Study On The Anomaly Of Stratospheric Polar Vortex And Its Impact On Troposphere

The troposphere and stratosphere interaction is a very basic subject in atmospheric science. The traditional view is that the stratosphere is a passive recipient of energy and waves from weather systems in the underlying troposphere, but recent evidence suggests there are feedback processes that stratospheric anomalies in turn alter weather patterns in the troposphere. In this paper the characteristics and abnormal changes of stratospheric polar vortex were studied by using isentropic potential vorticity (IPV) diagnosis. The influence on tropospheric circulation and region climate, especially focusing on the weather variations of China were discussed, and the mechanism of the downward propagation of stratospheric anomalies were explored with the view of potential vorticity conserves on isentropic surfaces. In addition, the relationships between polar vortex and planetary waves were analyzed by using E-P flux. More than that, the NCAR community atmosphere model CAM 3.0 was used to simulate the stratospheric polar vortex variations, compare with the observational data and investigate the influence on troposphere. The following results and new recognition were obtained.Firstly, IPV was calculated by NCEP reanalyzed data, the mean climate characteristics and the anomalies of polar vortex were analyzed, it suggested that the strong vortex collapsed later while the weak vortex collapsed earlier. Moreover, the principal mode of polar vortex and Northern Annular Mode (NAM) index were obtained from the leading EOF spatial pattern and EOF time series of geopotential height. The interdecadal variations of NAM index as well as the interdecadal variations of IPV indicated the increased frequency of weak polar vortex since the end of 20th century.Secondly, the strong and weak polar vortexes were separated by NAM index, and then strong polar vortexes were divided into two patterns with respect to the position of high IPV center over Northeast Asia (NEA) and North America (NA), respectively. According to the different patterns of anomalies, the circulation and regional weather in troposphere were discussed. The anomaly of stratospheric polar vortex led to the lag response of geopotential height and surface temperature, but the effect of each pattern was different. During weak polar vortex, East Asia was warmer except some north regions and the warming spread from north to south gradually, but it is colder during strong polar vortex, and the cold center moved towards the south and brought about more influence in the south region in the pattern of NEA. The effect on North America was different from East Asia, especially when the center of stratospheric polar vortex was close to it during strong polar vortex, North America trough strengthened but East Asia trough weakened, the drop in temperature in the northeast of Northeast America was larger, wider, longer and recovered more slowly.Thirdly, the dry adiabatic air should moved along isentropic surface and its potential vorticity would conserved, this principle was used to explore the downward progress and mechanism of stratospheric polar vortex anomalies. The vertical structure of potential temperature and potential vorticity were obvious different between strong and weak vortex, the isentropic surfaces were higher in north polar regions and descended towards middle latitudes during strong polar vortex, therefore when polar vortex extended towards the south, the cold air with high potential vorticity would move along declining isentropic surfaces and bring the cold air from higher polar regions to lower middle latitudes, even to low latitudes in troposphere to form a sunken high potential vorticity center. While the isentropic surfaces were lower in north polar regions and ascended towards middle latitudes during weak polar vortex, cold air with high potential vorticity was limited to polar regions and surrounding middle latitudes, the descending air would bring high potential vorticity to higt latitudes so that it is difficult to form and keep high potential vorticity center in mid or low latitudes in troposhere. The pattern of polar vortex in January, 2008 was NEA, the meridianal extension of high potential vorticity air from polar regions reached to low latitudes in troposphere above South China, high potential vorticity center formed and developed, obviously related to the widen snow calamity at the same time in South China. The strong polar vortex in January, 2009 had some influence on the cold weather in North China, but vortex split and became very weak soon after, stratospheric high potential vorticity air extended poleward, cold air was limited to arctic and surrounding regions and could not expand towards the south, so that it is warm in South China.Fourthly, the amplitude of planetary waves of wave-number 1 (wave-1) and wave-number 2 (wave-2) have several types of variance during weak polar vortex. In February, 2009, a mass of upward propagation of wave-2 played very important role for causing weakened polar vortex and SSW. The amplitude of wave-1 and wave-2 had a sudden increase during mid and late strong polar vortex, its increase appeared earlier in the pattern of the strong polar vortex in NEA than that in NA, and downward propagation of planetary waves was identified by the vertical variance of amplitude. The analysis of E-P flux suggested upward propagation of wave-1 strengthen at the end of strong vortex and meanwhile E-P flux of wave-2 converged in high latitude regions in stratosphere.Fifthly, the anomalies of stratospheric polar vortex in winter were simulated by CAM 3.0. The consistency of simulation and observation illustrates the model is applicable to simulate the variance of polar vortex and its regional impact in troposphere. The investigation of model result indicated that abnormal polar vortex led to different vertical structure of potential vorticity, dry adiabatic air with high potential vorticity descended along oblique isentropic surfaces and effected troposphere is one way of the downward control from stratosphere. Other than that, planetary waves and mean flows were discussed to elaborate different upward propagations of waves between strong and weak vortexes, and different range of influence on mean flow in stratosphere between wave-1 and wave-2. The variance of mean flow could propagate downward gradually, for some time it reached to surface and effected atmospheric circulation and weather progress.At last, we summed up the limitations of this paper and conceived the further research work.