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We present new sensitivity experiments that link observedanomalies of the mesosphere and lower thermosphere at highlatitudes during the MaCWAVE/MIDAS summer program 2002 to enhancedplanetary Rossby-wave activity in the austral winter troposphere.<P  style="line-height: 20px;">We employ the same general concept of a GCM having simplifiedrepresentations of radiative and latent heating as in a previousstudy by Becker et al. (2004). In the present version, however,the model includes no gravity wave (GW) parameterization. Insteadwe employ a high vertical and a moderate horizontal resolution inorder to describe GW effects explicitly. This is supported byadvanced, nonlinear momentum diffusion schemes that allow for aself-consistent generation of inertia and mid-frequency GWs in thelower atmosphere, their vertical propagation into the mesosphereand lower thermosphere, and their subsequent dissipation which isinduced by prescribed horizontal and vertical mixing lengths asfunctions of height.<P  style="line-height: 20px;">The main anomalies in northern summer 2002 consist of highertemperatures than usual above 82 km, an anomalous eastward meanzonal wind between 70 and 90 km, an altered meridional flow,enhanced turbulent dissipation below 80 km, and enhancedtemperature variations associated with GWs. These signals are allreasonably described by differences between two long-integrationperpetual model runs, one with normal July conditions, and anotherrun with modified latent heating in the tropics and Southern Hemisphere to mimic conditions that correspond to the unusualaustral winter 2002. The model response to the enhanced winterhemisphere Rossby-wave activity has resulted in both aninterhemispheric coupling through a downward shift of theGW-driven branch of the residual circulation and an increased GWactivity at high summer latitudes. Thus a quantitative explanationof the dynamical state of the northern mesosphere and lowerthermosphere during June-August 2002 requires an enhanced Lorenzenergy cycle and correspondingly enhanced GW sources in thetroposphere, which in the model show up in both hemispheres

Enhanced gravity-wave activity and interhemispheric coupling during the MaCWAVE/MIDAS northern summer program 2002