Intensified Impact of the Equatorial QBO in August–September on the Northern Stratospheric Polar Vortex in December–January since the Late 1990s

This study reveals an intensified impact of the equatorial quasi-biennial oscillation (QBO) in August–September (QBO_AS) on the northern stratospheric polar vortex (SPV) in December–January (SPV_DJ) since the late 1990s. The unstable relationship may be related to the differences in the deep convection anomaly over the tropical western Pacific and Indian Oceans in October–November (ON) related to the QBO_AS prior to and after the late 1990s. During 1998–2017, the easterly phase of the QBO_AS is accompanied by a colder tropical tropopause in ON, which enhances the deep convective activity over the tropical western Pacific and suppresses it over the Indian Ocean. The deep convection anomaly generates anomalous Rossby waves that propagate into the northern mid-to-high latitudes to constructively interfere with the climatological wavenumber-1 and wavenumber-2 components, thereby resulting in enhanced upward-propagating tropospheric planetary-scale waves and a weakened SPV_DJ anomaly. During 1979–1997, however, the deep convection anomaly over the tropical western Pacific and Indian Oceans in ON related to the easterly phase of the QBO_AS is weaker and shifts eastward, which excites the anomalous Rossby waves to constructively/destructively interfere with the climatological wavenumber-1 component in the midlatitudes/high latitudes, thereby weakening the upward-propagating planetary-scale waves and leading to a weaker linkage with the SPV_DJ. Further analyses reveal that the unstable relationship may be associated with the interdecadal differences in deep convection over the tropical western Pacific and Indian Oceans and the upward-propagating tropospheric planetary-scale waves in ON.