WACCM simulations of the mean circulation and trace species transport in the winter mesosphere

Downwelling of air in the high latitude winter middle atmosphere causes perturbations in chemical composition. Species with sources in the mesosphere and lower thermosphere, such as nitric oxide and carbon monoxide, can be locally enhanced in the polar region. We investigate the origin of the downwelling air and the timescale for transport using the Whole Atmosphere Community Climate Model (WACCM). Analysis of the middle atmosphere transformed Eulerian mean (TEM) circulation, also called the residual circulation, is presented. This circulation gives the net air motion in the meridional and vertical directions due to the combined effects of the zonally averaged winds and the wave transport. The summer to winter circulation in the upper mesosphere during the solstice seasons includes strong upward motion at the summer pole and downward motion at the winter pole. During most winters, the air that is transported down by the circulation originates from low to middle latitudes in the upper mesosphere, 85-95 km; the timescale for this transport is 2-3 months. Trace species transport by molecular and eddy diffusion can bring high concentrations of thermospheric molecules into the winter mesosphere, where they are then brought to lower altitudes by the circulation. The magnitudes of the net tendencies due to advection by the TEM circulation and diffusion are similar. A case study of an active NH winter shows that the origin of the downwelling air in the TEM circulation is similar to that during average winters although the downwelling velocity is stronger by about a factor of two.