Coordinated ozonesonde launches from the Intercontinental Transport Experiment (INTEX) Ozonesonde Network Study (IONS) (http://croc.gsfc.nasa.gov/intex/ions.html) in July‐August 2004 provided nearly 300 O3 profiles from eleven North American sites and the R/V Ronald H. Brown in the Gulf of Maine. With the IONS period dominated by low‐pressure conditions over northeastern North America (NENA), the free troposphere in that region was frequently enriched by stratospheric O3. Stratospheric O3 contributions to the NENA tropospheric O3 budget are computed through analyses of O3 laminae (Pierce and Grant, 1998; Teitelbaum et al., 1996), tracers (potential vorticity, water vapor), and trajectories. The lasting influence of stratospheric incursions into the troposphere is demonstrated, and the computed stratospheric contribution to tropospheric column O3 over the R/V Ronald H. Brown and six sites in Michigan, Virginia, Maryland, Rhode Island, and Nova Scotia, 23% ± 3%, is similar to summertime budgets derived from European O3 profiles (Collette and Ancellet, 2005). Analysis of potential vorticity, Wallops ozonesondes (37.9°N, 75.5°W), and Measurements of Ozone by Airbus In‐service Aircraft (MOZAIC) O3 profiles for NENA airports in June‐July‐August 1996–2004 shows that the stratospheric fraction in 2004 may be typical. Boundary layer O3 at Wallops and northeast U.S. sites during IONS also resembled O3 climatology (June‐July‐August 1996–2003). However, statistical classification of Wallops O3 profiles shows the frequency of profiles with background, nonpolluted boundary layer O3 was greater than normal during IONS.

This paper discusses Rossby wave breaking on the isentropic surfaces that intersect the subtropical tropopause, using winds and isentropic potential vorticity from high-resolution meteorological analysis. The focus is both on particular aspects of individual wave breaking events, as well as on more general aspects such as the spatial and temporal distribution of the mixing associated with these events. The direction and intensity of wave breaking is shown to exhibit the same dependence on stagnation points in the wind field as that seen in previous highly idealized numerical model studies. Wave breaking that results in stratospheric intrusions into the troposphere can be categorized as weak or strong, depending on the development of filaments or larger, coherent vortices or cutoff lows. The events presented show a deep vertical structure that approximately spans the region between the 330-K and 370-K isentropic surfaces, where the tropopause is steeply sloping through the subtropical jets. This is in contrast with tropospheric intrusions into the stratosphere, which appear to be less directly related to wave breaking than to the interaction of coherent structures in the tropospheric circulation. Transport estimates during weak wave breaking are shown to be very sensitive to the definition of the tropopause. Contour stretching is used as a measure of the mixing properties at the tropopause associated with the Rossby wave breaking and reveals longitudinal inhomogeneities that are consistent with the different structure of the subtropical jets over the Atlantic and Pacific Oceans. A strong seasonal cycle and interannual variability are also present, with generally stronger mixing in the summer and weaker mixing over the western Pacific during the warm phase of the El Niño-Southern Oscillation.