The Mean-Meridional Transport Circulation of the Troposphere in an Idealized GCM

Abstract

Large numbers of particle trajectories are used to characterize the mean-meridional transport circulation of an idealized general circulation model (GCM). The GCM has a uniform land surface, no topography or land?sea contrasts, and no seasonal cycle. The trajectories are analyzed using both Lagrangian-mean statistics and an approach based on the Green's function of the tracer transport equation. It is shown that the distribution of particle trajectories provides an estimate of the ensemble-mean Green's function for the inviscid transport equation. Lagrangian means have a number of problems that render their interpretation difficult, including boundary effects and selection of the appropriate averaging period. The Green's function, on the other hand, provides a quantitative description of the transport circulation that is easy to visualize and interpret. The results demonstrate that within the idealized model the atmosphere can be divided into three parts: the Southern Hemisphere extratropics, the Tropics, and the Northern Hemisphere extratropics. Climatologically, particle dispersion within each part of the atmosphere is rapid, while exchange between the different parts is much slower. There is a semipermeable transport barrier between the Tropics and extratropics. The interhemispheric exchange rate in the model is toward the slower end of estimates from observations, possibly due to the higher degree of zonal symmetry in the model.