We investigate the process of Rossby wave breaking on the subtropical tropopause and the resulting filamentary structures of stratospheric air transported into the upper tropical troposphere. We make extensive use of the Measurements of Ozone by Airbus In-Service Aircraft (MOZAIC) flight database, both for identifying individual wave-breaking events and for analyzing the small-scale features of the filamentation process. Two models are used to provide a dynamically consistent description of the evolution of a particular wave-breaking event, one a limited area, high-resolution general circulation model, MesoNH, the other an isentropic contour advection model. The ability of both of these to represent the small-scale evolution of the filament is examined by explicit comparison with the MOZAIC ozone data; at larger scales, comparison is made with analyses from the European Center for Medium-Range Weather Forecasts. Two other wave-breaking events are presented briefly, on the one hand, verifying that the filamentary structure is ubiquitous, on the other hand, illustrating additional, distinct features of the evolution. A simple analysis of all the MOZAIC flights over the southern tropical Atlantic provides an indication of the frequency of wave-breaking events over this region. Together with the small-scale development suggested by the model studies and present in the MOZAIC data we infer a potentially significant contribution to the global stratosphere-troposphere exchange from the wave-breaking processes. ¿ 2001 American Geophysical Union