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Laboratory for Atmospheric and Space Physics

Middle Atmosphere Dynamics

The middle atmosphere extends from the tropopause to the mesopause. The polar tropopause is near 10 km and the tropical tropopause is near 18 km. The mesopause is near 100 km except in the polar summer when it is around 85 km. Stratospheric (~10-50 km) temperature increases with height due to the absorption of UV radiation by ozone. In the mesosphere (~50-100 km) temperatures decrease with altitude.

 

The zonal circulation in the middle atmosphere at solstice is shown in the figure below. In the winter hemisphere, a westerly jet flows around the polar vortex (See Polar Vortex Section). In the summer hemisphere an easterly jet encircles a polar anticyclone. These jets maximize in speed in the upper stratosphere and “close off” in the mesosphere due to breaking gravity waves.

Mean zonal wind for solstice conditions. (Courtesy of R.J. Reed)

Mean zonal wind for solstice conditions. (Courtesy of R.J. Reed)

 

The mean meridional and vertical motion in the atmosphere is given in the figure below. In the troposphere, the Hadley, Ferrell, and Polar cells are responsible for the north-south exchange of heat and momentum. In the middle atmosphere the mean meridional circulation is driven by breaking planetary and gravity waves. In the lower stratosphere the ‘Brewer-Dobson circulation’ consists of rising motion at the equator with two branches moving toward each of the poles. Air continues to rise as it is deflected into the summer hemisphere and wave-driven ascent is responsible for the cold summer polar mesopause. Air then drifts across the equator toward the winter pole where is descends into the polar vortex.

Meridional Circulation for solstice conditions (Courtesy of University of Albany).

Meridional Circulation for solstice conditions (Courtesy of University of Albany).

 

Keep in mind when looking at these mean zonal and meridional circulation figures is that the atmosphere is not static. On any given day the circulation can and does look very different than what is shown here. In fact, during sudden stratospheric warming events (see Sudden Stratospheric Warming section) both the zonal and the meridonal/vertical circulations can be opposite of the mean state.