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I. Introduction

Eddy transport barriers, resulting from the contribution of horizontal shear and isentropic gradient of PV, separate the tropical stratospheric reservoir from the mid-latitude stratosphere (Holton et al., 1995). Aerosol and trace gas steep gradients clearly materialise the edges of this reservoir (Grant et al., 1996). Sporadic disruptions of these barriers are occurring through planetary wave breaking and the generation of laminae. Yet, the height-latitude structure of dynamical and chemical tracer fields is complex and disruption mechanisms controlling the release of material between the tropics and mid-latitudes are still under study.

Moreover, the decrease of total ozone at the northern hemisphere middle latitudes is now well recognised (Scientific Assessment of Ozone Depletion : 1994, WMO, 1995). It is much larger in winter/spring (about 6% per decade) than in summer/autumn (3% per decade). It occurs mainly in the lower stratosphere. One possible cause of decrease in ozone concentration in the lower mid-latitude stratosphere is a change in the exchange rate of air between mid-latitudes and subtropics/tropics.

Knowledge of mass exchange between different parts of the atmosphere is then essential to understand the transport and distribution of long-lived atmospheric trace constituents, including ozone and atmospheric aerosols. It is also important for calculating the radiative budget and understanding the short- and long-term changes of anomalous phenomena, such as the "ozone hole" in the stratosphere over Antarctica.

Transport of air from troposphere to the stratosphere takes place mainly in the tropics. By studying satellite records of the dispersal of volcanic aerosols from tropical eruptions, Trepte and Hitchman (1992) concluded that there is a barrier inhibiting the transport of stratospheric air from the tropics to mid-latitudes. Thus, except for intrusions by planetary-scale waves and/or by tropospheric cyclonic-scale disturbances, the tropics are substantially isolated from the mid-latitudes throughout the lower stratosphere.

Randel et al. (1993) have suggested, through analyses of global maps of gas tracers obtained by the UARS experiments, that the transport from tropics to middle latitudes is similar to planetary wave erosion of the polar winter vortices, where material is pulled off the outer edge and mixed into mid-latitudes.

Similarly, transport out of the tropics occurs in Rossby wave breaking events in which filaments of tropical air are drawn into middle latitudes in the form of large-scale "tongues" through the surf zone.

In the present paper we report on a case study of an observed tropic/extra-tropic air mass exchange event detected by LIDAR through aerosol loading in the low stratosphere, and analysed by the use of ECMWF fields, a high resolution contour advection model and water vapour infrared satellite images.

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