This approach is applied first to a simple chaotic advection flow. It is demonstrated that the effective diffusivity clearly identifies the transport and mixing structure. It also gives a useful quantification of the strength of the barriers in the flow
The same approach is then applied to atmospheric flows, defined by isentropic winds from large-scale meteorological datasets.
The effective diffusivity shows the evolution of the stratospheric polar vortex edge barrier and the mid-latitude surf zone. The lower limit of the vortex-edge barrier, i.e. the `sub-vortex' transition, is shown to vary in altitude (in the range 350--380 K in the Antarctic, and 400--450 K in the Arctic) throughout the winter. The effective diffusivity also shows the seasonal evolution of the tropical reservoir region, which takes different forms at 400 K, 450--600 K and above 650 K, determined by the relative influences of tropospheric synoptic eddies and stratospheric planetary waves. A multi-year integration shows a clear signal at low latitudes associated with the phase of the Quasi-Biennial Oscillation (QBO), consistent with previous modelling studies. Comparison of the effective diffusivity calculated from satellite-observed (MLS) ozone data with that from a simulated ozone field from a 3D chemical transport model driven by ECMWF winds, suggests the QBO is not fully represented in the analysed winds.
Local minima in effective diffusivity in the upper troposphere and lower stratosphere, in the range 330--400 K, indicate transport barriers associated with the extratropical tropopause. These barriers are strongest in winter and considerably weakened by the monsoon circulations in summer. The minimum value of effective diffusivity is proposed as a new definition of the tropopause, more generally applicable than traditional definitions based on potential vorticity (PV) values. Comparison with PV-based definitions suggests that a value of +/-2 PVU corresponds most closely to the minimum effective diffusivity at 330 K but values of +/-2.5 PVU and +/-4.5 PVU are more appropriate at 350 K and 370 K respectively.
Thus effective diffusivity has been used to present a global picture of the temporal and spatial variation of the mixing and transport structure of the troposphere and stratosphere.