A151-0018
Wave Guide of the 2019 Stratospheric Sudden Warming and Tropospheric Double Jets in the Southern Hemisphere
Wave Guide of the 2019 Stratospheric Sudden Warming and Tropospheric Double Jets in the Southern Hemisphere
Monday, 14 December 2020
Poster
Abstract:
This study investigates observational features of the displacement-type minor sudden stratospheric warming (SSW) event associated with double jets in the troposphere which occurred in the Southern Hemisphere (SH) in September 2019. The primary tools employed here are zonal wind, temperature, and Eliassen-Palm (EP) flux diagnostics based on the transformed Eulerian-mean formulation from daily-averaged JRA-55 reanalysis data. Daily climatologies are created for averages on a calendar date during 1979-2019, after applying Lanczos lowpass-filter with a cutoff period of 60 days.
SSW events are quite rare in SH, with only two in recorded times in 1988 and 2002, even if minor events are included. The 2019 SSW is characterized as the strongest Antarctic warming on record with a maximum polar temperature of 264 K at 10 hPa, 80-90S on 19 September and by the strongest upward EP flux at 100 hPa with zonal wavenumber 1 in August-to-September. In September 2019 the SH circulation exhibits large variability in the troposphere and stratosphere, associated with double jets centered around 70S and 30S in the upper troposphere, intensified in 3-11 September. The eddy driven jet around 70S is located in anomalously higher latitudes than that observed in other years, i.e., 60S. The vertical profile of momentum budget at 60-70S during this period shows that the overall balance between EP flux divergence and the Coriolis force due to the induced residual mean meridional circulation holds throughout the troposphere and that their small residuals result in westerly acceleration of the eddy driven jet; note that EP flux divergence occurred in the tropopause layer, while EP flux convergence was observed in the mid troposphere but the contribution of the Coriolis force slightly exceeded that of the EP flux convergence there. These results imply that the poleward movement of the synoptic wave-induced meridional circulation causes a change in the mean flow geometry in the subpolar mid-troposphere, and in the layer adjacently above the anomalously modulated westerly associated with the eddy driven jet is capable of emanating wave activities away equatorward as well as upward. Therefore, we find that this upward wave activity leads to the strongest stratospheric warming.
SSW events are quite rare in SH, with only two in recorded times in 1988 and 2002, even if minor events are included. The 2019 SSW is characterized as the strongest Antarctic warming on record with a maximum polar temperature of 264 K at 10 hPa, 80-90S on 19 September and by the strongest upward EP flux at 100 hPa with zonal wavenumber 1 in August-to-September. In September 2019 the SH circulation exhibits large variability in the troposphere and stratosphere, associated with double jets centered around 70S and 30S in the upper troposphere, intensified in 3-11 September. The eddy driven jet around 70S is located in anomalously higher latitudes than that observed in other years, i.e., 60S. The vertical profile of momentum budget at 60-70S during this period shows that the overall balance between EP flux divergence and the Coriolis force due to the induced residual mean meridional circulation holds throughout the troposphere and that their small residuals result in westerly acceleration of the eddy driven jet; note that EP flux divergence occurred in the tropopause layer, while EP flux convergence was observed in the mid troposphere but the contribution of the Coriolis force slightly exceeded that of the EP flux convergence there. These results imply that the poleward movement of the synoptic wave-induced meridional circulation causes a change in the mean flow geometry in the subpolar mid-troposphere, and in the layer adjacently above the anomalously modulated westerly associated with the eddy driven jet is capable of emanating wave activities away equatorward as well as upward. Therefore, we find that this upward wave activity leads to the strongest stratospheric warming.