Interactions between Orographic Gravity Wave Drag and Forced Stationary Planetary Waves in the Winter Northern Hemisphere Middle Atmosphere

Abstract

A quasigeostrophic model is used to study the combined interaction among orographically generated gravity wave drag, forced planetary waves, and zonal mean flows in the Northern Hemisphere winter stratosphere and mesosphere. The localized gravity wave drag is shown to generate planetary waves in the mesosphere that, in turn, exert a substantial drag on the zonal mean flow via the Eliassen-Palm flux divergence. The amount of planetary wave drag is found to depend not only on the presence of the localized source of orographic gravity wave drag but also on the presence of upward-propagating planetary waves in the lower stratosphere. The zonal mean wind field exhibits a split jet structure with the larger jet maximum situated in the upper stratosphere at 30°N. This feature is shown to arise from the presence of weak winds above the subtropical tropospheric jet maximum, which results in a region of low-level gravity wave breaking and reduced drag and larger winds above.