The Influence of Climate Variability and Change on Stratospheric Intrusions of Ozone over North America

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Ground-level ozone is harmful to human health, and ozone-producing activities must sometimes be curtailed to avoid dangerous ozone concentrations. Efforts to maintain safe ozone levels by limiting production are complicated by the presence of ozone from sources other than local human activity, as higher levels of this "background" ozone necessitate more stringent production caps. The background concentration poses a challenge for air quality management as background levels vary from place to place and also change over time. Thus the need to manage and mitigate ground-level ozone motivates research on the behavior of the background and the fundamental processes that govern its variability in time and space.A significant portion of the background ozone comes from the stratosphere, where ozone is produced naturally by ultraviolet sunlight and can be transported over great distances. The stratosphere extends upward from roughly the flight level of long-distance air travel and is thus well separated from ground-level ozone production and impacts. But ozone-rich air can descend into the troposphere during stratospheric intrusions, which occur as a result of Rossby wave breaking. In this process the large-scale flow becomes strongly sheared and stratospheric air forms filaments which extend and fold into the troposphere and mix with ambient air. The stratospheric contribution implies that the ozone background can be influenced by several forms of atmospheric and climate variability that are known to affect the stratosphere, including El Nino/Southern Oscillation (ENSO) events, the Northern Annular Mode (NAM), and the Quasi-Biennial Oscillation (QBO).Research performed here seeks to understand the processes which control the location and amount of stratospheric ozone entering the troposphere over North America. Two factors are considered, the first of which is the amount of ozone present in the lowest layer of the stratosphere where it can be mixed into the troposphere by intrusions. The amount of ozone in this reservoir is controlled by processes including the equator-to-pole overturning circulation of the stratosphere which can potentially be influenced by ENSO, NAM, and QBO variability. The second factor is north-south shifts of the North Pacific jet stream and accompanying storm track, which modulate the frequency and location of Rossby wave breaking events that produce intrusions over western North America. Such jet shifts are known to occur as a consequence of ENSO and NAM variability. The research is conducted through a combination of observational analysis, working from satellite data and reanalysis products, and computer simulations using the Whole Atmosphere Community Climate Model (WACCM, developed as part of the Community Earth System Model).As noted above, the work has broader impacts due to the need to inform mitigation actions to address the health impacts of ground-level ozone pollution. In addition to pollution concerns the work has relevance to several forms of surface weather that are affected by stratospheric intrusions. In particular, deep intrusions can modulate thunderstorm activity and increase surface winds leading to enhanced wildfire occurrence.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.