Recent stratospheric warmings at mid-latitudes and planetary-scale wave activity

Summary

Recent stratospheric warmings are herewith examined in association with the influence of long-period planetary waves. Investigations on height and time variations of geopotential and wind data obtained in the winters of 1980–87 at different tropo-stratospheric levels from messages of the radiosondes situated in the Central Europe take necessarily into account the propagation of quasi-stationary and free planetary waves which in proper conditions mutually interfere with each other. The definition and evolution of the different types of warming are connected with the structure of these waves and enable one to interpret more completely coupling phenomena among different regions of the neutal atmosphere during the most disturbed winters. Distinct spectral analysis methods permit to extract from geopotential data series prominent wave periods within 1÷3 weeks. Westerly winds in the stratosphere reach unusual amplitudes before a sudden stratospheric warming, and this amplification is seen to be in connection with transient normal modes which are able to interact even with the mean circulation of the mesosphere and lower thermosphere.