اثر گرمایش ناگهانی پوشن‌سپهر در تغییرات ارتفاع وردایست گرمایی در نیمکره شمالی(2020-1979)

A sudden stratospheric warming (SSW) represent large scale perturbations of the polar winter stratosphere, which substantively influence the temperature and circulation of the middle atmosphere and also the contents of atmospheric species. SSW occurs mostly in middle and late winter and almost exclusively in the Northern Hemisphere. During an event, the polar stratospheric temperature increases by several tens of degree Celsius within a few days and eventually becomes warmer than that of the mid latitudes, reversing the climatological temperature gradient. At the same time, the prevailing westerly wind speed decreases rapidly and becomes easterly.
The tropopause is a transition layer between the troposphere and the stratosphere. The occasional exchange of air, water vapor, trace gases, and energy between the troposphere and the stratosphere occurs in this layer. Based on some concepts; two different tropopause in the name of thermal tropopause and dynamical tropopause are defined. The conventional definition is the thermal tropopause which is detected based on the mark disruption of the vertical temperature lapse rate. The thermal tropopause definition is based on the fact that the stratosphere is more stably stratified than the troposphere. The thermal tropopause is defined as the lowest level at which the lapse rate decreases to 2 K/km or less, provided that also the average lapse rate between this level and all higher levels within 2 km does not exceed 2 K/km. The original concept of the dynamical tropopause was based on the isentropic gradient of potential vorticity. The dynamical tropopause is typically determined in a thin layer with absolute PV values within 1 pvu and 4 pvu.
The vertical temperature stratification of the atmosphere plays a basic role in atmospheric motions. In this paper, the Brunt–Väisälä frequency (N²) value is used to detect the change of stratospheric static stability. In this work the NCEP/NCAR reanalysis daily data including temperature at different pressure levels (1000hPa-10hPa), the tropopuse temperature and pressure from 1th of January 1961 to 31th of December 2020 in northern hemisphere are used. The study region covers 0° to 357.5° geographical longitudes and 0°N to 90°N geographical latitudes. The northern hemisphere is divide into three 30° none overlapping latitudinal band width called as the tropical bands (0°N-27.5°N), the middle latitude bands (30°N-57.5°N) and polar bands (60°N-90°N) regions. First of all the potential temperature and Brunt-Väisälä frequency (N²) at different pressure levels are calculated; then the average zonal mean temperatures at 10hPa, the tropopause temperatures, the tropopuse pressures and the values of N² in three former introduced regions are obtained. To represent the tropopuse's height variations during the sudden stratospheric warming, the daily anomaly of these parameters in the regions are calculated and analyzed.
The daily average mean zonal tropopause temperatures and pressure changes in the three meridian divided regions during eighteen major and one minor sudden stratospheric warming (SSW) events are analyzed in this study. The results show that all 19 SSW events in the statistical period of 1979-1920 are associated with positive anomaly of the zonal mean temperature and pressure of tropopuse along with increase of the tropopuse temperature and lowering its height which causes downward development of the stratosphere and thinning the depth of the troposphere. In addition, the tropopuse height reduction in the polar band region is greater than in the middle latitude band. It was also shown that, the static stability (positive anomaly) increment in the stratosphere started before the SSW and decreases during SSW (negative). These changes are greater in the polar cap band with respect to the middle latitudes band. This result reveals that the static stability structure in the lower stratosphere and upper troposphere in the polar cap are more affected by SSW with respect to other regions.