A study of air particle motions during a stratospheric warming and their influence on photochemistry

Abstract

Middle stratospheric ten-day trajectories are calculated using satellite data for air particles initially situated on the edge of the circumpolar vortex during the minor warming of January 1979. The trajectory behaviour is consistent with the observed evolution of the isentropic distribution of potential vorticity. Further, from an analysis of a large number of trajectories calculated from closely spaced initial positions it is argued that the trajectories are realistic indicators of transport. In particular, the trajectories divide naturally into a limited number of distinct categories representing either vortex erosion or the continued advection of air around the distorted vortex edge. None of the categories indicate horizontal transport of air into the vortex. A comprehensive photochemical model is integrated along the trajectories and the results are compared with similar integrations along trajectories calculated for a suitably chosen axially symmetric reference atmosphere. In this way it is established that, for the late January warming, the distortion of the vortex from zonal symmetry led to substantial decreases in the destruction of ozone and in the production of nitric acid on the edge of the vortex. Once the particles are stripped from the vortex by the wave breaking, they experience only small changes in photochemistry if the particles remain in middle latitudes, or much larger changes if they are transported as far as the subtropics. Results from the photochemical model are also compared with co-located satellite observations of the constituents ozone, water vapour and nitric acid for the different categories of trajectories. In many cases there are statistically significant discrepancies between observed and modelled concentrations. It is suggested that these discrepancies are due to the presence of other particles transported along trajectories from different locations, and therefore with different chemical properties, contaminating the coarse resolution co-located observations. The findings are entirely consistent with the view that outside the vortex the wave breaking will mix or generate small unresolvable scales in the tracer fields. Some implications of the results for the photochemical modelling of the stratosphere are discussed.