Stratosphere–troposphere exchange and surface ozone pollution over tropical regions: a case study of Rossby wave breaking and tropopause folding

Abstract. Stratosphere-troposphere exchange (STE) is a key process by which ozone-rich stratospheric air enters the troposphere, influencing surface air quality. This study analyses an atypical STE event over North America between 6 and 14 March 2016, coinciding with a Phase I ozone contingency in Mexico City. Using ERA5 reanalysis, potential vorticity (PV) diagnostics, ozone tracers, Lagrangian trajectories, and isentropic analyses, the event is linked to anticyclonic Rossby wave breaking, a cut-off low, and a persistent tropopause fold. Deep intrusions of high-PV air reached mid- and lower-tropospheric levels, with maximum downward transport one day before the contingency. Equatorward wave amplification enabled coherent isentropic transport, allowing ozone-rich air to descend efficiently over elevated basins in Mexico. Backward trajectories confirmed stratospheric origins, while isentropic advection quantified quasi-horizontal transport along 320–340 K surfaces. Tropopause folding, strengthened by the subtropical jet and local topography, contributed an ozone mixing ratio of ~8 × 10-8 kg kg-1 near the surface, acting as a precursor to exceedance levels. The study also identifies recurrent tropopause folds preceding high-ozone episodes, underscoring the recurring influence of STE on regional air quality. These findings highlight how topography, Rossby wave dynamics, and quasi-horizontal transport pathways modulate surface ozone at low tropical latitudes. These emphasize the importance of monitoring synoptic precursors and incorporating STE diagnostics into high-resolution air quality forecasts to improve prediction in complex subtropical environments. This case demonstrates how mid-latitude disturbances can directly affect tropical air quality during boreal winter–spring.