Descent and mixing in the 1999-2000 northern polar vortex inferred from in situ tracer measurements

In situ measurements from the Lightweight Airborne Chromatograph Experiment (LACE) and the Airborne Chromatograph for Atmospheric Trace Species (ACATS-IV) taken during the SAGE III Ozone Loss and Validation Experiment (SOLVE) are used to examine the descent and mixing in the 1999-2000 northern stratospheric polar vortex. LACE was flown twice on an in situ balloon platform, in November 1999 just after the vortex formed and in March 2000 near the end of the vortex lifetime. The aim of this paper is to use simple models of vortex transport to try to explain the changes seen in the long-lived tracers measured by LACE between the two vortex flights. It is the high precision and long-term accuracy of the observations that allow small differences in the data from a number of species obtained on different flights to be used to infer transport processes. Changes in tracer profiles as a function of height or potential temperature can be attributed, to first order, to descent in the vortex. A calculation which used six tracers shows that the total descent was a strong function of potential temperature, from 200 K in the middle stratosphere to 50 K in the lower stratosphere over the nearly 4-month period between flights. Observed changes in long-lived tracer-tracer correlations require a mixing process to have occurred since descent alone cannot change the correlations. Two simple models of mixing are used to examine the tracer correlation changes. One model simulates entrainment of midlatitude air across the vortex edge with relatively efficient mixing within the vortex. A second model simulates the effect of differential descent and subsequent efficient mixing within a vortex that is entirely isolated from the midlatitudes. It is shown that the differential descent and mixing calculation produces results which are much more consistent among all the tracer correlations compared to the results from midlatitude entrainment. The unique sensitivity of SF6 as a tracer of mesospheric air makes it an outlier in both the descent and mixing calculations. The inclusion of a small fraction of air from the mesosphere is necessary to bring the SF6 mixing and descent calculations into agreement with the other tracers. The conclusions regarding descent and descent rates in the vortex are consistent with other modeling studies. The results indicate that mixing of midlatitude air into the winter vortex is not a significant contributor to the observed ozone changes in the 1999/2000 season.