctic Oscillation; Antarctic vortex; Climatic extremes; Earth and Environmental Science; Earth Sciences; Earth System Sciences; El Nino; El Nino phenomena; Environmental risk; Extreme values; Fires; Geochemistry; Geology; Geophysics/Geodesy; Jet stream; Jet streams (meteorology); Ozone; Polar vortex; Polarity; Spring; Spring (season); Statistical analysis; Statistical analysis of data; Statistical methods; Stratospheric polar vortexes; Stratospheric vortices; Stratospheric warming; Troposphere; Tropospheric circulation; Vortices; Warm seasons; Water supply; Weather; Wildfires; Winter; Australia
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/s41561-019-0456-x

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.
Hot and dry climate extremes in Australia are linked to stratospheric polar vortex weakening, with potential implications for their predictability, according to statistical analyses of observational data from the past 40 years.