Modulation of the gravity wave driven meridional circulation in the mesosphere by planetary wave forcing in the troposphere

We estimate the climatological effects of orography and land-sea heating contrasts on the boreal winter climatology of the mesosphere. This is done by analyzing a condensed set of numerical experiments performed with an idealized general circulation model ranging from planetary boundary layer to the mesopause. Parameterization of gravity waves in included according to the Lindzen scheme.

In the stratosphere, present model results confirm that the boreal winter zonal-mean climate is controlled by planetary wave activity, hence by the combined action of oropgraphy and land-sea heating contrasts. Since in turn the propagation and breaking of internal gravity waves is strongly modulated by the background horizontal winds, the mesospheric response to low level stationary wave forcing turns out be substantial as well. It is found that in the climatological zonal mean, a warmer polar night stratosphere is accompanied by cooling at higher altitudes up to about 80 km. In addition, the upper mesosphere/lower thermosphere is globally heated up. These effects are interpreted in terms of first, diminished gravity wave-induced turbulence and energy deposition in the winter mesosphere and second, a globally weakened summer to winter pole gravity-wave driven residual circulation.

Present model results suggest that a winter-summer asymmetry in gravity wave break-down, well-known from the northern hemisphere, might be absent in the southern mesosphere.