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The Southern Hemisphere Sudden Stratospheric Warming in September 2019 and its predictions in S2S Models
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  • Jian Rao,
  • Chaim I Garfinkel,
  • Ian White,
  • Chen Schwartz
Jian Rao
Nanjing University of Information Science and Technology

Corresponding Author:raojian@nuist.edu.cn

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Chaim I Garfinkel
Hebrew University of Jerusalem
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Ian White
Hebrew University of Jerusalem
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Chen Schwartz
Hebrew University of Jurusalem
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A sudden stratospheric warming (SSW) happened in September 2019 in the Southern Hemisphere (SH) with winds at 10hPa, 60°S reaching their minimum value on September 18. The evolution, favorable conditions, and predictability for this SSW event are explored. The favorable conditions include easterly equatorial quasi biennial oscillation (QBO) winds at 10hPa, solar minimum, positive Indian Ocean Dipole (IOD) sea surface temperatures (SST), warm SST anomalies in the central Pacific, and a blocking high near the Antarctic Peninsula. The predictive limit to this SSW is around 18 days in some S2S models, and more than 50% of the ensemble members forecast the zonal wind deceleration in reforecasts initialized around 29 August. A vortex slowdown in evident in some initializations from around 22 August, while initializations later than 29 August capture the wave-like pattern in the troposphere. The ensemble spread in the magnitude of the vortex deceleration during the SSW is mainly explained by the ensemble spread in the magnitude of upward propagation of waves, with an underestimated tropospheric wave amplitude leading to a too-weak weakening of the vortex. The September 2019 SH SSW did not show a near-instantaneous downward impact on the tropospheric southern annular mode (SAM) in late September and early October 2019. The Australian drought and hot weather in September possibly associated with the positive IOD might have been exacerbated by the negative SAM in October and later months due to the weak stratospheric polar vortex. However, models tend to forecast a near-instantaneous tropospheric response to the SSW.
27 Jul 2020Published in Journal of Geophysical Research: Atmospheres volume 125 issue 14. 10.1029/2020JD032723