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  • Wilks, D. S., 2006: Statistical Methods in the Atmospheric Sciences. 2nd ed. Academic Press, 592 pp.

  • Yamazaki, Y., and et al. , 2020: September 2019 Antarctic sudden stratospheric warming: quasi-6-day wave burst and ionospheric effects. Geophys. Res. Lett., 47, e2019GL086577, https://doi.org/10.1029/2019GL086577.

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The 2019 Southern Hemisphere Stratospheric Polar Vortex Weakening and Its Impacts

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  • 1 Bureau of Meteorology, Melbourne, Australia
  • | 2 NOAA/Chemical Sciences Laboratory, Boulder, Colorado
  • | 3 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
  • | 4 CIRES, University of Colorado, and NOAA/Physical Sciences Laboratory, Boulder, Colorado
  • | 5 Met Office Hadley Centre, and College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, United Kingdom
  • | 6 Department of Meteorology, University of Reading, Reading, United Kingdom
  • | 7 European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom
  • | 8 Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan
  • | 9 Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan
  • | 10 Met Office Hadley Centre, Exeter, United Kingdom
  • | 11 NASA Goddard Space Flight Center, Greenbelt, and Science Systems and Applications, Inc., Lanham, Maryland
  • | 12 Bureau of Meteorology, Melbourne, Australia
  • | 13 Department of Geophysics, University of Chile, Santiago, Chile
  • | 14 NASA Goddard Space Flight Center, Greenbelt, Maryland
  • | 15 Bureau of Meteorology, Melbourne, Australia
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Abstract

This study offers an overview of the low-frequency (i.e., monthly to seasonal) evolution, dynamics, predictability, and surface impacts of a rare Southern Hemisphere (SH) stratospheric warming that occurred in austral spring 2019. Between late August and mid-September 2019, the stratospheric circumpolar westerly jet weakened rapidly, and Antarctic stratospheric temperatures rose dramatically. The deceleration of the vortex at 10 hPa was as drastic as that of the first-ever-observed major sudden stratospheric warming in the SH during 2002, while the mean Antarctic warming over the course of spring 2019 broke the previous record of 2002 by ∼50% in the midstratosphere. This event was preceded by a poleward shift of the SH polar night jet in the uppermost stratosphere in early winter, which was then followed by record-strong planetary wave-1 activity propagating upward from the troposphere in August that acted to dramatically weaken the polar vortex throughout the depth of the stratosphere. The weakened vortex winds and elevated temperatures moved downward to the surface from mid-October to December, promoting a record strong swing of the southern annular mode (SAM) to its negative phase. This record-negative SAM appeared to be a primary driver of the extreme hot and dry conditions over subtropical eastern Australia that accompanied the severe wildfires that occurred in late spring 2019. State-of-the-art dynamical seasonal forecast systems skillfully predicted the significant vortex weakening of spring 2019 and subsequent development of negative SAM from as early as late July.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Eun-Pa Lim, eun-pa.lim@bom.gov.au

Abstract

This study offers an overview of the low-frequency (i.e., monthly to seasonal) evolution, dynamics, predictability, and surface impacts of a rare Southern Hemisphere (SH) stratospheric warming that occurred in austral spring 2019. Between late August and mid-September 2019, the stratospheric circumpolar westerly jet weakened rapidly, and Antarctic stratospheric temperatures rose dramatically. The deceleration of the vortex at 10 hPa was as drastic as that of the first-ever-observed major sudden stratospheric warming in the SH during 2002, while the mean Antarctic warming over the course of spring 2019 broke the previous record of 2002 by ∼50% in the midstratosphere. This event was preceded by a poleward shift of the SH polar night jet in the uppermost stratosphere in early winter, which was then followed by record-strong planetary wave-1 activity propagating upward from the troposphere in August that acted to dramatically weaken the polar vortex throughout the depth of the stratosphere. The weakened vortex winds and elevated temperatures moved downward to the surface from mid-October to December, promoting a record strong swing of the southern annular mode (SAM) to its negative phase. This record-negative SAM appeared to be a primary driver of the extreme hot and dry conditions over subtropical eastern Australia that accompanied the severe wildfires that occurred in late spring 2019. State-of-the-art dynamical seasonal forecast systems skillfully predicted the significant vortex weakening of spring 2019 and subsequent development of negative SAM from as early as late July.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Eun-Pa Lim, eun-pa.lim@bom.gov.au

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