A quasi-Lagrangian coordinate system based on high resolution tracer observations: implementation for the Antarctic polar vortex

Abstract. In order to quantitatively analyse the chemical and dynamical evolution of the polar vortex it has proven extremely useful to work with coordinate systems that follow the vortex flow. We propose here a two-dimensional quasi-Lagrangian coordinate system {χ_i, Δχ_i}, based on the mixing ratio of a long-lived stratospheric trace gas i, and its systematic use with i = N₂O, in order to describe the structure of a well-developed Antarctic polar vortex. In the coordinate system {χ_i, Δχ_i} the mixing ratio χ_i is the vertical coordinate and Δχ_i = χ_i(Θ)−χ_i^vort(Θ) is the meridional coordinate (χ_i^vort(Θ) being a vertical reference profile in the vortex core). The quasi-Lagrangian coordinates {χ_i, Δχ_i} persist for much longer time than standard isentropic coordinates, potential temperature Θ and equivalent latitude φ_e, do not require explicit reference to geographic space, and can be derived directly from high-resolution in situ measurements. They are therefore well-suited for studying the evolution of the Antarctic polar vortex throughout the polar winter with respect to the relevant chemical and microphysical processes. By using the introduced coordinate system {χ_N2O, Δχ_N2O} we analyze the well-developed Antarctic vortex investigated during the APE-GAIA (Airborne Polar Experiment – Geophysica Aircraft in Antarctica – 1999) campaign (Carli et al., 2000). A criterion, which uses the local in-situ measurements of χ_i=χ_i(Θ) and attributes the inner vortex edge to a rapid change (δ-step) in the meridional profile of the mixing ratio χ_i, is developed to determine the (Antarctic) inner vortex edge. In turn, we suggest that the outer vortex edge of a well-developed Antarctic vortex can be attributed to the position of a local minimum of the χ_H2O gradient in the polar vortex area. For a well-developed Antarctic vortex, the Δχ_N2O-parametrization of tracer-tracer relationships allows to distinguish the tracer inter-relationships in the vortex core, vortex boundary region and surf zone and to examine their meridional variation throughout these regions. This is illustrated by analyzing the tracer-tracer relationships χ_i : χ_N2O obtained from the in-situ data of the APE-GAIA campaign for i = CFC-11, CFC-12, H-1211 and SF₆. A number of solitary anomalous points in the CFC-11 : N₂O correlation, observed in the Antarctic vortex core, are interpreted in terms of small-scale cross-isentropic dispersion.