Tracer‐based determination of vortex descent in the 1999/2000 Arctic winter

Abstract

A detailed analysis of available in situ and remotely sensed N 2 O and CH 4 data measured in the 1999/2000 winter Arctic vortex has been performed in order to quantify the temporal evolution of vortex descent. Differences in potential temperature (θ) among balloon and aircraft vertical profiles (an average of 19–23 K on a given N 2 O or CH 4 isopleth) indicated significant vortex inhomogeneity in late fall as compared with late winter profiles. A composite fall vortex profile was constructed for 26 November 1999, whose error bars encompassed the observed variability. High‐latitude extravortex profiles measured in different years and seasons revealed substantial variability in N 2 O and CH 4 on θ surfaces, but all were clearly distinguishable from the first vortex profiles measured in late fall 1999. From these extravortex‐vortex differences we inferred descent prior to 26 November: as much as 397 ± 15 K (1σ) at 30 ppbv N 2 O and 640 ppbv CH 4 , and falling to 28 ± 13 K above 200 ppbv N 2 O and 1280 ppbv CH 4 . Changes in θ were determined on five N 2 O and CH 4 isopleths from 26 November through 12 March, and descent rates were calculated on each N 2 O isopleth for several time intervals. The maximum descent rates were seen between 26 November and 27 January: 0.82 ± 0.20 K/day averaged over 50–250 ppbv N 2 O. By late winter (26 February to 12 March), the average rate had decreased to 0.10 ± 0.25 K/day. Descent rates also decreased with increasing N 2 O; the winter average (26 November to 5 March) descent rate varied from 0.75 ± 0.10 K/day at 50 ppbv to 0.40 ± 0.11 K/day at 250 ppbv. Comparison of these results with observations and models of descent in prior years showed very good overall agreement. Two models of the 1999/2000 vortex descent, SLIMCAT and REPROBUS, despite θ offsets with respect to observed profiles of up to 20 K on most tracer isopleths, produced descent rates that agreed very favorably with the inferred rates from observation.