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Found 27 results

  1. The Corresponding Tropospheric Environments during Downward-extending and Non-downward-extending Events of Stratospheric Northern Annular Mode Anomalies Authors: Zhang, Wenshou Tian, Jiankai Zhang, Jinlong Huang, Fei Xie and Mian Xu Published: 25th January, 2019 Abstract: Using the NCEP/NCAR reanalysis dataset, this study classifies stratospheric Northern Annular Mode (NAM) anomalies during negative/positive phase into two categories—anomalies extending into the troposphere (referred as negative/positive TEs) and those not extending into the troposphere (referred as negative/positive NTEs), and the corresponding tropospheric environments during the TEs and NTEs are identified. Compared with that for the negative NTEs, the upward wave fluxes entering the stratosphere are stronger and more persistent during the negative TEs. Furthermore, the stronger and more persistent upward wave fluxes during the negative TEs are due to more favorable conditions for upward wave propagation, which is manifested by fewer occurrences of negative refractive index squared in the mid-high latitude troposphere and stronger wave intensity in the mid-high latitude troposphere. However, the tropospheric wave intensity plays a more important role than the tropospheric conditions of planetary wave-propagation in modulating the upward wave fluxes into the stratosphere. Stronger and more persistent upward wave fluxes in the negative TEs, particularly wave-1 fluxes, are closely related to the negative geopotential height anomalies over the North Pacific and positive geopotential height anomalies over the Euro-Atlantic sectors. These negative/positive geopotential height anomalies over the North Pacific/Euro-Atlantic are related to the positive/negative diabatic heating anomalies and the decreased/increased blocking activities in the mid-high latitudes. The subtropical diabatic heating could also impact on the strength of the mid-high latitude geopotential height anomalies through modulating horizontal wave fluxes. For positive NAM events, the results are roughly similar to those for negative NAM events, but with opposite signal. Link to full paper:
  2. Observed and Simulated Teleconnections Between the Stratospheric Quasi‐Biennial Oscillation and Northern Hemisphere Winter Atmospheric Circulation Authors: Martin B. Andrews , Jeff R. Knight, Adam A. Scaife, Yixiong Lu, Tongwen Wu, Lesley J. Gray and Verena Schenzinger Published: 15th January, 2019 Abstract: The Quasi‐Biennial Oscillation (QBO) is the dominant mode of interannual variability in the tropical stratosphere, with easterly and westerly zonal wind regimes alternating over a period of about 28 months. It appears to influence the Northern Hemisphere winter stratospheric polar vortex and atmospheric circulation near the Earth's surface. However, the short observational record makes unequivocal identification of these surface connections challenging. To overcome this, we use a multicentury control simulation of a climate model with a realistic, spontaneously generated QBO to examine teleconnections with extratropical winter surface pressure patterns. Using a 30‐hPa index of the QBO, we demonstrate that the observed teleconnection with the Arctic Oscillation (AO) is likely to be real, and a teleconnection with the North Atlantic Oscillation (NAO) is probable, but not certain. Simulated QBO‐AO teleconnections are robust, but appear weaker than in observations. Despite this, inconsistency with the observational record cannot be formally demonstrated. To assess the robustness of our results, we use an alternative measure of the QBO, which selects QBO phases with westerly or easterly winds extending over a wider range of altitudes than phases selected by the single‐level index. We find increased strength and significance for both the AO and NAO responses, and better reproduction of the observed surface teleconnection patterns. Further, this QBO metric reveals that the simulated AO response is indeed likely to be weaker than observed. We conclude that the QBO can potentially provide another source of skill for Northern Hemisphere winter prediction, if its surface teleconnections can be accurately simulated. Link to full paper:
  3. Enhanced Stratosphere/Troposphere Coupling During Extreme Warm Stratospheric Events with Strong Polar-Night Jet Oscillation Author: Dieter H.W. Peter, Andrea Schneidereit and Alexey Y. Karpechko Published: November 29th, 2018 Abstract: Extreme warm stratospheric events during polar winters from ERA-Interim reanalysis and CMIP5-ESM-LR runs were separated by duration and strength of the polar-night jet oscillation (PJO) using a high statistical confidence level of three standard deviations (strong-PJO events). With a composite analysis, we demonstrate that strong-PJO events show a significantly stronger downward propagating signal in both, northern annular mode (NAM) and zonal mean zonal wind anomaly in the stratosphere in comparison with non-PJO events. The lower stratospheric EP-flux-divergence difference in ERA-Interim was stronger in comparison to long-term CMIP5-ESM-LR runs (by a factor of four). This suggests that stratosphere–troposphere coupling is stronger in ERA-Interim than in CMIP5-ESM-LR. During the 60 days following the central date (CD), the Arctic oscillation signal was more intense during strong-PJO events than during non-PJO events in ERA-Interim data in comparison to CMIP5-ESM-LR runs. During the 15-day phase after CD, strong PJO events had a significant increase in stratospheric ozone, upper tropospheric zonally asymmetric impact, and a regional surface impact in ERA-Interim. Finally, we conclude that the applied high statistical threshold gives a clearer separation of extreme warm stratospheric events into strong-PJO events and non-PJO events including their different downward propagating NAM signal and tropospheric impacts. Link to full paper: Credit goes to @sebastiaan1973 for finding this paper
