The Evolutions and Large-Scale Mechanisms of Summer Stratospheric Ozone Intrusion Across Global Hotspots

Stratospheric ozone intrusions can have a significant impact on regional near-surface ozone levels. Especially in summer, intrusions can contribute to extreme ozone events because of preexisting high ozone levels near the surface and cause serious health issues. Considering the increasing trend of surface ozone level, an understanding of stratospheric ozone intrusion is necessary. From a 19-year Whole Atmosphere Community Climate Model, version 6 simulation and a stratospheric origin ozone tracer, we identify the global hotspots of stratospheric intrusions based on extreme tracer concentrations near the surface: North America, Africa, the Mediterranean, and the Middle East. We investigate the common underlying large-scale mechanisms of the stratospheric intrusions over the identified hotspots from the lower stratosphere to the lower troposphere. From the trajectory analysis, we find that the upper-level jet drives isentropic mixing near the jet axis and initiates stratospheric ozone intrusion. Subsequently, climatological descent at the lower troposphere brings the ozone down to the surface, which explains the spatial preference of summertime stratospheric intrusion events. High ozone concentration near the surface is harmful to human health. Occasionally, a significant amount of ozone in the stratosphere intrudes deep into the troposphere and increases the surface ozone levels. During summer, as background ozone concentration is high, it is easy for the ozone level to surpass the health threshold with additional contribution from stratospheric ozone intrusion. In this study, we advanced our understanding of the summer stratospheric intrusions, where they happen frequently, and what drives them. We identified four global hotspots of stratospheric ozone intrusion: North America, Africa, the Mediterranean, and the Middle East, which cover areas not well known to be significantly affected in previous studies. We found that upper tropospheric jet dynamics and lower tropospheric descents both play a role in the stratospheric ozone intrusions and determine the locations affected. Based on the mechanisms, we expect to improve our ability to predict when and where summer stratospheric intrusions may occur. Thereby, our findings can also contribute to the establishment of an early warning system for extreme ozone events in summer. There are four hotspots of summer ozone extremes due to stratospheric ozone, North America, Africa, the Mediterranean, and the Middle East Summer stratospheric intrusions initiate in the jet axis region near tropopause by isentropic mixing Climatological descent drives vertical transport in the lower troposphere and determines the location of the hotspots