Several studies of tracer transport in the lower stratosphere have shown that horizontal tracer evolution is primarily controlled by the large-scale, low-frequency component of the flow. This behaviour is consistent with the concept of "chaotic advection", wherein the Eulerian velocity field is spatially coherent and temporally quasi-regular on time scales over which the Lagrangian evolution is chaotic. In this study, winds from a middle atmosphere general circulation model are used to study tracer evolution in the stratosphere and mesosphere. It is found that the concept of chaotic advection is relevant in the stratosphere but not in the mesosphere. The explanation for this behaviour is the increased strength of gravity-wave activity in the mesosphere as compared with the stratosphere, which is reflected in shallower energy spectra and a stronger divergent component to the flow.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics