DOWNWARD CONTROL OF THE MEAN MERIDIONAL CIRCULATION AND TEMPERATURE DISTRIBUTION OF THE POLAR WINTER STRATOSPHERE

According to the ''downward control'' principle, the extratropical mea n vertical velocity on a given pressure level is approximately proport ional to the meridional gradient of the vertically integrated zonal fo rce per unit mass exerted by waves above that level. In this paper, a simple numerical model that includes parameterizations of both planeta ry and gravity wave breaking is used to explore the influence of gravi ty wave breaking in the mesosphere on the mean meridional circulation and temperature distribution at lower levels in the polar winter strat osphere. The results of these calculations suggest that gravity wave d rag in the mesosphere can affect the state of the polar winter stratos phere down to altitudes below 30 km. The effect is most important when planetary wave driving is relatively weak: that is, during southern w inter and in early northern winter. In southern winter, downwelling we akens by a factor of 2 near the stratopause and by 20% at 30 km when g ravity wave drag is not included in the calculations. As a consequence , temperatures decrease considerably throughout the polar winter strat osphere (over 20 K above 40 km and as much as 8 K at 30 km, where the effect is enhanced by the long radiative relaxation timescale). The po lar winter states obtained when gravity wave drag is omitted in this s imple model resemble the results of simulations with some general circ ulation models and suggest that some of the shortcomings of the latter may be due to a deficit in mesospheric momentum deposition by small-s cale gravity waves.