The middle-atmosphere Hadley circulation and equatorial inertial adjustment

In the tropical middle atmosphere the climatological radiative equilibrium temperature is inconsistent with gradient-wind balance and the available an gular momentum, especially during solstice seasons. Adjustment toward a bal anced state results in a type of Hadley circulation that lies outside the " downward control'' view of zonally averaged dynamics. This middle-atmospher e Hadley circulation is reexamined here using a zonally symmetric balance m odel driven through an annual cycle. It is found that the inclusion of a re alistic radiation scheme leads to a concentration of the circulation near t he stratopause and to its closing off in the mesosphere, with no need for r elaxational damping or a rigid lid. The evolving zonal flow is inertially u nstable, leading to a rapid process of inertial adjustment, which becomes s ignificant in the mesosphere. This short-circuits the slower process of ang ular momentum homogenization by the Hadley circulation itself, thereby weak ening the latter. The effect of the meridional circulation associated with extratropical wave drag on the Hadley circulation is considered. It is shown that the two cir culations are independent for linear (quasigeostrophic) zonal-mean dynamics , and interact primarily through the advection of temperature and angular m omentum. There appears to be no significant coupling in the deep Tropics vi a temperature advection since the wave-driven circulation is unable to alte r meridional temperature gradients in this region. However, the wave-driven circulation can affect the Hadley circulation by advecting angular momentu m out of the Tropics. The validity of the zonally symmetric balance model with parameterized iner tial adjustment is tested by comparison with a three-dimensional primitive equations model. Fields from a middle-atmosphere GCM are also examined for evidence of these processes. While many aspects of the GCM circulation are indicative of the middle-atmosphere Hadley circulation, particularly in the upper stratosphere, it appears that the circulation is obscured in the mes osphere and lower stratosphere by other processes.