Changes in the Hadley circulation with climate in a simple axisymmetric model of the tropical atmosphere

Climate change scenarios predict a weakening and poleward expansion of the Hadley circulation for the 21st century. Why these changes in the Hadley circulation occur is not well understood. Both thermodynamic changes in low latitudes and changes in eddy fluxes in the subtropics are likely to play a role. The complexity and the limited range of climates simulated by full general circulation models (GCMs) have impeded our ability to formulate an analytical theory for what controls the strength and extent of the Hadley circulation. Here we propose a dual approach in which the behavior of the Hadley circulation in an idealized GCM is compared to that in a single-layer model (SLM). The SLM solves for the dynamics of an axisymmetric atmosphere in which meridional transport is confined to thin layers just above the surface and below the tropical tropopause. Closures for eddy fluxes of dry static energy, moisture and momentum are introduced. A simple representation of changes in the moisture content of the troposphere is employed to constrain changes of the depth of the troposphere and the efficiency of moisture fluxes. The SLM is forced by a set of thermal and radiative parameters that depend only on the insolation contrast and longwave optical opacity for each climate. We find that the SLM captures the effects of changing longwave optical opacity and insolation contrast on the dynamics of the Hadley circulation when compared to the idealized GCM. In particular, the SLM predicts a weakening and widening of the Hadley circulation as climate warms, as seen in the GCM simulations. We describe these variations over a wide range of climates and elucidate their causes using numerical and analytical solutions provided by the SLM.