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The present two-part paper provides a Lagrangian perspective of the finalsouthern warming in 2002, during which the stratospheric polar vortex (SPV)experienced a unique splitting. Part I focuses on the understanding offundamental processes for filamentation and ultimately for vortex splitting ona selected isentropic surface in the middle stratosphere. Part II discusses thethree-dimensional evolution of the selected sudden warming event. We approachthe subject from a dynamical systems viewpoint and search for Lagrangiancoherent structures using a Lagrangian descriptor as a tool. In this Part I wework in the idealized framework of a kinematic model that allows for anunderstanding of the contributing elements of the flow in late September duringthe splitting. We introduce a definition of kinematic SPV boundary based on acriterion for binning parcels inside and outside the vortex according to theirvalues of the Lagrangian descriptor and associated PDF. This definition isjustified by using arguments that go beyond heuristic considerations based onthe potential vorticity. Next, we turn to the filamentation proceses along thiskinematic boundary, and the role of Lagrangian structures in the SPV splitting.We determine a criterion for splitting based on the structure of the evolvingunstable and unstable manifolds.

Lagrangian study of the final warming in the southern stratosphere during 2002: Part I. The Vortex Splitting at Upper Levels