The Southern Hemisphere Sudden Stratospheric Warming in September 2019
and its predictions in S2S Models
Abstract
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.