  4. Eurasian snow cover variability and links to winter climate in the CMIP5 models Author: Jason C. Furtado, Judah L. Cohen, Amy H. Butler, Emily E. Riddle and Arun Kumar Published: 31st January, 2015 Abstract: Observational studies and modeling experiments illustrate that variability in October Eurasian snow cover extent impacts boreal wintertime conditions over the Northern Hemisphere (NH) through a dynamical pathway involving the stratosphere and changes in the surface-based Arctic Oscillation (AO). In this paper, we conduct a comprehensive study of the Eurasian snow–AO relationship in twenty coupled climate models run under pre-industrial conditions from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our analyses indicate that the coupled climate models, individually and collectively, do not capture well the observed snow–AO relationship. The models lack a robust lagged response between October Eurasian snow cover and several NH wintertime variables (e.g., vertically propagating waves and geopotential heights). Additionally, the CMIP5 models do not simulate the observed spatial distribution and statistics of boreal fall snow cover across the NH including Eurasia. However, when analyzing individual 40-year time slices of the models, there are periods of time in select models when the observed snow–AO relationship emerges. This finding suggests that internal variability may play a significant role in the observed relationship. Further analysis demonstrates that the models poorly capture the downward propagation of stratospheric anomalies into the troposphere, a key facet of NH wintertime climate variability irrespective of the influence of Eurasian snow cover. A weak downward propagation signal may be related to several factors including too few stratospheric vortex disruptions and weaker-than-observed tropospheric wave driving. The analyses presented can be used as a roadmap for model evaluations in future studies involving NH wintertime climate variability, including those considering future climate change. Link to full paper: Credit goes to Tom @Isotherm for recommending this excellent paper.
  5. Transfer of the solar signal from the stratosphere to the troposphere: Northern Winter Authors: Katja Matthes, Yuhji Kuroda, Kunihiko Kodera, Ulrike Langematz Published: March 2006 Abstract: The atmospheric response to the solar cycle has been previously investigated with the Freie Universität Berlin Climate Middle Atmosphere Model (FUB‐CMAM) using prescribed spectral solar UV and ozone changes as well as prescribed equatorial, QBO‐like winds. The solar signal is transferred from the upper to the lower stratosphere through a modulation of the polar night jet and the Brewer‐Dobson circulation. These model experiments are further investigated here to show the transfer of the solar signal from the lower stratosphere to the troposphere and down to the surface during Northern Hemisphere winter. Analysis focuses on the transition from significant stratospheric effects in October and November to significant tropospheric effects in December and January. The results highlight the importance of stratospheric circulation changes for the troposphere. Together with the poleward‐downward movement of zonal wind anomalies and enhanced equatorward planetary wave propagation, an AO‐like pattern develops in the troposphere in December and January during solar maximum. In the middle of November, one third of eddy‐forced tropospheric mean meridional circulation and surface pressure tendency changes can be attributed to the stratosphere, whereas most of the polar surface pressure tendency changes from the end of November through the middle of December are related to tropospheric mechanical forcing changes. The weakening of the Brewer‐Dobson circulation during solar maximum leads to dynamical heating in the tropical lower stratosphere, inducing circulation changes in the tropical troposphere and down to the surface that are strongest in January. The simulated tropospheric effects are identified as indirect effects from the stratosphere because the sea surface temperatures are identical in the solar maximum and minimum experiment. These results confirm those from other simplified model studies as well as results from observations. Link to full paper:
  6. Progress in research of stratosphere-troposphere interactions: Application of isentropic potential vorticity dynamics and the effects of the Tibetan Plateau Authors: Rongcai Ren, Guoxiong Wu, Ming Cai, Shuyue Sun, Xin Liu and Weiping Li Published: 19th October, 2014 Abstract: This paper reviews recent progress in understanding isentropic potential vorticity (PV) dynamics during interactions between the stratosphere and troposphere, including the spatial and temporal propagation of circulation anomalies associated with the winter polar vortex oscillation and the mechanisms of stratosphere-troposphere coupling in the global mass circulation framework. The origins and mechanisms of interannual variability in the stratospheric circulation are also reviewed. Particular attention is paid to the role of the Tibetan Plateau as a PV source (via its thermal forcing) in the global and East Asian atmospheric circulation. Diagnosis of meridional isentropic PV advection over the Tibetan Plateau and East Asia indicates that the distributions of potential temperature and PV over the east flank of the Tibetan Plateau and East Asia favor a downward and southward isentropic transport of high PV from the stratosphere to the troposphere. This transport manifests the possible influence of the Tibetan Plateau on the dynamic coupling between the stratosphere and troposphere during summer, and may provide a new framework for understanding the climatic effects of the Tibetan Plateau. Link to full paper: Ren Wu etc JMR_QXXB.pdf
  7. Linking stratospheric circulation extremes and minimum Arctic sea ice extent Workshop Presentation: Aspen Global Change Institute, Aspen, Colorado Workshop Programme: “Understanding the Causes and Consequences of Polar Amplification” - June 12th -16th, 2017 Presenter: Karen Smith, Lorenzo Polvani and Bruno Tremblay Presentation Date: 14th June, 2017 Link to full presentation (32 minute video): Link to presentation (slides and charts only): Link to full agenda and presentations:
  8. Preconditioning of Arctic Stratospheric Polar Vortex Shift Events Authors: Jinlong Huang and Wenshou Tian Published: 23rd March, 2018 Abstract: This study examines the preconditioning of events in which the Arctic stratospheric polar vortex shifts toward Eurasia (EUR events), North America (NA events), and the Atlantic (ATL events) using composite analysis. An increase in blocking days over northern Europe and a decrease in blocking days over the Bering Strait favor the movement of the vortex toward Eurasia, while the opposite changes in blocking days over those regions favor the movement of the vortex toward North America. An increase in blocking days over the eastern North Atlantic and a decrease in blocking days over the Bering Strait are conducive to movement of the stratospheric polar vortex toward the Atlantic. These anomalous precursor blocking patterns are interpreted in terms of the anomalous zonal wave-1 or wave-2 planetary wave fluxes into the stratosphere that are known to influence the vortex position and strength. In addition, the polar vortex shift events are further classified into events with small and large polar vortex deformation, since the two types of events are likely to have a different impact at the surface. A significant difference in the zonal wave-2 heat flux into the lower stratosphere exists prior to the two types of events and this is linked to anomalous blocking patterns. This study further defines three types of tropospheric blocking events in which the spatial patterns of blocking frequency anomalies are similar to the blocking patterns prior to EUR, NA, and ATL events, respectively, and our reanalysis reveals that the polar vortex is indeed more likely to shift toward Eurasia, North America, and the Atlantic in the presence of the above three defined tropospheric blocking events. These shifts of the polar vortex toward Eurasia, North America, and the Atlantic lead to statistically significant negative height anomalies near the tropopause and corresponding surface cooling anomalies over these three regions. Link to full paper: Please note that the full paper is currently behind an AMS paywall. Link to the AMS website:
  9. Role of gravity waves in vertical coupling during sudden stratospheric warmings Authors: Erdal Yiğit and Alexander S. Medvedev Published: 24th August, 2016 Abstract: Gravity waves are primarily generated in the lower atmosphere, and can reach thermospheric heights in the course of their propagation. This paper reviews the recent progress in understanding the role of gravity waves in vertical coupling during sudden stratospheric warmings. Modeling of gravity wave effects is briefly reviewed, and the recent developments in the field are presented. Then, the impact of these waves on the general circulation of the upper atmosphere is outlined. Finally, the role of gravity waves in vertical coupling between the lower and the upper atmosphere is discussed in the context of sudden stratospheric warmings. Link to full paper:
