Reconstruction of the constituent distribution and trends in the Antarctic polar vortex from ER‐2 flight observations

Abstract

During the Airborne Antarctic Ozone Experiment, the high‐altitude ER‐2 aircraft measured ozone, chlorine monoxide, and nitrous oxide concentrations in the south polar region. These measurements have been analyzed using conservative coordinate transformations to potential temperature‐N 2 O and potential temperature‐potential vorticity space. The latter transformation is equivalent to interpreting trace species observations within the modified Lagrangian mean (MLM) coordinate system. With certain assumptions about zonal symmetry, this transformation allows for the interpretation of chemical changes of constituents independent of meteorological variability. The method also allows for a reasonable reconstruction of constituent distributions outside the aircraft flight track. Our analysis shows that the MLM transformed ozone concentration decreases at about 0.06 ppmv (parts per million by volume) per day between 20 and 16 km altitude inside the polar vortex during the mid‐August to mid‐September period. These ozone changes must be chemical in origin; they are also collocated with the region of high ClO. Outside the CPR (chemically perturbed region) at the highest aircraft altitudes, ozone systematically increases, suggesting a diabatic cooling of the order of 0.3–0.6 K/d. Within the CPR the cooling rate appears to be less than 0.2 K/d. The MLM analysis technique creates a picture of the general chemical structure of the Austral polar vortex which shows that air deep within the chemically perturbed region has subsided substantially in relation to the air outside. However, there is also a tongue of high ozone air which extends from mid‐latitudes downward along the stratospheric jet at 65°W and 6O°S. An examination of the last three flight days, September 20–22, 1987, shows that during this period the polar vortex shifts systematically equatorward along the Antarctic Peninsula. Apparent changes in the constituents measured over this period result from sampling air progressively further into the vortex.