Future Changes in Stratospheric Ozone Intrusions in CESM

Stratospheric ozone intrusions represent a significant contribution to the tropospheric ozone budget, particularly in the Western United States. The factors controlling the frequency and intensity of these intrusions are not well understood. Potential impacts of climate change on these intrusions are even less explored but are essential for predicting and regulating the tropospheric ozone budget in the future. We quantify these changes in two 40-year time-slice simulations of the Community Earth System Model with WACCM (CESM-WACCM) version 1.5. One simulation is a pre-industrial control run with 1850s chemical concentrations; the other an RCP 8.5 scenario with projected 2100 greenhouse gas concentrations. The runs include climatological SSTs fixed to the pre-industrial or future period, removing ENSO and other significant modes of internal variability. Analysis of the runs shows that, in the pre-industrial scenario, the cold season Northern Annular Mode is a significant forcing on the spring intrusion budget in the western United States. Further analysis of the impact of climate change on the spatial structure of these relationships is conducted using the RCP 8.5 scenario simulation, highlighting the emergence of a region of increased intrusion and transport activity in the North Pacific. However, this particular model configuration results in declining jet modulated intrusion activity over the Southwestern United States, indicating high dynamic variability in the spatial distribution of intrusion phenomena.