Wave Breaking and Mixing at the Subtropical Tropopause

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.