The strength of the equinox Hadley circulation over a wide range of climates in a hierarchy of models

Simulations of 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. But 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 two-layer model (TLM). The top layer is an atmosphere with axisymmetric dynamics and a simple representation of moist thermodynamics. The flow in the top layer is desribed by a shallow-water model representing the zonal momentum balance in the troposphere, which assumes that the meridional circulation is confined just below the tropopause and just above the surface and that the zonal wind increases linearly with height. The bottom layer is a uniform ocean slab that interacts with the atmosphere through radiative, sensible heat, and moisture fluxes. The enthalpy budget in the top layer assumes that the tropospheric thermal structure is always close to a moist pseudo-adiabat. In addition, the enthalpy of the ocean and the atmosphere are convectively-coupled wherever a thermal instability criterion is met. We also constrain the flow in the top layer by simple representations for the eddy momentum and heat fluxes, which are found to hold approximately in the idealized GCM over a wide range of simulations.

We show that the TLM can represent successfully changes in the strength of the Hadley circulation and is able to reproduce the various dynamical regimes obtained in the GCM simulations over a wide range of climates. Using the TLM, we describe the sensitivity of the Hadley circulation to critical external parameters, such as the insolation gradient and optical depth, both in axisymmetric and in eddy-permitting simulations, and we compare these to results obtained in the idealized GCM simulations. Our preliminary interpretations for the behavior of the Hadley circulation in the idealized GCM simulations are presented using the simplified analytics of the TLM.