Influence of a middle-latitude cyclone on tropospheric ozone distributions during a period of TRACE A

A middle-latitude cyclone occurring during the Transport and Atmospheric Chemistry near the Equator-Atlantic (TRACE A) experiment is examined to determine its influence on distributions of tropospheric ozone over the South Atlantic Ocean. A maximum of tropospheric ozone is located in the vicinity of this cyclone on October 3, 1992. Flight level data and meteorological analyses indicate a downward protrusion of dry, ozone-rich stratospheric air near the cyclone, i.e., a tropopause fold. Backward trajectories show that air parcels arriving in the upper troposphere of the cyclone originate in the stratosphere. Forward trajectories are calculated from these locations having stratospheric histories. They indicate that some air is transported as far north as 22 degrees S, subsiding into the middle troposphere along the southern fringes of a region of enhanced tropospheric ozone that is located west of Africa on October 6. Backward trajectories then are computed along the Greenwich meridian over much of the South Atlantic Ocean. This axis passes through the tropospheric ozone maximum west of Africa and the region of strong horizontal ozone gradients along its southern border. Results indicate that most air parcels arriving north of 20 degrees S (in the ozone-rich region) originate over Africa. Conversely, most parcels arriving south of 20 degrees S (where there is less ozone) originate from the west, passing over the southern half of South America. Thus the tropospheric ozone maximum west of Africa on October 6 appears to be attributable to outflow from Africa, with stratospheric transport being much less important. Formerly stratospheric air near the cyclone on October 3 also is transported forward into the middle troposphere near Madagascar where there is a second maximum of tropospheric ozone on October 6. Backward trajectories from this region indicate that middle-latitude systems exert a much greater influence here than over the South Atlantic. This area experiences relatively little outflow from Africa during our period of study.