Effects of El Niño on summertime ozone air quality in the eastern United States.

We investigate the effect of El Niño on maximum daily 8-hour average surface ozone over the eastern United States in summer during 1980-2016. El Niño can influence the extra-tropical climate through the propagation of stationary waves, leading to (1) reduced transport of moist, clean air into the mid- and southern Atlantic states and greater subsidence, reduced precipitation, and increased surface solar radiation in this region, as well as (2) intensified southerly flow into the south central states, which here enhances flux of moist and clean air. As a result, each standard deviation increase in the Niño 1+2 index is associated with an increase of 1-2 ppbv ozone in the Atlantic states and a decrease of 0.5-2 ppbv ozone in the south central states. These influences can be predicted 4 month in advance. We show that U.S. summertime ozone responds differently to eastern-type El Niño events compared to central-type events.