  10. Coupling of Stratospheric Warmings with Mesospheric Coolings in Observations and Simulations Authors: Christoph Zülicke, Erich Becker, Vivien Matthias, and Dieter H. W. Peters Published: 19th January, 2018 Abstract: The vertical coupling between the stratosphere and the mesosphere is diagnosed from polar cap temperatures averaged over 60°–90°N with a new method: the joint occurrence of a warm stratosphere at 10 hPa and a cold mesosphere at 0.01 hPa. The investigation of an 11-yr-long dataset (2004–15) from Aura-MLS observations shows that such mesospheric coupling days appear in 7% of the winter. During major sudden stratospheric warming events mesospheric couplings are present with an enhanced average daily frequency of 22%. This daily frequency changes from event to event but broadly results in five of seven major warmings being classified as mesospheric couplings (2006, 2008, 2009, 2010, and 2013). The observed fraction of mesospheric coupling events (71%) is compared with simulations of the Kühlungsborn Mechanistic Circulation Model (KMCM), the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), and the Whole Atmosphere Community Climate Model (WACCM). The simulated fraction of mesospheric coupling events ranges between 57% and 94%, which fits the observations. In searching for causal relations weak evidence is found that major warming events with strong intensity or split vortices favor their coupling with the upper mesosphere. More evidence is found with a conceptual model: an effective vertical coupling between 10 and 0.01 hPa is provided by deep zonal-mean easterlies at 60°N, which are acting as a gravity-wave guide. The explained variance is above 40% in the four datasets, which indicates a near-realistic simulation of this process. Link to full paper: Please note that the full paper is currently behind an AMS paywall Link to the AMS website:
  11. Three Types of Synoptic Events and Their Associated Troposphere-Stratosphere Coupling Authors: Hannah Attard Presentation: 28th June, 2017 Abstract: Events that perturb the extratropical waveguide have the potential to excite Rossby waves. These Rossby waves can disperse and propagate horizontally, impacting the downstream weather, and vertically inducing extreme stratospheric conditions (e.g., sudden stratospheric warmings [SSWs]), potentially impacting surface weather sometime later. Studies have shown that synoptic-scale blocking events are one such type of waveguide perturbation event that can induce these vertically propagating Rossby waves prior to SSWs. This work examines the role of blocking and two other types of tropospheric synoptic events, bombing extratropical cyclones and the extratropical transition (ET) of tropical cyclones, that can excite tropopause waveguide perturbations and produce vertically propagating Rossby waves. Utilizing NASA’s MERRA-2 dataset the tropopause-level zonal-mean meridional eddy heat flux anomaly was calculated with respect to the climatological mean for all events that occurred in the Northern Hemisphere winter months from 1980-2015. The event lists included 288 blocking events, 876 bombing events, and 128 ET events. The zonal mean meridional eddy heat flux is utilized as it is directly proportional to the vertical component of the Eliassen-Palm (EP) flux vector. The zonal mean meridional eddy heat flux anomaly thus describes when there is anomalous upward wave propagation. The goal of this study is to understand the characteristics of the subset of synoptic events that are associated with the largest (and smallest) upward EP flux anomaly to elucidate the types of synoptic events that have the potential to precede extreme stratospheric conditions. The results show that each type of synoptic event can be associated with upward EP flux but there is variability in the magnitude of upward EP flux anomaly between and within the event types. The largest 100-hPa zonal-mean meridional eddy heat flux anomaly (46.5 K m s-1) averaged over the 11 days following event identification was associated with a blocking event but the smallest 100-hPa zonal-mean meridional eddy heat flux anomaly (−30.4 K m s-1) averaged over the 11 days following event identification was associated with a bombing event. In a composite sense, blocking events that occurred in Europe (West Pacific) were followed by a statistically significant maximum (minimum) in the 100-hPa zonal-mean meridional eddy heat flux anomaly. Bombing events that occurred in the West Pacific (Atlantic) were followed by a statistically significant maximum (minimum) in the 100-hPa zonal-mean meridional eddy heat flux anomaly. The presented results explore the variability in the synoptic-event associated tropopause zonal-mean meridional eddy heat flux anomaly from a climatological and composite perspective. Link to full paper: Please note that the full paper is behind a paywall but this was presented at the "19th Conference on Middle Atmosphere" hosted by the University of California, Department of Atmospheric Sciences, UCLA, Los Angeles. Session 9 entitled "The Polar Vortices & Planetary Waves: Upward Coupling" contained six excellent presentations 28th June 2017. Link to Conference Presentation Video:
  12. Comparing Extreme Heat Flux Events for Wavenumber 1 and 2 Authors: Andreas Miller and R. A. Plumb Presentation: 28th June, 2017 Abstract: Many studies of stratosphere-troposphere coupling have focused on composites based on the state of the polar stratospheric vortex (e.g. Charlton and Polvani (2007); Garnkelet al. (2010); Mitchell et al. (2013)). A smaller number of studies (Polvani and Waugh, 2004; Dunn-Sigouin and Shaw, 2015; Watt-Meyer and Kushner, 2015) have instead centered their attention on the flux into the stratosphere. We add to the latter class of studies by comparing composites of the largest positive and negative anomalous heat fluxes at 100 hPa for wavenumbers 1 and 2. Using MERRA 1 reanalysis data, we show that longer averaging times generally lead to fewer extreme events while the correlation to wave amplitudes of geopotential height and the zonal mean wind in the stratosphere becomes more