The influence of tropical oscillations on the thermodynamics of the middle and upper
atmosphere at high latitudes was studied using a nonlinear model of the general circulation
of the middle and upper atmosphere (MUAM). The observed oscillations include the
quasi-biennial oscillation of the zonal wind in the equatorial stratosphere (QBO) and
the El Niño–Southern Oscillation (ENSO). The main focus of this work is to study the
influence of these oscillations on the strength and persistence of the stratospheric polar
vortex. Four ensemble calculations were carried out (10 runs for each QBO and ENSO
phase combination) for January–February. It was shown that the polar vortex and Eliassen–
Palm (EP) flux divergence were especially strong under La Niña and the westerly QBO
phase (wQBO). This was accompanied by a strengthening of the residual mean circulation
(RMC) from the summer to the winter hemisphere, causing positive temperature anomalies
in the polar mesosphere and negative anomalies in the stratosphere. The greatest RMC
weakening and the weakest and warmest polar vortex occurred during El Niño and eQBO
conditions in January and during El Niño and wQBO conditions in February. Such diverse
manifestations of tropical oscillations via teleconnections can provide valuable information
for predicting the frequency and intensity of sudden stratospheric warmings (SSWs) and
subsequent extreme cold wave events in the troposphere. Specifically, SSWs are the least
probable during La Niña and wQBO conditions in both January and February. The QBO
phase most significantly influences the polar vortex during El Niño events in both months.
We conclude that SSW development is more favorable during eQBO in January and wQBO
in February under El Niño conditions.