May 12th, 2015
Exposure to ground-level ozone is associated with numerous effects on human health. It can also have harmful effects on sensitive vegetation and ecosystems. There is mounting evidence that deep stratospheric intrusions (when “good” ozone is forced from the stratosphere into the troposphere by strong winds) can elevate surface ozone to unhealthy levels at high-elevation western U.S. regions during spring. This study demonstrates a link between strong La Niña winters and late spring stratospheric intrusions in the western Rockies. This link is important for developing seasonal forecasts a few months in advance, to aid in western U.S. air quality planning and for effective implementation of U.S. ozone standards.
Inter-annual climate variability modulates the frequency of these episodes of stratospheric influence on Western U.S. surface ozone. Specifically, late spring deep stratospheric intrusions occur more frequently when the polar jet stream meanders towards the central western United States, as occurs following strong La Niña winters in the tropical Pacific Ocean. While El Niño leads to enhancements of upper tropospheric ozone, the authors find this influence does not extend to the western U.S. surface. Fewer and weaker events followed in the two springs after the 1991 volcanic eruption of Mt. Pinatubo. The authors demonstrate that such connections occur coherently in observations and hindcast simulations with the NOAA GFDL’s new chemistry-climate model, AM3.
The authors underscore that ozone produced from U.S. anthropogenic emissions dominates pollution events during summer and at low-elevation U.S. regions. Nevertheless, stratospheric intrusions reaching surface air can occur with sufficient frequency in spring when the polar jet is unusually contorted, as to confound ozone measures in high-elevation western U.S. regions. While the Clean Air Act allows for screening of such “exceptional events” from counting towards non-attainment determinations, failure to identify them accurately would imply a need for additional controls on regional emissions from human activities in order to attain the standard.
Most El Niño and La Niña episodes develop in late spring to summer and peak near the end of the calendar year. Our finding that deep stratospheric intrusions reaching Western U.S. surface air occur more frequently in the spring following the mature winter phase of a strong La Niña episode raises the possibility of developing seasonal predictions several months in advance. Knowledge of the likelihood of an upcoming active stratospheric intrusion spring season could allow Western U.S. air quality planners to prepare accordingly, such as conducting daily forecasts for public health alerts, and deploying targeted measurements aimed at identifying exceptional events.