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Abstract. Today's Arctic is characterized by a lengthening of the sea ice melt season, as well as by fast and at times unseasonal melt events. Such anomalous melt cases have been identified in Pacific and Atlantic Arctic sector sea ice studies. Through observational analyses, we document an unprecedented, concurrent marginal ice zone melt event in the Bering Sea and Labrador Sea in March of 2023. Taken independently, variability in the cold-season ice edge at synoptic timescales is common. However, such anomalous, short-term ice loss over either region during the climatological sea ice maxima is uncommon, and the tandem ice loss that occurred qualifies this as a rare event. The atmospheric setting that supported the unseasonal melt events was preceded by a sudden stratospheric warming event amidst background La Niña conditions that led to positive tropospheric height anomalies across much of the Arctic and the development of anomalous mid-troposphere ridges over the ice loss regions. These large-scale anticyclonic centers funneled extremely warm and moist airstreams onto the ice causing melt. Further analysis identified the presence of atmospheric rivers within these warm airstreams whose characteristics likely contributed to this bi-regional ice melt event. Whether such a confluence of anomalous wintertime events associated with troposphere–stratosphere coupling may occur more often in a warming Arctic remains a research area ripe for further exploration. 

Abstract. Today’s Arctic is characterized by a lengthening of the sea ice melt season, but also by fast and at times unseasonal melt events. Such anomalous melt cases have been identified in Pacific and Atlantic Arctic sector sea ice studies. Through observational analyses, we document an unprecedented, simultaneous marginal ice zone melt event in the Bering Sea and Labrador Sea in March of 2023. Taken independently, variability in the cold season ice edge at synoptic time scales is common. However, such anomalous, short-term ice loss over either region during the climatological sea ice maxima is uncommon, and the tandem ice loss that occurred qualifies this as a rare event. The atmospheric setting that supported the unseasonal melt events was preceded by a sudden stratospheric warming event that, along with ongoing La Niña teleconnections, led to positive tropospheric height anomalies across much of the Arctic and the development of anomalous mid-troposphere ridges over the ice loss regions. These large-scale anticyclonic centers funneled extremely warm and moist airstreams onto the ice causing melt. Further analysis identified the presence of atmospheric rivers within these warm airstreams whose characteristics likely contributed to this bi-regional ice melt event. Whether such a confluence of anomalous wintertime events associated with troposphere-stratosphere coupling may occur more often in a warming Arctic remains a research area ripe for further exploration. 

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. 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. 

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. 

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. 

Abstract The stratospheric quasi-biennial oscillation (QBO) induces temperature anomalies in the lower stratosphere and tropical tropopause layer (TTL) that are cold when lower-stratospheric winds are easterly and warm when winds are westerly. Recent literature has indicated that these QBO temperature anomalies are potentially important in influencing the tropical troposphere, and particularly in explaining the relationship between the QBO and the Madden–Julian oscillation (MJO). The authors examine the variability of QBO temperature anomalies across several time scales using reanalysis and observational datasets. The authors find that, in boreal winter relative to other seasons, QBO temperature anomalies are significantly stronger (i.e., colder in the easterly phase of the QBO and warmer in the westerly phase of the QBO) on the equator, but weaker off the equator. The equatorial and subtropical changes compensate such that meridional temperature gradients and thus (by thermal wind balance) equatorial zonal wind anomalies do not vary in amplitude as the temperature anomalies do. The same pattern of stronger on-equatorial and weaker off-equatorial QBO temperature anomalies is found on decadal time scales: stronger anomalies are seen for 1999–2019 compared to 1979–99. The causes of these changes to QBO temperature anomalies, as well as their possible relevance to the MJO–QBO relationship, are not known. 

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.