Zonal-mean transport characteristics of ECMWF re-analysis data

Abstract

Transport characteristics for a dataset from a global data assimilation system (European Centre for Medium-Range Weather Forecasts re-analyses for 1993) are analysed by constructing an advection-diffusion operator with tracer-transport properties as close as possible to the zonally and seasonally averaged tracer advection. This is achieved by computing a best-fit flux-gradient relation for a large number of artificial tracers. The meridional circulation of the advection-diffusion operator (the transport velocity) is thermally direct almost everywhere. It is similar to the Transformed Eulerian Mean (TEM) residual circulation in most of the stratosphere, but is much weaker than this circulation in the extratropical troposphere. The diffusivity tensor has a complicated structure where transport barriers and 'surf zones' can be identified. The structure is strongly connected to the zonal wind, with distinct minima in the jet-stream areas. The zonal acceleration due to effects other than the transport circulation is studied. It is shown that this acceleration has a tendency to be strong when the mixing is strong and vice versa.