Wave Breaking and Mixing at the Subtropical Tropopause

Abstract

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.

Transport of aerosols into the UTLS and their impact on the Asian monsoon region as seen in a global model simulation

“…It is likely that large-scale Rossby wave mixing processes are acting to transport some of these aerosols towards the equator (e.g. Scott and Cammas, 2002;Homeyer and Bowman, 2013).…”

The generation of zonal jets by large-scale mixing

“…The situation is similar to the predominantly one-way wave breaking at the edge of the winter stratospheric polar vortex, in which polar air is entrained and mixed into the surf zone, 18,21,22 or on the subtropical tropopause. 23 A corresponding entrainment of the fluid from the region below the lower edge of the mixed zone at y 2 results in the growth of the extent of the mixed zone, while its average potential vorticity remains unchanged. Related behaviour was observed in a spherical shallow water model in which an initial critical layer in a pre-existing background flow exhibited a poleward migration in conjunction with the development of the mixed zone.…”

Global large-scale stratosphere–troposphere exchange in modern reanalyses

“…One of the most well-known subtropical STE processes is Rossby wave breaking, which is a quasiisentropic transport process that can be predominantly poleward (TST), equatorward (STT), or bidirectional (e.g., Seo and Bowman, 2001;Scott and Cammas, 2002;Homeyer and Bowman, 2013). Stratospheric intrusions along the subtropical jet stream occur outside of the tropics but can bring extratropical lower stratosphere air into the tropical middle and upper troposphere (Waugh and Polvani, 2000).…”