The Roles of Mean Meridional Motions and Large-Scale Eddies in Zonally Averaged Circulations

Abstract

A hierarchy of zonally averaged atmospheric models is used to study the role of mean meridional motions and large-scale eddies in determining the zonal climate. Five models are developed: a radiative-convective equilibrium model (no large-scale motion), a zonally uniform model (no longitudinal asymmetries), an energy balance model (parameterized energy transport), a model that combines the physics of the two previous models, and a full statistical-dynamic model (with parameterizations of eddy momentum transport as well as eddy sensible heat transport). In the most complete model, the zonally averaged primitive equations are solved after parameterizing the eddies, friction and the diabatic heating rates. All the models have two layers in the vertical and a latitudinal grid resolution of 5°. For simplicity, we treat a “dry earth” case and calculate annual-average equilibrium states with each of the five models. We find that in the tropics a parameterized energy transport commonly used in energy balance models does not accurately simulate the energy transport as determined by an explicit calculation of the Hadley cell. The explicitly calculated Hadley transport is generally greater than the parameterized transport and leads to very small horizontal temperature gradients in the tropics. The strength of the Hadley cell is determined by both the local heating distribution and extratropical heat and momentum transport by the eddies. The extratropical mean meridional motions are primarily driven by the requirements of the momentum budget. An indirect (Ferrel) cell appears only when eddy momentum transport is included in the model.