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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. 

The teleconnection between the Quasi‐Biennial Oscillation (QBO) and the Arctic polar vortex is investigated using Coupled Model Intercomparison Project 6 (CMIP6) models. Output from 14 CMIP6 models is compared with reanalysis, three experiments with prescribed QBOs, one of which has no free polar stratospheric variability, and transient experiments in which a QBO is prescribed in runs previously devoid of a QBO. Each CMIP6 model underestimates the Holton‐Tan effect (HTE), the weakening of the polar vortex expected with QBO easterlies in the tropical lower stratosphere. To establish why, potential vorticity maps are used to investigate longitudinal variations in the teleconnection. Prescribing easterly QBO in the transient experiments promotes more high‐latitude planetary wave breaking by influencing the mid‐latitude stratospheric circulation, particularly over Asia. CMIP6 models that better simulate this response over Asia better simulate the HTE. These models also have stronger 10 hPa QBO westerlies.

 

The teleconnection between the Quasi‐Biennial Oscillation (QBO) and the boreal winter polar vortex, the Holton–Tan effect, is analyzed in the Whole Atmosphere Community Climate Model (WACCM) with a focus on how stationary wave propagation varies by QBO phase. These signals are difficult to isolate in reanalyses because of large internal variability in short observational records, especially when decomposing the data by QBO phase. A 1,500‐year ensemble is leveraged by defining the QBO index at five different isobars between 10 and 70 hPa. The Holton–Tan effect is a robust part of the atmospheric response to the QBO in WACCM with warming of the polar stratosphere during easterly QBO (QBOE). A nudging technique is used to reduce polar stratospheric variability in one simulation. This enables isolation of the impact of the QBO on the atmosphere in the absence of a polar stratospheric response to the QBO: referred to as the “direct effect” and the polar stratospheric response, “indirect effect.” This simulation reveals that the polar stratospheric warming during QBOE pushes the tropospheric jet equatorward, opposing the poleward shift of the jet by the QBOE, especially over the North Pacific. The Holton–Tan effect varies over longitude. The QBO induces stronger planetary wave forcing to the mean flow in the extratropical lower stratosphere between Indonesia and Alaska. The North Pacific polar stratosphere responds to this before other longitudes. What follows is a shift in the position of the polar vortex toward Eurasia (North America) during easterly (westerly) QBO. This initiates downstream planetary wave responses over North America, the North Atlantic, and Siberia. This spatiotemporal evolution is found in transient simulations in which QBO nudging is “switched on.” The North Pacific lower stratosphere seems more intrinsically linked to the QBO while other longitudes appear more dependent on the mutual interaction between the QBO and polar stratosphere.