Observations of gravity wave activity during stratospheric sudden warmings in the Northern Hemisphere

Abstract

Arctic stratospheric sudden warming (SSW) with polar vortex displacement and splitting occurred in 2008 and 2009, respectively. The temperature profiles derived from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) along with NCEP/NCAR reanalysis are used to investigate the gravity wave (GW) variations during these SSW events. Some phenomena are characterized and possible mechanisms are discussed. The GW variations during both types of SSW events shared some similarities. During the warming events, enhanced zonal mean GW amplitudes are detected. The enhancements of GW amplitudes first appeared at higher altitudes and then progressed downward during the major warming events. The downward progressions correspond well with the height variations of the zonal wind reversal, and the enhancements seem to occur at the same time with the wind reversal. The spatial variations of GWs showed evident correlation to the polar vortex. Generally, GWs were enhanced at the edge of the polar vortex where the background wind was strong, whereas the areas with the largest GW amplitudes did not always correspond with those with the strongest background wind. The differences during the two types of SSW are also noticeable. In the 2008 major event, the magnitudes of the zonal mean GW enhancements during the downward progression were relatively stable, whereas in the 2009 major event, a clear reduction was detected as the GW enhancement was progressing downward. Also, the recovery time of the polar vortex was longer in 2009. These differences indicate that different types of SSWs may have different impacts on the GW.