عنوان مقاله [English]
A major sudden stratospheric warmings (SSWs) represent large-scale perturbations of the polar winter stratosphere, which substantively influence the temperature and circulation of the middle atmosphere and contents of atmospheric species. There are two types of sudden stratospheric warmings: minor warmings and major warmings. In a minor (major) warming the temperature gradient reverses over a range of altitude at or below 010hPa and zonal wind at 010hPa weakens but does not change its direction (reverse direction). Moreover, SSWs have been classified into vortex displacement events and vortex splitting events based on the shape and continuity of the polar vortex.
When a major sudden stratospheric warmings events occur, the zonal mean temperature at 10hPa around the polar cap for latitudes north of 60°N suddenly rises and increases by the 25K over period of several days and zonal-mean zonal wind reverses at 10hPa and 60oN during winter from November to March. In this paper, we consider just the major sudden stratospheric warming effect with displacement or split of the polar vortex toward mid latitudes in 1979-2019.
We have used the daily mean data from the MERRA2 assimilated data and NCEP-NCAR reanalysis data. From the MERRA2 data, zonal mean zonal wind and zonal mean temperature were obtained at 010hPa from 01 January 1979 to 04 June 2020.The temperature is averaged around the polar cap for latitudes north of 60°N and zonal wind is averaged for 60°N. From the NCEP–NCAR data, geopotential height is presented by a horizontal resolution of 2.5° ×2.5° at 10hPa from 1 January 1949 to 31 December 2019.
We first examined the time evolution of zonally averaged temperature and zonal wind at 60°N and 010hPa to identify the warmings onset day, zero day and decay day for the trace of SSWs and its relation with displacement or splitting of the polar vortex. Then we determined three periods: before and after the zero day and the time of recovery for warming event. Also we determined the threshold center latitude for investigating the polar vertex in 1949-2019. Finally, we determine the most probable directions of the polar stratospheric vortex displacement or splitting as a result of major SSWs.
Investigation of the zonal mean temperature and zonal-mean zonal wind at 10hPa detected 19 and 13 major and minor SSWs events respectively. From the total number of major SSWs (19), 11(58%) were classified as warmings with splitting of the stratospheric vortex and 8(44%) as warmings with displacement of the vortex. The mentioned 8 cases show that the polar vortex is displaced to the south and weakened after shifting and the other 11 cases show that the polar vortex is split into two cells. Of these 11 cases, 8 cases are of the complete splitting type and 3 cases are of the incomplete splitting type.
The SSWs events of 11 February 2001 (displacement type), 8 December 1987, 15 December 1998, 15 January 2004 (incomplete split type), 21 February 1989, 26 February 1999 and 24 January 2009 (complete split type) were selected to represent displacement, incomplete split and complete split type, respectively. During vortex displacements the polar vortex is shifted off the pole and during vortex splits the polar vortex is split into two pieces of comparable size.