Response of the mesopause region dynamics to the February 2001 stratospheric warming

Abstract

The response of the mesosphere/lower-thermosphere (MLT) region to a major stratospheric warming in Europe during winter 2000/2001 has been investigated using mesopause-region winds measured by meteor radar or the LF-D1 method over three stations (Castle Eaton, 52°N; Collm, 52°N; and Esrange 68°N). The vertical wind structure measured over the three sites, and its time evolution, are found to be quite similar despite the different techniques used in the measurements. The effects of stratospheric warming are very clear, and are similar over both the mid-latitude and high-latitude sites. The warming resulted in a reversal of both the zonal and meridional wind. In the zonal component, this reversal was apparently associated with a planetary-wave oscillation with a period of ∼10 days. The effect was most conspicuous in the vertical prevailing wind gradients. The mesopause-region effects thus seem to be the results of a superposition of an intensifying planetary wave and a slow overall decrease in the strength of the zonal prevailing winds.

Introduction

Stratospheric warmings can dramatically change the structure of the entire middle-atmosphere wind and temperature fields up to the height of the mesosphere/lower thermosphere (MLT) region. This is particularly so in the case of major warmings, which occur when the meridional temperature gradient in the winter high-latitude stratosphere actually reverses as part of a complete breakdown of the stratospheric polar vortex.

The effects of stratospheric warmings on the MLT region have been observed since the 1970s, when measurements of pressure and wind variations revealed a connection between these two levels of the atmosphere (e.g. Lysenko et al., 1975; Lauter and Schminder, 1976). These MLT-region reactions to stratospheric warmings have been interpreted as representing a breakdown of the winter circulation and a return to quasi-summer conditions (Lauter and Entzian, 1982; Von Cossart et al., 1982). A number of subsequent studies have examined the influence of stratospheric warmings on MLT-region circulation. Often, these have taken the form of case studies examining time series or selected profiles of mesopause-region mean winds measured during one or a few winters. These studies have frequently reported that stratospheric warmings lead to a decrease in the westerly zonal prevailing wind, or even to a wind reversal (Gregory and Manson, 1975; Schminder and Kürschner 1981a, Schminder and Kürschner 1981b, Schminder and Kürschner 1990; Greisiger et al., 1984; Muller et al., 1985; Lysenko et al., 1990; Kazimirowsky, 1994; Jacobi et al., 1997). Some studies have also reported changes in the meridional prevailing winds (Schminder and Kürschner, 1981b; Muller et al., 1985).

The semidiurnal tide is arguably the most conspicuous feature of the dynamics of the mid- and high-latitude MLT region. However, in contrast to the distinct response of the MLT-region mean winds to stratospheric warmings, the influence that such warmings have on the semidiurnal tide is less well understood. Lauter and Schminder (1976) reported that the amplitude of the semidiurnal tide increased during the periods when MLT-region pressure fluctuated in connection with stratospheric warmings. Schminder and Kürschner (1981b) presented examples when the semidiurnal tidal phase changed during stratospheric warmings, although in other years the tide seemed not to be influenced by the warming events (Schminder and Kürschner, 1981a; Manson and Meek, 1985).

Kazimirowsky (1994) reported measurements of winds over Siberia and showed that, in some years, the phase of the semidiurnal tide increased during the time when the MLT-region zonal winds decrease or reverse during a stratospheric warming. However, he also noted that the particular interaction between stratosphere and upper mesosphere during stratospheric warmings depends on the peculiarities of the specific warming event. Jacobi et al. (1997) also noted this effect in several case studies of the effect of major stratospheric warmings on the MLT-region winds measured over Collm. These latter authors considered simultaneous measurements of stratospheric temperature and mesopause region winds and reported that the stratospheric warmings regularly are indeed connected with a strongly disturbed zonal wind field in the MLT region, but that the breakdown of the stratospheric polar vortex is not necessarily connected with the observation of easterly zonal winds in the upper mesosphere and lower thermosphere.

At greater altitudes, strong westerly winds are measured during some stratospheric warmings—which may be called a “compensation” effect. This effect may result from the vertical structure of stratospheric warmings. In particular, an upward motion can occur above the level of the strongest poleward acceleration due to planetary-wave/mean-flow interaction, which may lead to a cooling of the lower thermosphere, and subsequently to strong westerlies. Alternatively, changes in the filtering of ascending gravity waves due to the reversed flow in the stratosphere may lead to easterly (westward) winds in the lower thermosphere. Jacobi et al. (2001) showed that during the occurrence of this compensation effect the stratospheric warming influence on the MLT region also varies with longitude.

In contrast to the behaviour of the stratosphere, the “compensation effect” during (and also after) stratospheric warmings may lead to a particularly cold upper mesosphere at high latitudes during stratospheric warmings (Labitzke, 1972). Thus, the easterly winds of the disturbed stratosphere may decrease with height, and particularly strong westerly winds in the mesopause region may actually be observed in some cases. In fact, a recent comprehensive study using radar measurements made over 12 years found that the particular response of the MLT-region to individual warming events differed notably from case to case, and moreover, varied significantly with latitude and longitude (Hoffmann et al., 2002). Hence, although in many cases the beginning of a stratospheric warming is marked by a decrease in the prevailing westerly zonal wind of the upper mesosphere, this observation probably cannot be used to indicate further development of the stratospheric warming event in the course of the following days or weeks.

In summary, while the basic behaviour of stratospheric warmings is reasonably well understood, their effect on the MLT region is still uncertain, and highlights the need for additional measurements and case studies. In the following work, we shall present MLT-region wind measurements made over three European sites during the February 2001 major stratospheric warming event.