The origin of relatively high surface ozone concentrations measured at Izana Observatory (Canary Islands) during the end of March 1996 is studied using a coupled chemistry-GCM (ECHAM4) at T63 resolution (1.875¿¿1.875¿). Meteorological fields (geopotential height, potential vorticity, specific humidity), and a model-simulated stratospheric ozone tracer as well as 3-D back trajectories, show the stratospheric origin of these relatively high surface ozone values caused by cross-tropopause exchange at the western flank of an upper level trough/cut-off low (COL) over the extratropical North-Atlantic Ocean. The good agreement between observations and model results (within 10--15%) indicates that the high resolution chemistry-GCM is a useful tool towards the understanding of natural sources controlling background surface ozone variability. The results underscore the importance of stratosphere-troposphere exchange (STE) during late winter/early spring for lower free tropospheric ozone at subtropical latitudes. ¿ 2000 American Geophysical Union