SUBTROPICAL TROPOPAUSE BREAK AS A POSSIBLE STRATOSPHERIC SOURCE OF OZONE IN THE TROPICAL TROPOSPHERE

The understanding of the transport of trace chemical species between t he stratosphere and the troposphere is necessary for global change pre diction. Until recently it was believed that stratospheric inputs, thr ough jet streaks and tropopause folding, should occur only at extratro pical latitudes. A case study of a tropopause fold was reported at Poi nte-a-Pitre by Gouget et al. (1996). We presently corroborate this fir st case study by new observations in the Indian Ocean suggesting that stratosphere-troposphere exchanges, induced by the subtropical jet, ar e actually occurring near the edge of the tropics. Key to these exchan ges is the crucial region of the junction between the lowermost strato sphere and the tropical and extratropical tropospheres, defined by Hol ton (1996) as the intersection zone of the 2 PVU potential vorticity s urface with the 380 K potential temperature level. In this paper, the wind and ozone climatological context is given using vertical radiosou nding data from Reunion Island (France, 21 degrees S, 55 degrees E) an d Irene (South Africa, 25 degrees S, 28 degrees E). A case of subtropi cal tropopause fold occurring between Madagascar and Reunion Islands i s presented using ECMWF and TOMS data. Observations are found to be fa irly accordant with a dynamical jet analysis and suggest that stratosp heric air intrusions are possible during winter in the sub-tropics edg es. (C) 1998 Elsevier Science Ltd. All rights reserved.