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 withcoordinate systems that follow the vortex flow. We propose here atwo-dimensional quasi-Lagrangian coordinate system {chii,Deltachii}, based on the mixing ratio of a long-livedstratospheric trace gas i, and its systematic use with i =N2O, in order to describe the structure of a well-developedAntarctic polar vortex. In the coordinate system {chii,Deltachii} the mixing ratio chii is thevertical coordinate and Deltachii =chii(Theta)-chiivort(Theta)is the meridional coordinate (chiivort(Theta)being a vertical reference profile in the vortex core). Thequasi-Lagrangian coordinates {chii,Deltachii} persist for much longer time than standardisentropic coordinates, potential temperature Theta and equivalentlatitude phie, do not require explicit reference togeographic space, and can be derived directly from high-resolution insitu measurements. They are therefore well-suited for studying theevolution of the Antarctic polar vortex throughout the polar winter withrespect to the relevant chemical and microphysical processes. By usingthe introduced coordinate system {chiN2O,DeltachiN2O} we analyze the well-developedAntarctic vortex investigated during the APE-GAIA (Airborne PolarExperiment - Geophysica Aircraft in Antarctica - 1999) campaign (Carliet al., 2000). A criterion, which uses the local in-situ measurements ofchii=chii(Theta) and attributes the innervortex edge to a rapid change (delta-step) in the meridional profileof the mixing ratio chii, is developed to determine the(Antarctic) inner vortex edge. In turn, we suggest that the outer vortexedge of a well-developed Antarctic vortex can be attributed to theposition of a local minimum of the chiH2Ogradient in the polar vortex area. For a well-developed Antarcticvortex, the DeltachiN2O-parametrization oftracer-tracer relationships allows to distinguish the tracerinter-relationships in the vortex core, vortex boundary region and surfzone and to examine their meridional variation throughout these regions.This is illustrated by analyzing the tracer-tracer relationshipschii : chiN2O obtained from thein-situ data of the APE-GAIA campaign for i = CFC-11, CFC-12, H-1211 andSF6. A number of solitary anomalous points in the CFC-11 :N2O correlation, observed in the Antarctic vortex core, areinterpreted in terms of small-scale cross-isentropic dispersion.