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Abstract. Stratosphere-to-troposphere transport (STT) is an important sourceof ozone for the troposphere, particularly over western North America. STTin this region is predominantly controlled by a combination of thevariability and location of the Pacific jet stream and the amount of ozonein the lower stratosphere, two factors which are likely to change ifgreenhouse gas concentrations continue to increase. Here we use WholeAtmosphere Community Climate Model experiments with a tracer ofstratospheric ozone (O3S) to study how end-of-the-century RepresentativeConcentration Pathway (RCP) 8.5 sea surface temperatures (SSTs) andgreenhouse gases (GHGs), in isolation and in combination, influence STT ofozone over western North America relative to a preindustrial controlbackground state. We find that O3S increases by up to 37 % during late winter at 700 hPaover western North America in response to RCP8.5 forcing, with the increasestapering off somewhat during spring and summer. When this response to RCP8.5greenhouse gas forcing is decomposed into the contributions made by futureSSTs alone versus future GHGs alone, the latter are found to be primarilyresponsible for these O3S changes. Both the future SSTs alone and the futureGHGs alone accelerate the Brewer–Dobson circulation, which modifiesextratropical lower-stratospheric ozone mixing ratios. While the future GHGsalone promote a more zonally symmetric lower-stratospheric ozone change dueto enhanced ozone production and some transport, the future SSTs aloneincrease lower-stratospheric ozone predominantly over the North Pacific viatransport associated with a stationary planetary-scale wave. Ozoneaccumulates in the trough of this anomalous wave and is reduced over thewave's ridges, illustrating that the composition of the lower-stratosphericozone reservoir in the future is dependent on the phase and position of thestationary planetary-scale wave response to future SSTs alone, in additionto the poleward mass transport provided by the accelerated Brewer–Dobsoncirculation. Further, the future SSTs alone account for most changes to thelarge-scale circulation in the troposphere and stratosphere compared to theeffect of future GHGs alone. These changes include modifying the positionand speed of the future North Pacific jet, lifting the tropopause,accelerating both the Brewer–Dobson circulation's shallow and deep branches,and enhancing two-way isentropic mixing in the stratosphere. 

Abstract. The El Niño–Southern Oscillation (ENSO) is known to modulate the strength and frequency of stratosphere-to-troposphere transport (STT) of ozone over the Pacific–North American region during late winter to early summer. Dynamical processes that have been proposed to account for this variability include variations in the amount of ozone in the lowermoststratosphere that is available for STT and tropospheric circulation-relatedvariations in the frequency and geographic distribution of individual STTevents. Here we use a large ensemble of Whole Atmosphere Community Climate Model(WACCM) simulations (forced by sea-surface temperature (SST) boundaryconditions consistent with each phase of ENSO) to show that variability inlower-stratospheric ozone and shifts in the Pacific tropospheric jetconstructively contribute to the amount of STT of ozone in the NorthAmerican region during both ENSO phases. In terms of stratosphericvariability, ENSO drives ozone anomalies resembling the Pacific–NorthAmerican teleconnection pattern that span much of the lower stratospherebelow 50 hPa. These ozone anomalies, which dominate over other ENSO-drivenchanges in the Brewer–Dobson circulation (including changes due to both thestratospheric residual circulation and quasi-isentropic mixing), stronglymodulate the amount of ozone available for STT transport. As a result,during late winter (February–March), the stratospheric ozone response to theteleconnections constructively reinforces anomalous ENSO-jet-driven STT ofozone. However, as ENSO forcing weakens as spring progresses into summer(April–June), the direct effects of the ENSO-jet-driven STT transportweaken. Nevertheless, the residual impacts of the teleconnections on theamount of ozone in the lower stratosphere persist, and these anomalies inturn continue to cause anomalous STT of ozone. These results should provehelpful for interpreting the utility of ENSO as a subseasonal predictor ofboth free-tropospheric ozone and the probability of stratospheric ozoneintrusion events that may cause exceedances in surface air qualitystandards.