The atmosphere is divided by transport barriers into regions of different chemical characteristics. These barriers are generally not perfect but leaky, and are associated with regions of weak mixing, separating regions of strong mixing.
The mixing properties of a flow may be characterised by calculating the `effective diffusivity' (Nakamura: JAS 53 1524--1537 1996) of an advected tracer. The effective diffusivity characterises the geometric structure of a tracer, which is most complex where the mixing is strongest. Effective diffusivity has been calculated for idealised and observed flows.
Effective diffusivity calculated from tracer fields advected by idealised flows has been compared with other measures of transport and mixing. Simple chaotic advection flows that contain mixing regions separated by barriers whose leakiness depends on the parameters defining the flow have been used as the basis for this comparison.
Effective diffusivity calculated from tracer fields advected by observed winds has been used to produce a complete quantitative characterisation of the transport and mixing structure of the troposphere and stratosphere. This has shown the variation in height and latitude and the seasonal evolution of the barriers to transport at the edge of the stratospheric polar vortex, at the tropopause and in the subtropics. The monsoons have been shown to have a significant influence on the strength and latitudinal position of the tropopause barrier.
Other regions, for example the summer lower stratosphere and the sub-vortex region, have also been examined. Whereas in the Antarctic the vortex edge barrier extends into the upper troposphere and prevents polar air from leaving the subvortex region and entering mid-latitudes, this barrier does not extend to such low levels in the Arctic with resulting implications for ozone depletion. Effects of the QBO on tropical mixing have also been investigated.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics