Characteristics Of Zonal Wind In Lower Stratosphere And Diagnosis Of A Special Category Of Stratospheric Sudden Warming Events

Based on the ERA-40 and ERA Interim reanalysis data, the characters of zonal wind within multiple time scales in lower stratosphere are studied. Firstly, the three dimensions structures of the zonal wind of lower stratosphere in different seasons are analyzed. Furthermore, the intraseason oscillation (ISO) of Stratospheric wind filed is examined distinctly, and then the influence of Quasi-biennial Oscillation on the interannual variation are studied. According to the study mentioned, the results are summarized as following.According to the different forming mechanism, the Stratospheric Quasi-zero Wind Layers (QZWL) can be classified into two categories:the first kind of QZWL which due to different phases lay of the QBO and the second kind of QZWL which is caused by the reversal of meridional temperature gradient in lower stratosphere. Based on this classification, the first kind of QZWL can be founded at tropical zone in winter, whereas the second exists at extratropical area in summer and over the Pacific Ocean in middle and lower latitude (20°40°N) in winter. The main height of QZWL is settled from 16km to 27km. The height of QZWL in summer is 3 km lower and more stable than that in winter. Except the variance of different seasons, there are intraseason oscillation of stratosphere both in winters and summers. The ISO in winter is more obvious than it in summer and it is related to the polar vortex. What's more, obvious interannual variations of the ISO of zonal wind in lower stratosphere are found in the study.QBO influences the QZWL both in summer and winter. However, the effect of QBO on QZWL in winter is more obvious than that in summer. The case study and composite analysis show that the frequency of QZWL showing up at 20°40°N during winter in the westerly phase of QBO is more than in the easterly phase of QBO. This conclusion is limited in the years with a strong QBO index and weak indices of other factors like ENSO and 11-year solar cycle signals.The common characters of the chosen SSW cases can be summarized as the long vortex disruption in lower stratosphere, the slow progression of the warming from upper to lower stratosphere and the strong and rapid recovery in the upper stratosphere. The planetary wave continuously propagates from the troposphere to the upper stratosphere before the warming period, which induces the reversion of the polar zonal wind in stratosphere. The high latitude easterlies in upper stratosphere limit the vertical propagation of planetary wave and make it accumulate in lower layers in stratosphere. Therefore, the disruption of polar vortex in lower stratosphere usually remains over one month and polar vortex intensifications happen after that.