Meteorological analysis of tropospheric ozone profiles at Bermuda

As part of the North Atlantic Regional Experiment (NARE) a number of ozonesonde profiles were obtained from Bermuda during the spring and summer of 1993. We present meteorological case studies of two instances of elevated O-3 mixing ratio in the middle and upper troposphere which took place during the summer. The ozonesonde profile of July 9, 1993, indicated ozone mixing ratios exceeding 100 parts per billion by volume (ppbv) from similar to 5 km to above 15 km above sea level. A series of profiles for August 2-4 indicated elevated ozone in a layer which initially extended from 8 to 9 km and thickened to cover the 8- to 14-km range, Isentropic trajectories lead back to potential source areas over North America 2 to 5 days prior to the events, Objective analyses based on radiosonde data demonstrate that the air masses, as indicated by the trajectories, pass through areas of elevated isentropic potential vorticity located in troughs in the tropopause-level geopotential height field. The conformation and temporal development suggest that active planetary wave breaking was responsible for stratosphere-troposphere exchange in these systems, These results suggest an important role for O-3 of stratospheric origin in the western North Atlantic Ocean area, transported into the troposphere upstream over or near North America, It is notable that these events took place in the summer months of July and August, when such events had been considered unlikely.