Long-lived tracer transport in the Antarctic stratosphere

Recent observations made by the Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosphere Research Satellite (UARS) indicate that during the austral fall, CH4 zonal mean isopleths in the Antarctic vortex appear to descend more rapidly than those of N2O. How is this possible in an isolated region such as the vortex when photochemical sinks are insignificant? To understand these observations, we have run a simulation of the 1992 austral fall using the Goddard Global Spectral Mechanistic Model (GSMM) and the three-dimensional Chemistry and Transport Model (CTM). Model tracer fields show good agreement with the observations over a 4-month period beginning in mid-February. Both the observations and the simulation show that the apparent differential descent occurs during periods of wave activity, while during quiet vortex periods N2O and CH4 descend at approximately the same rate. To understand how wave activity may drive the observed differences between N2O and CH4 behavior, we calculate the terms of the zonal mean tracer tendency equation in the transformed Eulerian Mean (TEM) formulation of Andrews et al. [1987]. Analysis of the terms of the tracer tendency equation shows that each tracer's response to eddy forcing is the source of the apparent differential descent. While descent is the dominant motion inside the vortex, horizontal mixing becomes significant during periods of wave activity and differences in the tracers' meridional gradients affect the relative amounts of each tracer transported into the vortex. This analysis demonstrates the relationship between wave activity, eddy transport, and tracer mixing ratios inside the vortex throughout the fall. In addition, CLAES observations deep in the vortex (70 degrees-80 degrees S) show gradually increasing CH4 mixing ratios from March to September, implying the importance of eddy-driven mixing within the vortex in winter.