Abstract
The necessary and sufficient conditions for Lagrangian hyperbolicity recently derived in the literature are reviewed in the light of older concepts of effective local rotation in strain coordinates. In particular, we introduce the simple interpretation of the necessary condition as a constraint on the local angular displacement in strain coordinates. These mathematically rigorous conditions are applied to the winter stratospheric circulation of the southern hemisphere, using analysed wind data from the European Centre for Medium-Range Weather Forecasts. Our results demonstrate that the sufficient condition is too strong and the necessary condition is too weak, so that both conditions fail to identify hyperbolic lines in the stratosphere. However a phenomenological, non-rigorous, criterion based on the necessary condition reveals the hyperbolic structure of the flow. Another (still non-rigorous) alternative is the Finite-Size Lyapunov Exponent (FSLE) which is shown to produce good candidates for hyperbolic lines. In addition, we also tested the sufficient condition for Lagrangian ellipticity and found that it is too weak to detect elliptic coherent structures (ECS) in the stratosphere, of which the polar vortex is an obvious candidate. Yet, the FSLE method reveals a clear ECS-like mixing barrier along the polar vortex edge. Further theoretical advancement is needed to explain the apparent success of non-rigorous methods, such as the FSLE approach, so as to achieve a sound kinematic understanding of chaotic mixing in the winter stratosphere and other geophysical flows.
published in June 2002, in Chaos: An Interdisciplinary Journal of
Nonlinear Science, Vol. 12, No. 2, pp. 382-394.
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