A closer look at chaotic advection in the stratosphere. Part 1: Geometric structure

The relevance of chaotic advection to stratospheric mixing and transport is addressed in the context of (i) a numerical model of forced shallow-water how on the sphere, and (ii) a middle-atmosphere general circulation model. It is argued that chaotic advection applies to both these models if there i s suitable large-scale spatial structure in the velocity field and if the v elocity held is temporally quasi-regular. This spatial structure is manifes ted in the form of "cat's eyes" in the surf zone. such as are commonly seen in numerical simulations of Rossby wave critical layers; by analogy with t he heteroclinic structure of a temporally aperiodic chaotic system the cat' s eyes may be thought of as an "organizing structure" for mixing and transp ort in the surf zone. When this organizing structure exists, Eulerian and L agrangian autocorrelations of the velocity derivatives indicate that veloci ty derivatives decorrelate more rapidly along particle trajectories than at fixed spatial locations (i.e., the velocity field is temporally quasi-regu lar). This phenomenon is referred to as Lagrangian random strain.