The western Pacific pattern precursor of major stratospheric sudden warmings and the ENSO modulation

Previous studies show that there is an apparent inconsistency between the impact of opposite phases of El Nino-Southern Oscillation (ENSO) on the seasonal-mean vortex and on major stratospheric sudden warmings (SSWs): El Nino (EN) leads to an anomalously warm, and La Nina (LN) leads to an anomalously cool seasonal-mean polar stratospheric state, but both phases of ENSO lead to an increased SSW frequency. This study examines the dynamics underlying this apparent paradox. First, this study demonstrates that 30 out of 35 SSWs occurred in 1957/1958-2012/2013 winters may be triggered by the western Pacific patterns (WPs). WPs can interact constructively with the climatological stationary waves and enhance wavenumber-1 waves and the associated upward wave activity fluxes, which leads to the occurrence of SSWs. This finding suggests that WPs may serve as a good precursor of SSWs. Furthermore, this study also demonstrates that WPs have larger amplitudes and longer durations during EN winters than ENSO neutral (NE) winters, which contributes dominantly to the increased frequency of SSWs from NE winters to EN winters. While for LN winters, both the ENSO modulation on WPs and the blockings over the eastern Pacific contribute to the SSW frequency increase from NE winters to LN winters. Finally, the question how EN modulates WPs is explored. It is found that more WPs with larger amplitude and longer duration develop from PNAs during EN than NE winters, which contributes to the relatively larger amplitude and longer duration of WPs observed in EN, compared to NE.