Lagrangian flow in the middle atmosphere

Abstract

AbstractThe Lagrangian view of air flow in the middle atmosphere is studied by computing tens of thousands of particle trajectories in a global primitive‐equation model. This approach focuses attention on the long‐term fate of air and provides a high‐resolution view of air transport. Simulations are carried out for both the northern and southern winters to permit an inter‐hemispheric comparison. The first part of the paper concentrates on transport in the meridional plane. Trajectories followed for many months show patterns which are well organized on a global scale. Ensembles of particles followed for a much shorter time are used to calculate a Lagrangian mean meridional circulation which shows close similarities to published diabatic circulations calculated from observational data. Maps of meridional dispersion reveal a broad mid‐latitude band, with well defined boundaries, in which rapid dispersion occurs. A more detailed visualization of transport in and around the southern polar vortex in late winter shows that small scales are generated in this mid‐latitude region, but not in the vortex nor at low latitudes. The ability of the vortex to retain a labelled air mass for many months is clearly demonstrated, and the relevance of the results to understanding springtime ozone depletion is discussed.