Relationship Between Traveling and Stationary Planetary Waves in the Northern Hemisphere Winter Middle Atmosphere

It is a well-established overview that planetary waves propagating upward from the troposphere dissipate in the stratosphere to drive the poleward meridional circulation in the winter hemisphere, while the meridional circulation in the mesosphere is driven mainly by gravity waves. Quite recently, however, the importance of traveling planetary waves has been underscored in the onset and/or the recovery periods of sudden stratospheric warmings (SSWs); traveling planetary waves could be generated within the middle atmosphere due to barotropic and/or baroclinic instability to bring about significant impacts on the mean flow through their generation and dissipation. However, observational studies are insufficient for the region, so that detailed features are still unclear. Hence, we investigate planetary wave behavior in the middle atmosphere, which includes the upper mesosphere and lower thermosphere, during the Northern Hemisphere winter by using TIMED/SABER satellite data. Resultantly, it is found that Eliassen-Palm (EP) flux convergence regions climatologically appear with two separate peaks in the middle atmosphere around the stratopause (50N, 50km) and in the polar mesosphere (65N, 80km). When we divided planetary waves into components of monthly mean stationary planetary waves (STPWs) and the remaining ones of traveling planetary waves (TRPWs), it is found that EP fluxes of STPWs converge in the widely extended region around the polar stratopause. However, those of TRPWs diverge in the same region and propagate upward and/or toward lower latitudes to converge in the above mentioned two convergence regions. It is also found that the divergence region is coincident with the region fulfilling the necessary conditions for zonal flow instability. Moreover, interannual variations of the convergence of STPWs and the divergence of TRPWs around the polar stratopause are intimately correlated each other, and these features in EP fluxes are outstanding for winters without major SSWs. Thus, the detailed analysis implies that the cancellation between the STPW convergence and the TRPW divergence could contribute to maintain the observed features of the mean circulation for a longer period.