Climatological Effects of Orography and Land–Sea Heating Contrasts on the Gravity Wave–Driven Circulation of the Mesosphere

Abstract

On the basis of permanent January simulations performed with an idealized general circulation model for the troposphere and middle atmosphere, the sensibility of the general circulation to orographic and thermal forcing of large-scale stationary waves is assessed. Gravity waves are parameterized following Lindzen's saturation theory. Up to the stratopause, present model results coincide with earlier estimates, confirming that the boreal winter zonal-mean climate does crucially depend on the combined action of orography and land?sea heating contrasts. Since, in turn, the propagation and breakdown of internal gravity waves is strongly modulated by the background horizontal winds, the mesospheric response to 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 lower temperatures in the mesosphere up to about 80 km. The temperature signal induced by stationary wave forcing changes sign again in the upper mesosphere/lower thermosphere, which, except for the polar night region, is globally heated up by 10?20 K. This heating is weaker if the assumed Prandtl number for gravity wave?induced vertical diffusion is raised from 3 to 6. The thermal effects in the mesosphere are interpreted in terms of a global weakening of the summer-to-winter-pole residual circulation that occurs along with strongly diminished gravity wave drag, turbulent diffusion, and energy deposition in the northern winter mesosphere. The weakening of gravity wave effects in the presence of quasi-stationary planetary waves is dominated by reduced efficiency of gravity wave saturation in the mesosphere. That is, due to the more variable and, on average, reduced planetary-scale horizontal winds, gravity wave saturation is distributed over a greater depth and drops in altitude. On the other hand, enhanced critical level absorption of gravity waves in the lower stratosphere plays at most a secondary role. Furthermore, present model results suggest that the winter?summer asymmetry in gravity wave breakdown, which is well known from the northern mesosphere, may be absent or even reversed in the southern mesosphere.