Impact of downward propagating planetary waves associated with sudden stratospheric warming on the extratropical troposphere: zonal wavenumber 1 component

To elucidate dynamics of significant downward propagation (SDP) events of zonal wavenumber 1 (WN1) planetary waves from the stratosphere after sudden stratospheric warming (SSW) events and their influence on the extratropical troposphere, a case study of an SDP event in March 2023 and composite analyses for SDP events with and without SSW using JRA-3Q reanalysis are conducted.The March 2023 event is characterized by the equatorial propagation of enhanced WN1 components in the troposphere, which follows the WN1 downward propagation after the SSW. Exceptionally warm anomalies in East Asia including Japan in early March are associated with the equatorial propagation. A statistical investigation on the timing of all SDP and SSW events reveals that an SDP event is significantly more likely to occur after an SSW. The composite of the SDP event with SSW is characterized by stratospheric easterlies, in contrast to that without SSW, in which westerlies prevail in the stratosphere. Downward propagated WN1 components in the troposphere propagate equatorward and produce temperature anomalies in the extratropics, depending on the WN1 phase in high latitudes for both SDP events. When the WN1 ridge is positioned around the date line in SDP events with SSW, cold anomalies tend to cover East Asia including Japan. The WN1 component during the March 2023 event has the largest amplitude among SDP events with SSW and a ridge location around 90°W, which is far apart from the rest. These peculiar characteristics of the WN1 component would contribute to the extraordinary warm anomalies near Japan.