STRATOSPHERIC CIRCULATION OF THE NORTHERN-HEMISPHERE WINTER DURING 1989-90

The stratospheric circulation in winter 1989/90 is investigated with the aid of the transformed Eulerian mean (TEM) equations. The winter 1989/90 is characterized by a very cold mid-winter and a pronounced minor warming following it. The sudden warming was caused by the planetary wave of zonal wavenumber 1 vertically propagating from the troposphere. The temperature increase itself was fairly large but the westerly polar night jet did not reverse to the easterly at the lower stratosphere. This is consistent with the hypothesis shown by LABITZKE and VAN LooN (1988) that major mid-winter warmings are hard to occur in the easterly phase of the quasi-biennial oscillation (QBO) during weak solar activity.