Observed boreal fall season (September--November) 1991 surface CO data from Mace Head, Ireland, are characterized by particularly high mixing ratios, as evidence by high means, medians, and maxima for those months, relative to the same data for boreal fall 1992. Air parcels characterized by elevated CO during fall 1991 are attributed to European sources on the basis of isentropic back trajectory analysis. A histogram of the 1991 data shows a bimodal distribution representing two discrete source regions, North Atlantic and European, while the 1992 data show only one mode, representing primarily zonal westerly flow over the North Atlantic Ocean. A similar distinction exists in O3 data between the two years. Considerable interannual variability has important implications for observationalists and modelers alike; an inherent uncertainty is introduced by basing any determination of trend from only a few years of data. An isentropic flow climatology for Mace Head illustrates significant differences in the regional-scale flow patterns to Mace Head between the 1991 and the 1992 fall seasons. These differences have been examined in terms of general dynamical principles and tropical/extratropical coupling. There is evidence of the existence of Rossby wave links with the tropical upper troposphere over South America and the central Pacific Ocean which are responsible for the climatic forcing giving rise to the observed interannual variation in large-scale flow patterns and influencing the chemical character of air parcels reaching Mace Head. Using CO as a tracer for short-lived continental anthropogenic O3 precursors and calculating ΔO3/ΔCO for air parcel trajectories following anticyclonic paths over western Europe during the late summer and fall season of 1991, we estimate O3 production over western Europe at about 66 (40--96) billion moles of O3 per summer (~3 Tg O3 per summer), based on 1985 CO emission inventory data (37 Tg Co yr-1 for western Europe). ¿ American Geophysical Union 1994