The meteorological environment of the tropospheric ozone maximum over the tropical South Atlantic Ocean

Abstract

Atmospheric flow patterns are examined over the South Atlantic Ocean where a maximum of tropospheric ozone has been observed just west of southern Africa. We investigate the flow climatology during October and perform a case study for 6 days during October 1989. Analyses from the European Center for Medium‐Range Weather Forecasting are employed, and a high‐resolution global spectral model is used to prepare forecasts during the period. Horizontal and vertical motions are examined and used to prepare three‐dimensional backward trajectories from the region of greatest ozone. An initially zonally symmetric distribution of ozone is treated as a passive tracer and advected by three‐dimensional flows forecast by the global model. Results from the passive tracer simulation indicate that three‐dimensional advection alone can produce a maximum of tropospheric ozone in the observed location. In addition, the trajectories suggest that by‐products of biomass burning could be transported to the area of maximum ozone. Low‐level flow from commonly observed regions of burning in Africa streams westward to the area of interest. Over Brazil, if the burning by‐products are carried into the upper troposphere by convective process, they then could be transported eastward to the ozone feature in approximately 5 days. There is considerable subsidence over the tropical southern Atlantic, such that stratospheric influences also are a factor in producing the ozone maximum. Both planetary‐scale and transient synoptic‐scale circulation features play major roles in the various transport processes that influence the region. In summary, the observed tropospheric ozone maximum appears to be caused by a complex set of horizontal and vertical advections, transport from regions of biomass burning, and stratospheric influences.