title: Climate variability modulates western US ozone air quality in spring via deep stratospheric intrusions.
abstract: Evidence suggests deep stratospheric intrusions can elevate western US surface ozone to unhealthy levels during spring. These intrusions can be classified as 'exceptional events', which are not counted towards non-attainment determinations. Understanding the factors driving the year-to-year variability of these intrusions is thus relevant for effective implementation of the US ozone air quality standard. Here we use observations and model simulations to link these events to modes of climate variability. We show more frequent late spring stratospheric intrusions when the polar jet meanders towards the western United States, such as occurs following strong La Niña winters (Niño3.4<-1.0 °C). While El Niño leads to enhancements of upper tropospheric ozone, we find this influence does not reach surface air. Fewer and weaker intrusion events follow in the two springs after the 1991 volcanic eruption of Mt. Pinatubo. The linkage between La Niña and western US stratospheric intrusions can be exploited to provide a few months of lead time during which preparations could be made to deploy targeted measurements aimed at identifying these exceptional events.
authors.affiliation: 1] Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey 08540, USA [2] NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08540, USA.
authors.forename: Meiyun
authors.initials: M
authors.lastname: Lin
authors.affiliation: 1] Department of Earth and Environmental Sciences, Columbia University, New York City, New York 10027, USA [2] Lamont-Doherty Earth-Observatory, Columbia University, Palisades, New York 10964, USA.
authors.forename: Arlene M
authors.initials: AM
authors.lastname: Fiore
authors.affiliation: NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08540, USA.
authors.forename: Larry W
authors.initials: LW
authors.lastname: Horowitz
authors.affiliation: NOAA Earth System Research Laboratory, Boulder, Colorado 80305, USA.
authors.forename: Andrew O
authors.initials: AO
authors.lastname: Langford
authors.affiliation: 1] NOAA Earth System Research Laboratory, Boulder, Colorado 80305, USA [2] Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80305, USA.
authors.forename: Samuel J
authors.initials: SJ
authors.lastname: Oltmans
authors.affiliation: Experimental Studies Research Division, Meteorological Service of Canada, Environment Canada, Ontario, Canada M3H5T4.
authors.forename: David
authors.initials: D
authors.lastname: Tarasick
authors.affiliation: 1] Lamont-Doherty Earth-Observatory, Columbia University, Palisades, New York 10964, USA [2] Wegener Center for Climate and Global Change and IGAM/Institute of Physics, University of Graz, Graz 8010, Austria.
authors.forename: Harald E
authors.initials: HE
authors.lastname: Rieder