The dominant intraseasonal variability mode of tropical convection in the 2 weeks before a Northern Hemisphere extreme stratospheric polar vortex.

The Madden–Julian Oscillation (MJO) phase 7 has been recognized as a driver for Northern Hemisphere (NH) sudden stratospheric warmings (SSWs) in the literature recently. However, uncertainties remain on the exact relationship between MJO phase 3 and the stratospheric polar vortex. Here, we re‐examined and compared the observed evolution of the MJO before the two types of NH extreme stratospheric polar vortex events (i.e. strong vortex (SV) events and weak vortex (WV) events) in this study. Our analysis reveals that MJO phase 3 (7) is the dominant intraseasonal variability mode in about 2 weeks before onset of the SV (WV) events, suggesting these two phases are closely linked to variation in the intensity of the stratospheric polar vortex. MJO phase 3 (7) leads to a strengthened (weakened) polar vortex due to the suppressed (enhanced) wave‐number 1 wave activity in the extratropical stratosphere resulting from destructive (constructive) interference between the negative (positive) Pacific–North American (PNA)‐like pattern induced by MJO phase 3 (7) and the background field. Note that the time‐scales for the peak responses of wave activity in the extratropical stratosphere to MJO phases 3 and 7 are around 2 weeks and 1 week, respectively. Thus, within 2 weeks, MJO phase 3 (7) can be treated as the precursor to the strengthened (weakened) polar vortex. [ABSTRACT FROM AUTHOR]