Subtropical tropopause break as a possible stratospheric source of ozone in the tropical troposphere

The understanding of the transport of trace chemical species between the stratosphere and the troposphere is necessary for global change prediction. Until recently it was believed that stratospheric inputs, through jet streaks and tropopause folding, should occur only at extratropical latitudes. A case study of a tropopause fold was reported at Pointe-a-Pitre by Gouget et al. (1996). We presently corroborate this first case study by new observations in the Indian Ocean suggesting that stratosphere-troposphere exchanges, induced by the subtropical jet, are actually occurring near the edge of the tropics. Key to these exchanges is the crucial region of the junction between the lowermost stratosphere and the tropical and extratropical tropospheres, defined by Holton (1996) as the intersection zone of the 2 PVU potential vorticity surface with the 380 K potential temperature level. In this paper, the wind and ozone climatological context is given using vertical radiosounding data from Reunion Island (France, 21 degrees S, 55 degrees E) and Irene (South Africa, 25 degrees S, 28 degrees E). A case of subtropical tropopause fold occurring between Madagascar and Reunion Islands is presented using ECMWF and TOMS data. Observations are found to be fairly accordant with a dynamical jet analysis and suggest that stratospheric air intrusions are possible during winter in the sub-tropics edges. (C) 1998 Elsevier Science Ltd. All rights reserved.