Rapid changes in surface ozone mixing ratios (up to 15 ppbv over 12 hours) at the Cerro Tololo (CT) station (70¿W, 30¿S, 2200 m) over the period 1996--2000 are analyzed. These changes explain most of the wintertime variability of the ozone data at CT. Since the wintertime data show no significant diurnal cycle, local circulations and in situ photochemical processes cannot account for the sudden changes in ozone. Rather, a synoptic-scale forcing of the changes is described on the basis of composite analyses of reanalysis data. Synoptic configurations associated with sudden changes in ozone were separated into W and D cases, according to the local evolution of humidity. W cases are characterized by humidity values in excess of 4 hPa lasting one or two days before the ozone maximum. D cases show no significant change in humidity before the ozone maximum. Both in W and in D cases, stratospheric air appears to be advected from polar latitudes to midlatitudes and subtropical latitudes during the development of cutoff lows and deep troughs. The origin of the air arriving at CT during a W and a D case study was assessed using mesoscale numerical simulations. The model calculations for the W case show a deep stratospheric intrusion reaching the lower troposphere in subtropical latitudes during the amplification and mature stages of a cutoff low. For D cases the stratospheric intrusion is mostly confined to midlatitudes and to an oceanic area far from the site of study. Analyses of trajectories show that in both cases, high ozone mixing ratios at the surface occur in connection with air parcels having a stratospheric history.