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1 Introduction :
The Rossby wave breaking events participate directly to the transport of air masses from the tropics to the poles, via the Brewer-Dobson circulation marked by large-scale air rising in the tropics and by sinking in the poles [Holton et al., 1995]. These wave breaking events to midlatitudes also generate air filaments born in the tropics, stretched isentropically and ultimately mixed in the surf zone [Waugh et al., 1994a, 1994b]. Conversely, other filament structures do an opposite transport, from the surf zone to the tropics. The behaviour of long lived chemical species and of potential vorticity implies the existence of a dynamic stratospheric subtropical barrier that controls and limits isentropic horizontal exchanges between tropical and extratropical stratospheres[Grant et al., 1996]
The seasonal variation of the intensity of planetary wave breakings is characterised by a maximum in the winter hemisphere. Hence, larger gradients on the Tropical Stratospheric Reservoir (TSR) edges and a more important filamentary activity are expected during this period. The seasonal variation of intensity and location of the subtropical barrier is also function of altitude and depends significantly on the QBO phase [Randel et al., 1998; Dunkerton et al., 1996 ; Schroeberl et al., 1997].
From a case study based on Reunion island data, this work presents a preliminary analysis of isentropic transport trough the subtropical stratospheric barrier. Reunion Island [21°S, 55°E] is a French territory in the south western part of Indian Ocean. Reunion is one of the few stations steadily recording atmospheric measurements in the southern tropical zone.
From ozone anomaly profiles obtained from ECC radiosoundings and from DIAL O3 lidar measurements, an example of isentropic exchange between 550K and 700K is presented. This case study exchange occurred around the 12th July 2000. A high resolution PV contour advection code (MIMOSA) and a dynamic barrier locating code taking advantage of an area coordinate (LOBADY) are used as a diagnostoc tools. These codes are working on potential vorticity data from ECMWF fields.