The Classification And Characteristics Of Stratospheric Northern Annular Model Anomalies And Its Relationship With The Troposphere

The daily NCEP/NCAR reanalysis data and northern annular mode index have been used to divide the stratospheric northern annular mode into two types. Then the developmental and diagnostic differences of two types , and its relation with wave-flow interaction are measured.The major results are as follows: the features of anomalies are very similar in the stratosphere, while great differences appeared in the troposphere. During type S, the zonal mean wind anomalies have the tendency of downward propagating from the stratosphere to the troposphere, and have continuity in the two levels. While during type D, the downward propagating trend disappeared, all the anomalies of zonal mean wind are confined to the stratosphere, at the same time, it displayed opposite anomalies in the troposphere.In the latitude-height cross sections of zonal mean anomaly fields, zonal mean winds also have the continuity in the stratosphere and troposphere during type S, and the critical wind velocity line run through the two levels. Moreover, the maximum wind speed axis sloped towards high latitude with height. While its features during type D have distinct differences with type S, such as, zonal mean wind anomalies are opposite in the stratosphere and troposphere. At the same time, the critical wind velocity line was confine to the stratosphere.Different types of NAM anomalies are closely correlated with the wave activity and the structure of zonal-mean flow. During type S, the activity of wave advanced the downward propagation of stratospheric anomalies, it promotes the continuity between stratosphere and troposphere. While the wave activity during type D inhibited the propagation of NAM in the stratosphere, so the feature of atmospheric circulation showed inverse trend, and it was especially clear in the later period of the whole events. By means of analyzing the developmental process of stratospheric polar warming, which is closely related to the stratospheric NAM anomalies, we can conclude that wave forcing and the transporting of energy from the troposphere is crucial to the propagation of polar warm temperature anomalies. If the wave forcing in the stratosphere is very strong and can arrive lower levels, meanwhile the heat can transport from the troposphere to the stratosphere, the polar warming can spread downward. Otherwise it is difficult for the downward propagation of stratospheric polar warming. In this regard, it is similar to the stratospheric NAM.Generally speaking, whether an upper-stratospheric anomaly propagates down depends on how planetary waves interact with stratospheric zonal flow. Specifically, there are three elements that are important for downward propagation. First, the wave forcing must be strong enough to reverse the polar westerlies below certain levels. Second, after the wind reverses, the critical line must descend to lower altitudes. Because waves cannot propagate upward in easterly winds, the descending critical line forces wave energy to be deposited at lower altitudes. Consequently, the warm anomaly gradually propagates downward. But this mechanism of wave-mean flow interaction only works above the lowest altitude of the critical line. For further downward propagation, the third element that is required is continuous wave transport into the polar stratosphere.At last, in the geopotential height and temperature anomaly fields, we can see that the most remarkable characteristics in type S are the inverse phase oscillation between polar region and middle latitude in the horizontal direction, and the equivalent barotropic structure from the stratosphere to the troposphere in the vertical direction. Therefore, the type S demonstrated a typical feature of Arctic Oscillation. While in the type D, it has similar features in the upper-stratospheric with S type, while the prominent discrepancy reflected in the lower-stratosphere and the troposphere. First, the inverse phase oscillation structure disappeared, the height in two levels even emerged entirely opposite anomalies. Second, the equivalent barotropic structure disappeared too, the continuity faded away along with the time.