INTERACTIONS BETWEEN OROGRAPHIC GRAVITY-WAVE DRAG AND FORCED STATIONARY PLANETARY-WAVES IN THE WINTER NORTHERN-HEMISPHERE MIDDLE ATMOSPHERE

A quasigeostrophic model is used to study the combined interaction amo ng orographically generated gravity wave drag, forced planetary waves, and zonal mean flows in the Northern Hemisphere winter stratosphere a nd mesosphere. The localized gravity wave drag is shown to generate pl anetary waves in the mesosphere that, in turn, exert a substantial dra g on the zonal mean flow via the Eliassen-Palm flux divergence. The am ount of planetary wave drag is found to depend not only on the presenc e of the localized source of orographic gravity wave drag but also on the presence of upward-propagating planetary waves in the lower strato sphere. The zonal mean wind field exhibits a split jet structure with the larger jet maximum situated in the upper stratosphere at 30-degree s-N. This feature is shown to arise from the presence of weak winds ab ove the subtropical tropospheric jet maximum, which results in a regio n of low-level gravity wave breaking and reduced drag and larger winds above.