Stationary Planetary Waves in Upper Mesospheric Winds

Abstract

Quasi-stationary planetary-scale longitudinal variations are found in the upper mesospheric winds measured during winter by the HRDI satellite instrument. These are negatively correlated with eddy winds in the stratosphere. Two different mechanisms are proposed to explain the mesospheric perturbation winds and their anticorrelation with stratospheric winds: 1) Planetary waves propagate through the stratosphere and into the mesosphere, with a phase shift of one-half cycle and 2) mesospheric perturbations are forced in situ by gravity waves whose spectrum has longitudinal asymmetries due to filtering by planetary waves in the stratosphere. The first mechanism is more consistent with the observations during Southern Hemisphere late winter (August) and also may explain the observations during Northern Hemisphere early winter (December). The second mechanism gives a more consistent explanation for the Northern Hemisphere late winter observations, as previously shown by the author. The hemispheric differences are reproduced in two linear model calculations in which the only difference is the background zonal mean wind.