The tropopause is one of the basic features of the earth's atmosphere.It separates the troposphere,which is marked by a relatively low static stability,from the stably stratified stratosphere.The height of the tropopause is often considered to be set primarily by the combined effects of a dynamically active troposphere and a stratosphere in near-radiative equilibrium.The sudden stratospheric warming(SSW)is a violent large-scale thermo-dynamical phenomenon in the winter polar region,which strongly affects the middle atmosphere.They are triggered by anomalous wave activity propagating from the upper troposphere and may,in turn,affect tropospheric weather patterns.The SSW evolution in the spring of 2009 is shown with the Modern-Era Retrospective analysis for Research and Applications(MERRA)reanalysis data.It is found that there is a prominent SSW in the second half of January 2009 occurred after a cold and undisturbed early winter.The warming was accompanied by clear splitting of the polar vortex due to unusual development of wave number 2 planetary waves.Furthermore,the temporal changes of the buoyancy frequency are different among different latitudes.Descent of strong downward flow depends on latitude and the descent is faster in higher latitude.Using the method of the Birner(2010),we calculated the static stability tendency around the tropopause during the SSW 2009.It is found that the vertical residual flow is a crucial factor in linking the residual circulation to the static stability structure near the tropopause.The variations of tropopause are studied with GPS/COSMIC temperature data.The tropopause height decreases accompanied by the tropopause temperature increases during the SSW 2009.Besides,the tropopause inversion layer also increases during the SSW 2009.