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Creators/Authors contains: "Butler, Amy H."

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  1. Free, publicly-accessible full text available May 28, 2023
  2. 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 asmore »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.« less
  3. 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 whichmore »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.« less
  4. 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.
  5. 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.
  6. 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.
  7. Abstract This study offers an overview of the low-frequency (i.e., monthly to seasonal) evolution, dynamics, predictability, and surface impacts of a rare Southern Hemisphere (SH) stratospheric warming that occurred in austral spring 2019. Between late August and mid-September 2019, the stratospheric circumpolar westerly jet weakened rapidly, and Antarctic stratospheric temperatures rose dramatically. The deceleration of the vortex at 10 hPa was as drastic as that of the first-ever-observed major sudden stratospheric warming in the SH during 2002, while the mean Antarctic warming over the course of spring 2019 broke the previous record of 2002 by ∼50% in the midstratosphere. This event was preceded by a poleward shift of the SH polar night jet in the uppermost stratosphere in early winter, which was then followed by record-strong planetary wave-1 activity propagating upward from the troposphere in August that acted to dramatically weaken the polar vortex throughout the depth of the stratosphere. The weakened vortex winds and elevated temperatures moved downward to the surface from mid-October to December, promoting a record strong swing of the southern annular mode (SAM) to its negative phase. This record-negative SAM appeared to be a primary driver of the extreme hot and dry conditions over subtropical easternmore »Australia that accompanied the severe wildfires that occurred in late spring 2019. State-of-the-art dynamical seasonal forecast systems skillfully predicted the significant vortex weakening of spring 2019 and subsequent development of negative SAM from as early as late July.« less