Polar night vortex breakdown and large-scale stirring in the southern stratosphere
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The present paper examines the vortex breakdown and large-scale stirring during the final warming of the Southern Hemisphere stratosphere during the spring of 2005. A unique set of in situ observations collected by 27 superpressure balloons (SPBs) is used. The balloons, which were launched from McMurdo, Antarctica, by the Stratéole/VORCORE project, drifted for several weeks on two different isopycnic levels in the lower stratosphere. We describe balloon trajectories and compare them with simulations obtained on the basis of the velocity field from the GEOS-5 and NCEP/NCAR reanalyses performed with and without VORCORE data. To gain insight on the mechanisms responsible for the horizontal transport of air inside and outside the well-isolated vortex we examine the balloon trajectories in the framework of the Lagrangian properties of the stratospheric flow. Coherent structures of the flow are visualized by computing finite-time Lyapunov exponents (FTLE). A combination of isentropic analysis and FTLE distributions reveals that air is stripped away from the vortex’s interior as stable manifolds eventually cross the vortex’s edge. It is shown that two SPBs escaped from the vortex within high potential vorticity tongues that developed in association with wave breaking at locations along the vortex’s edge where forward and backward FTLE maxima approximately intersect. The trajectories of three SPBs flying as a group at the same isopycnic level are examined and their behavior is interpreted in reference to the FTLE field. These results support the concept of stable and unstable manifolds governing transport of air masses across the periphery of the stratospheric polar vortex.
KeywordsStratospheric polar vortex dynamics Vortex breakdown Large-scale stirring Finite-time Lyapunov exponents Hyperbolic manifolds
The authors especially thank Andrew V. Tangborn, from the National Aeronautics and Space Administration (NASA), for providing with the GEOS5 data and Lynette J. Gelinas for helpful discussion. The authors are also grateful to the two anonymous reviewers for their constructive and useful comments that help improve the manuscript. VORCORE was jointly supported by Centre National de la Reserche Scientifique (CNRS), France, the Centre National d’Études Spatiales (CNES), France, and the National Science Foundation (NSF), USA. The research was supported by NSF Grant ATM—0732222 and by Office of Naval Research Grants N00014040191 and N000140910418 and by the Spanish Ministry of Science and Innovation project CGL2008-06295.
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