On the estimation of transport characteristics of atmospheric data sets

Transport characteristics of an atmospheric general circulation model have been estimated by analysing 30 000 trajectories over several months. These trajectories have been used to construct an advection-diffusion operator for the zonally-averaged transport of passive tracers. Several different methods have been tried for this purpose. The most promising seems to be a method where eddy-flux statistics for a large number of tracers are generated and used to End an optimized flux-gradient relationship. An alternative approach based on Lagrangian averaging is shown to be much less robust with respect to changes in the details of the procedure. The meridional circulation of the advection-diffusion operator is direct almost everywhere, but is very weak outside the tropics. Diffusion coefficients show a quite complicated structure with minima near the jet maxima. Certain limitations of the advection-diffusion model have been identified. In particular, there seems to be problems related to Walker cells. The use of a large number of different tracers has also allowed us to improve on the flux-gradient relationship by including higher order terms. The relevance of the advection-diffusion approach for the transport of potential vorticity has been tested. The Stokes drift is shown to be very important for potential vorticity fluxes near the tropopause.