| Summary: | The stratospheric influence on summertime high surface ozone ( $\mathrm{O_3}$ ) events is examined using a twenty-year simulation from the Whole Atmosphere Community Climate Model. We find that $\mathrm{O_3}$ transported from the stratosphere makes a significant contribution to the surface $\mathrm{O_3}$ variability where background surface $\mathrm{O_3}$ exceeds the 95 th percentile, especially over western U.S. Maximum covariance analysis is applied to $\mathrm{O_3}$ anomalies paired with stratospheric $\mathrm{O_3}$ tracer anomalies to identify the stratospheric intrusion and the underlying dynamical mechanism. The first leading mode corresponds to deep stratospheric intrusions in the western and northern tier of the U.S., and intensified northeasterlies in the mid-to-lower troposphere along the west coast, which also facilitate the transport to the eastern Pacific Ocean. The second leading mode corresponds to deep intrusions over the Intermountain Regions. Both modes are associated with eastward propagating baroclinic systems, which are amplified near the end of the North Pacific storm tracks, leading to strong descents over the western U.S.
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