Nonlinear effect of compound extreme weather events on ozone formation over the United States

Abstract

Compound events (e.g., co-occurrence of two extreme weather events simultaneously) have the potential to aggravate the impacts of heat wave and stagnation on ozone through nonlinear effects. Using the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem), this study investigated the impact of compound extreme weather events on ozone enhancement in the contiguous U.S. The analyses are based on simulations for the historical (2001–2006) and future (2046–2055) periods previously performed, with emissions in the future following the A1B/B2 emission scenarios produced by the Technology Driver Model (TDM). While A1B reflects rapid economic growth, B2 emphasizes more on local environmental sustainability, hence lower anthropogenic emissions. With overall warming in the future, the frequency and intensity of compound extreme events increase more than that of heat waves. Thus compound events promote higher ozone concentration compared to the single extreme events of heat wave and stagnation in larger areas of the U.S. in the future relative to the present. However, averaged over the U.S., the amplification effect of compound events on ozone enhancement is diminished in the future. Sensitivity experiments to isolate the effect of climate change versus emission change showed that climate change under the future emission scenarios dominates the weakening amplification effect of compound events. Further analyses to examine the nonlinear effect of compound events (NLRE), defined as the difference between the enhanced effect on ozone during compound events and the additive effect of heat waves and stagnation relative to non-extreme, showed that regions prone to positive NLRE tend to have compound events marked by higher temperature and stronger stagnation than the single extreme events. Simulations showed reduced mean positive NLRE in the U.S. in the future, suggesting that future emission reduction may provide a previously unrecognized benefit in ozone pollution control by reducing the impacts of compound events on ozone.