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  1. 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. 
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  2. Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that stratospheric aerosol injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far, most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections, namely an equatorial injection, an annual injection of equal amounts of SO2 at 15∘ N and 15∘ S, an annual injection of equal amounts of SO2 at 30∘ N and 30∘ S, and a polar strategy injecting SO2 at 60∘ N and 60∘ S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres, and changes in the speed of the residual circulation. These impacts tend to maximise under the equatorial injection strategy and become smaller as the aerosols are injected away from the Equator into the subtropics and higher latitudes. In conjunction with the differences in direct radiative impacts at the surface, these different stratospheric changes drive different impacts on the extratropical modes of variability (Northern and Southern Annular modes), including important consequences on the northern winter surface climate, and on the intensity of tropical tropospheric Walker and Hadley circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical, and chemical processes driving the atmospheric circulation and ozone response to SAI and lay the foundation for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.

     
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  3. 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. 
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  4. Abstract. Stratospheric circulation is a critical part of the Arctic ozone cycle.Sudden stratospheric warming events (SSWs) manifest the strongest alterationof stratospheric dynamics. During SSWs, changes in planetary wavepropagation vigorously influence zonal mean zonal wind, temperature, andtracer concentrations in the stratosphere over the high latitudes. In thisstudy, we examine six persistent major SSWs from 2004 to 2020 using theModern-Era Retrospective analysis for Research and Applications, Version 2(MERRA-2). Using the unique density of observations around the Greenlandsector at high latitudes, we perform comprehensive comparisons of high-latitude observations with the MERRA-2 ozone dataset during the six majorSSWs. Our results show that MERRA-2 captures the high variability of mid-stratospheric ozone fluctuations during SSWs over high latitudes. However,larger uncertainties are observed in the lower stratosphere and troposphere.The zonally averaged stratospheric ozone shows a dramatic increase of9 %–29 % in total column ozone (TCO) near the time of each SSW, which lastsup to 2 months. This study shows that the average shape of the Arcticpolar vortex before SSWs influences the geographical extent, timing, andmagnitude of ozone changes. The SSWs exhibit a more significant impact onozone over high northern latitudes when the average polar vortex is mostlyelongated as seen in 2009 and 2018 compared to the events in which the polarvortex is displaced towards Europe. Strong correlation (R2=90  %) isobserved between the magnitude of change in average equivalent potentialvorticity before and after SSWs and the associated averaged total columnozone changes over high latitudes. This paper investigates the differentterms of the ozone continuity equation using MERRA-2 circulation, whichemphasizes the key role of vertical advection in mid-stratospheric ozoneduring the SSWs and the magnified vertical advection in elongated vortexshape as seen in 2009 and 2018. 
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  5. Abstract

    The impacts of Stratospheric Aerosol Injection (SAI) on the atmosphere and surface climate depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. We use a set of CESM2(WACCM6) SAI simulations achieving three different levels of global mean surface cooling and demonstrate that unlike some direct surface climate impacts driven by the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in the high latitude circulation and ozone are more complex and could be non‐linear. This manifests in our simulations by disproportionally larger Antarctic springtime ozone loss, significantly larger intra‐ensemble spread of the Arctic stratospheric jet and ozone responses, and non‐linear impacts on the extratropical modes of surface climate variability under the strongest‐cooling SAI scenario compared to the weakest one. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI.

