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

Stratosphere–troposphere exchange (STE) is a fundamental process in which ozone-rich stratospheric air enters the troposphere, modulating tropospheric ozone distributions and, under favorable conditions, influencing surface air quality. This study investigates an atypical STE event over North America from 6 to 14 March 2016, which coincided with a severe surface ozone pollution episode in Mexico City. Analyses using ERA5 reanalysis, potential vorticity (PV) diagnostics, ozone tracers, Lagrangian trajectories, and isentropic methods attribute the event to anticyclonic Rossby wave breaking, a PV cut-off low, and a persistent tropopause fold. Deep intrusions of high-PV air penetrated the troposphere, with maximum downward transport occurring approximately 1 d before the peak in surface ozone (O3) mixing ratios. Equatorward wave amplification facilitated coherent isentropic transport, enabling efficient descent of ozone-rich stratospheric air over the elevated basins of central Mexico. Backward trajectory analysis confirmed stratospheric origins, while isentropic advection quantified quasi-horizontal transport along 320–340 K surfaces. Tropopause folding, intensified by the subtropical jet and the elevated terrain of central Mexico, resulted in enhanced O$_3$ mixing ratios of 8 × 10$^{−8}$ kg kg$^{−1}$ at 750–700 hPa, preconditioning background ozone levels prior to surface exceedance rather than serving as a direct surface source. ... mehrThe analysis identifies recurrent tropopause folds preceding high-ozone episodes, highlighting the persistent role of STE as a dynamical preconditioning mechanism for regional air-quality extremes. These results indicate that the location of cities in elevated tropical basins, in combination with Rossby wave dynamics and quasi-horizontal isentropic transport pathways, determines the depth, persistence, and significance of STE.