Tropospheric Ozone Assessment Report II: Stratosphere–troposphere exchange of ozone

Stratosphere-troposphere exchange (STE) of ozone is a significant source of tropospheric ozone, influencing air quality, atmospheric chemistry and climate forcing. This assessment synthesizes recent advances in understanding the magnitude, mechanisms, variability and future evolution of ozone transport across the tropopause, updating earlier reviews using observations, reanalyses and global chemistry-climate models. Globally, monthly mean STE is driven by the large-scale circulation of the stratosphere, but on shorter time scales and synoptic scales episodic dynamical processes such as tropopause folds, cutoff lows and wave breaking are locally important. Based on a synthesis of published estimates for 2000-2020, the present-day global net STE of ozone is assessed to be 380 (350-450) Tg O3 yr-1. This net flux is composed of a strong downward transport from the stratosphere into the troposphere of approximately 540 Tg O3 yr-1 in the extratropics, partially offset by an upward transport of ozone from the troposphere into the stratosphere of about 190 Tg O3 yr-1 in the tropics. Around 60% of the downward extratropical transport occurs in the Northern Hemisphere. STE exhibits pronounced seasonality as well as interannual variability linked to climate modes including the El Niño-Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO). Analyses indicate a decline in STE of 2.7% decade-1 during the late twentieth century associated with ozone depletion, while future projections consistently project an increase in STE of around 4.7% decade-1 as global circulation responds to climate change and stratospheric ozone recovers. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.