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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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The spring transition of the North Pacific jet and its relation to deep stratosphere-to-troposphere mass transport over western North America
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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- Award ID(s):
- 1756958
- PAR ID:
- 10308573
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 21
- Issue:
- 4
- ISSN:
- 1680-7324
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-21-2781-2021
