We analyze the relationships between strato- spheric polar vortex anomalies and cooling events in eastern China using isentropic reanalysis data from the European Center for Medium-Range Weather Forecasts. Daily mean data from 2000 to 2011 are used to explore the effective stratospheric signals. First, diagnoses of the 2009/2010 winter show that after the stratospheric sudden warming （SSW） of the Atlantic-East Asian （AEA） pattern, the stratospheric high isentropic potential vorticity （IPV） center derived from the split polar vortex will move to the northeast of the Eurasian continent. The air mass, accom- panied by some southward and eastward movements and characterized by high IPV values, will be stretched verti- cally, leading to apparent reinforcements of the positive vorticity and the development of acold vortex system in the troposphere. The northerly wind on the western side of the cold vortex can transport cold air southward and down- ward, resulting in this distinct cooling process in eastern China. Secondly, the empirical orthogonal function ana- lyses of IPV anomalies on the 430 K isentropic surface during 2000-2011 winters indicate that the IPV distribution and time series of the first mode are able to represent the polar vortex variation features, which significantly influ- ence cold-air activity in eastern China, especially in the AEA-type SSW winter. When the time series increases significantly, the polar vortex will be split and the high-IPV center will move to the northeast of the Eurasian continent with downward and southward developments, inducing obvious cooling in eastern China. Moreover, all the four times SSW events of AEA pattern from 2000 to 2011 are reflected in the first time series, and after the strong polar vortex disturbances, cooling processes of different inten- sities are observed in eastern China. The cooling can sus- tain at least 1 week. For this reason, the first time series can be used as an available index of polar vortex oscillation and has the power to predict
The influences of tropospheric blocking high on the stratospheric sudden warming (SSW) and the SSW-induced feedback on the lower atmosphere are analyzed with NCEP (National Center for Environmental Prediction) 2 reanalysis data. Daily mean data from 1979 to 2010 are used to perform statistical and dynamical analyses. According to different distribution features of polar vortex, which can be ascribed to different activities of blocking highs, we have obtained two warming patterns in vortex splitting and displacement patterns. For vortex splitting events, in the Eurasian-North American (ENA) paratype, with disturbances of Atlantic and Aleutian blocking highs, polar vortex is split into two parts that locate at Eurasian and North American continents respectively, while in the Atlantic-East Asian (AEA) paratype, two low-pressure centers derived from the split vortex are situated in the Atlantic and East Asian regions, and two blocking systems occurring in the Urals and North American areas precede these splitting processes. For vortex displacement events, in the Aleutian-Intrusion (AI) paratype, the polar vortex is displaced to the west European and Atlantic areas by the intrusive Aleutian high and this pattern always corresponds to the blocking events occurring in the Pacific basin only. Similarly, the vortex is pushed to the west Eurasian continent by the intrusive North American high-pressure system in the North American-Intrusion (NAI) paratype, which is closely related to the blocking over these areas. The second subject of the research is that whether the anomalous stratospheric signals can be propagated to the lower atmosphere, which is depended on the intensity, duration and position of the disturbed vortex. According to our case studies, geopotential height anomalies can be propagated to the troposphere in strong SSW years, taking about 10-15 d for the decrease from 10 to 500 hPa, leading to apparent variations in the geopotential height and temperature fields.
We analyze the decadal variation of the stratosphere troposphere coupled system around the year 2000 by using the NCEP reanalysis-2 data.Specifically,the relationship between the Northern Hemisphere Annular Mode（NAM） and the tropospheric East Asian trough is investigated in order to find the effective stratospheric signals during cold air outbreaks in China.Statistical analyses and dynamic diagnoses both indicate that after 2000,increased stratospheric polar vortex disturbances occur and the NAM is mainly in negative phase.The tropospheric polar areas are directly affected by the polar vortex,and in the midlatitudes,the Ural blocking high and East Asian trough are more active,which lead to enhanced cold air activities in eastern and northern China.Further investigation reveals that under this circulation pattern,downward propagations of negative NAM index are closely related to the intensity variation of the East Asian trough.When negative NAM anomalies propagate down to the upper troposphere and reach a certain intensity（standardized NAM index less than 1）,they result in apparent reinforcement of the East Asian trough,which reaches its maximum intensity about one week later.The northerly wind behind the trough transports cold air southward and eastward,and the range of influence and the intensity are closely associated with the trough location.Therefore,the NAM index can be used as a measure of the signals from the disturbed stratosphere to give some indication of cold air activities in China.