     
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  6. Abstract. It has been suggested that increased stratospheric sulfate aerosol loadings following large, low latitude volcanic eruptions can lead to wintertime warming over Eurasia through dynamical stratosphere–troposphere coupling. We here investigate the proposedconnection in the context of hypothetical future stratospheric sulfategeoengineering in the Geoengineering Large Ensemble simulations. In thosegeoengineering simulations, we find that stratospheric circulation anomalies that resemble the positive phase of the Northern Annular Mode in winter are a distinguishing climate response which is absent when increasing greenhouse gases alone are prescribed. This stratospheric dynamical response projects onto the positive phase of the North Atlantic Oscillation, leading to associated side effects of this climate intervention strategy, such as continental Eurasian warming and precipitation changes. Seasonality is a key signature of the dynamically driven surface response. We find an opposite response of the North Atlantic Oscillation in summer, when no dynamical role of the stratosphere is expected. The robustness of the wintertime forced response stands in contrast to previously proposed volcanic responses. 
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  7. Abstract. Stratosphere-to-troposphere mass transport to the planetaryboundary layer (STT-PBL) peaks over the western United States during borealspring, when deep stratospheric intrusions are most frequent. Thetropopause-level jet structure modulates the frequency and character ofintrusions, although the precise relationship between STT-PBL and jetvariability has not been extensively investigated. In this study, wedemonstrate how the North Pacific jet transition from winter to summer leadsto the observed peak in STT-PBL. We show that the transition enhancesSTT-PBL through an increase in storm track activity which produceshighly amplified Rossby waves and more frequent deep stratosphericintrusions over western North America. This dynamic transition coincideswith the gradually deepening PBL, further facilitating STT-PBL in spring. Wefind that La Niña conditions in late winter are associated with anearlier jet transition and enhanced STT-PBL due to deeper and more frequenttropopause folds. An opposite response is found during El Niñoconditions. El Niño–SouthernOscillation (ENSO) conditions also influence STT-PBL in late spring or earlysummer, during which time La Niña conditions are associated with largerand more frequent tropopause folds than both El Niño and ENSO-neutralconditions. These results suggest that knowledge of ENSO state and the North Pacific jet structure in late winter could be leveraged for predicting thestrength of STT-PBL in the following months. 
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  8. Abstract. Forecasts of Pacific jet variability are used to predictstratosphere-to-troposphere transport (STT) and tropical-to-extratropicalmoisture export (TME) during boreal spring over the Pacific–North Americanregion. A retrospective analysis first documents the regionality of STT andTME for different Pacific jet patterns. Using these results as a guide,Pacific jet hindcasts, based on zonal-wind forecasts from the European Centrefor Medium-Range Weather Forecasting Integrated Forecasting System, areutilized to test whether STT and TME over specific geographic regions may bepredictable for subseasonal forecast leads (3–6 weeks ahead of time). Largeanomalies in STT to the mid-troposphere over the North Pacific, TME to thewest coast of the United States, and TME over Japan are found to have the bestpotential for subseasonal predictability using upper-level wind forecasts. STTto the planetary boundary layer over the intermountain west of the UnitedStates is also potentially predictable for subseasonal leads but likely onlyin the context of shifts in the probability of extreme events. While STT andTME forecasts match verifications quite well in terms of spatial structure andanomaly sign, the number of anomalous transport days is underestimatedcompared to observations. The underestimation of the number of anomaloustransport days exhibits a strong seasonal cycle, which becomes steadily worseas spring progresses into summer. 
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  9. Abstract

    The impacts of Stratospheric Aerosol Injection (SAI) strategies on the Southern Annular Mode (SAM) are analyzed with the Community Earth System Model. Using a set of simulations with fixed single‐point SO2injections we demonstrate the first‐order dependence of the SAM response on the latitude of injection, with the northern hemispheric and equatorial injections driving a response corresponding to a positive phase of SAM and the southern hemispheric injections driving a negative phase of SAM. We further demonstrate that the results can to first order explain the differences in the SAM responses diagnosed from the two recent large ensembles of geoengineering simulations utilizing more complex injection strategies – Geoengineering Large Ensemble and Assessing Responses and Impacts of Solar climate intervention on the Earth system with Stratospheric Aerosol Injection (GLENS and ARISE‐SAI) – as driven by the differences in the simulated sulfate aerosol distributions. Our results point to the meridional extent of aerosol‐induced lower stratospheric heating as an important driver of the sensitivity of the SAM response to the injection location.

     
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