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

Abstract

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.

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Fig. 1: The S–T coupled mode and its relationship with the SAM.
Fig. 2: Anomalous Australian climate conditions during the nine polar vortex weakening years.
Fig. 3: Changes in the likelihood of extreme high Tmax, low rainfall and high wildfire danger during the nine polar vortex weakening years.
Fig. 4: Monthly standardized SAM index anomalies during the nine polar vortex weakening years.
Fig. 5: Circulation and cloud cover changes over Australia during the nine polar vortex weakening years.

Data availability

The datasets used in this study are available online in the following locations: ERA-Interim reanalysis, https://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/; JRA-55 reanalysis, https://jra.kishou.go.jp/JRA-55/index_en.html; Hurrell et al. merged SST analysis, https://climatedataguide.ucar.edu/climate-data/merged-hadley-noaaoi-sea-surface-temperature-sea-ice-concentration-hurrell-et-al-2008; Reynolds OI v2 SST analysis, https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.html; AWAP analysis, http://www.bom.gov.au/climate/data-services/maps.shtml; CPC AAO index, https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/aao.shtml. FFDI data are available by contacting the contributing author A.J.D. (andrew.dowdy@bom.gov.au). All the other data used for the analysis in this study are available by contacting the corresponding author.

Code availability

All the codes and scripts used for the analysis in this study are available by contacting the corresponding author.

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Acknowledgements

This study is part of the Forewarned is Forearmed project, which is supported by funding from the Australian Government Department of Agriculture as part of its Rural R&D for Profit programme. G.B. and J.M.A. were supported by the Australian Research Council Centre of Excellence for Climate Extremes (grant CE170100023). J.M.A. was also supported by the Regional and Global Model Analysis component of the Earth and Environmental System Modeling Program of the US Department of Energy’s Office of Biological & Environmental Research via National Science Foundation IA 1947282. A.J.D. was supported by funding from the Australian Government’s National Environmental Science Program. D.W.J.T. was supported by the NSF Climate and Large-Scale Dynamics Program. We thank T. Cowan and B. Trewin at the Bureau of Meteorology for their constructive feedback on the manuscript. We also thank M. Griffiths at the Bureau of Meteorology for processing AWAP data for the analysis of the study. This research was undertaken at the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government. The NCAR Command Language (NCL; http://www.ncl.ucar.edu) version 6.4.0 was used for data analysis and visualization of the results. We also acknowledge NCAR/UCAR, NOAA, ECMWF and the Japanese Met Agent for producing and providing the Hurrell et al. (2008) SST analysis, the Reynolds OI v2 SST analysis, the ERA-Interim reanalysis and the JRA-55 reanalysis, respectively.

Author information

E.-P.L. and H.H.H. conceived ideas for the study, E.-P.L. and G.B. conducted data analysis, H.H.H. wrote the first version of the manuscript and all authors contributed to the interpretation of the results and the writing of the manuscript.

Correspondence to Eun-Pa Lim.

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Lim, E., Hendon, H.H., Boschat, G. et al. Australian hot and dry extremes induced by weakenings of the stratospheric polar vortex. Nat. Geosci. 12, 896–901 (2019). https://doi.org/10.1038/s41561-019-0456-x

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