The existence and the seasonal variability of the Atlantic subtropical potential vorticity barrier controlling the stratosphere-troposphere exchanges between the lowermost extratropical stratosphere and the upper equatorial troposphere are investigated using Measurements of Ozone by Airbus In-Service Aircraft (MOZAIC). The methodology is based on the relationship between strong gradients of potential vorticity and of ozone mixing ratio situated on the cyclonic-shear side of the subtropical jet stream. Episodes of high ozone mixing ratio sampled along MOZAIC flight tracks over the subtropical Atlantic with mixing ratio exceeding 100 ppbv on length scales larger than 200 km on flight levels between 11 and 12 km are studied. A total of 154 high ozone episodes is extracted from the MOZAIC database over the period August 1994 to April 1997. All these high ozone episodes are observed north of 15 ¿N and have lengths ranging from subsynoptic to synoptic scales. It is shown that this barrier effect at 15 ¿N over the central/eastern Atlantic fits with the southernmost latitude of the subtropical jet stream during the period of interest. South of the subtropical jet stream within the latitude band where the Intertropical Convergence Zone oscillates, tens of ozone-rich transients (high-ozone episodes with length scales smaller than 80 km) are sampled within the upper equatorial Atlantic troposphere (9 to 12 km). At present, the origin of these tropical ozone-rich transients is still not clear. Some outlooks are given to investigate the possibility that some of the ozone-rich transients may be interpreted as the final result of tropopause foldings and small scale mixing processes. The seasonal variability of the subtropical barrier is captured when determining the subtropical tropopause break (STB) point for each flight, that is, the southernmost latitude of the southernmost high-ozone episode of a flight, and classifying STB points on a monthly basis. A sinusoidal evolution of STB points appears, reflecting the northernmost (southernmost) position of the dynamical barrier in summer (winter) boreal months. This seasonal variability clearly agrees with that of the position of the subtropical jet stream as derived from mean isotach analysis. ¿ 1998 American Geophysical Union