Abstract
Reanalysis data are used to study the El Niño–Southern Oscillation (ENSO) signal in the troposphere and stratosphere during the late fall to midwinter period. Warm ENSO events have extratropical tropospheric teleconnections that increase the wave 1 eddies and reduce the wave 2 eddies, as compared to cold ENSO. The increase in wave 1 overwhelms the decrease in wave 2, so the net effect is a weakened vortex. This modification in tropospheric wave forcing is induced by a deepening of the wintertime Aleutian low via the Pacific–North America pattern (PNA). Model results are also used to verify that the PNA is the primary mechanism through which ENSO modulates the vortex. During easterly Quasi‐Biennial Oscillation (EQBO), warm ENSO does not show a PNA response in the observational record. Consequently, the polar vortex does not show a strong response to the different phases of ENSO under EQBO, nor to the different phases of QBO under WENSO. It is not clear whether the lack of a PNA response to warm ENSO during EQBO is a real physical phenomenon or a feature of the limited data record we have.
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