robust. Integrating over the polar cap, wefind that the 100 hPa heat flux is well correlated with the flux at all levels in the stratosphere. However, the connection to tropospheric heat fluxes is limited to positive heat flux anomalies. A more detailed look at the longitudinal and temporal structure of the positive heat flux events allows us to identify statistically signicant anomalies in the geopotential heightfields associated with the anomalous fluxes. In the troposphere, wave-1 events are dominated by negative height anomalies over the Aleutian and positive values across most of the Atlantic. Wave-2 events are characterized by a more complicated NAM-like structure over the Atlantic and positive heights over the Aleutian. These results are compared to the results of Martius et al. (2009) and Colucci and Kelleher (2015) on the connection of blockings to stratospheric dynamics. Finally, we look at proles of planetary wave amplitudes and phases over the life cycle of the events. We find remarkably low variance in the phase as a function of height for both wave 1 and 2. This result is not sensitive to removing the climatological height field, further supporting the idea of linear interference as discussed by Watt-Meyer and Kushner (2015). Link to full paper: Please note that the full paper is behind a paywall but this was presented at the "19th Conference on Middle Atmosphere" hosted by the University of California, Department of Atmospheric Sciences, UCLA, Los Angeles. Session 9 entitled "The Polar Vortices & Planetary Waves: Upward Coupling" contained six excellent presentations 28th June 2017. Link to Conference Presentation Video:
  13. Why are Upward EP-Flux and Temperature Positively Skewed in the Stratosphere? Authors: Oliver Watt-Meyer and Paul J. Kushner Published: 11th December, 2017 Abstract: The distribution of temperatures in the wintertime polar stratosphere is significantly positively skewed, which has important implications for the characteristics of ozone chemistry and stratosphere–troposphere coupling. The typical argument for why the temperature distribution is skewed is that radiative balance sets a firm lower limit, while planetary wave driving can force much larger positive anomalies in temperature. However, the distribution of the upward Eliassen–Palm (EP) flux is also positively skewed, and this suggests that dynamics may play an important role in setting the skewness of the temperature distribution. This study explains the skewness of the upward EP flux distribution by appealing to the ideas of linear interference. In this framework, fluxes are decomposed into a linear term (LIN) that measures the coherence of the wave anomaly and the climatological wave and an additional nonlinear term (NONLIN) that depends only on the wave anomaly. It is shown that when filtered by wavenumber, there is a clear nonlinear dependence between LIN and NONLIN: the terms cancel when LIN is negative, but they reinforce each other when LIN is positive. This leads to the positive skewness of the upward wave activity flux. A toy model of wave interference is constructed, and it is shown that the westward vertical tilt of the climatological wave is the key ingredient to a positively skewed upward EP flux distribution. The causes of the skews of the LIN and NONLIN distributions themselves are shown to be related to relationships between wave phase and amplitude, and wave phase and vertical tilt, respectively. Link to full paper: Please note that the full paper is behind a paywall but this was presented at the "19th Conference on Middle Atmosphere" hosted by the University of California, Department of Atmospheric Sciences, UCLA, Los Angeles. Session 9 entitled "The Polar Vortices & Planetary Waves: Upward Coupling" contained six excellent presentations 28th June 2017. Link to Conference Presentation Video:
  14. Interannual variability in the gravity wave drag – vertical coupling and possible climate links Authors: Petr Šácha, Jiri Miksovsky and Petr Pisoft Published: 24th May, 2018 Abstract: Gravity wave drag (GWD) is an important driver of the middle atmospheric dynamics. However, there are almost no observational constraints on its strength and distribution (especially horizontal). In this study we analyze orographic GWD (OGWD) output from Canadian Middle Atmosphere Model simulation with specified dynamics (CMAM-sd) to illustrate the interannual variability in the OGWD distribution at particular pressure levels in the stratosphere and its relation to major climate oscillations. We have found significant changes in the OGWD distribution and strength depending on the phase of the North Atlantic Oscillation (NAO), quasi-biennial oscillation (QBO) and El Niño–Southern Oscillation. The OGWD variability is shown to be induced by lower-tropospheric wind variations to a large extent, and there is also significant variability detected in near-surface momentum fluxes. We argue that the orographic gravity waves (OGWs) and gravity waves (GWs) in general can be a quick mediator of the tropospheric variability into the stratosphere as the modifications of the OGWD distribution can result in different impacts on the stratospheric dynamics during different phases of the studied climate oscillations. Link to full paper: Link to the Supplement:
  15. Tropospheric and Stratospheric Precursors to the January 2013 Sudden Stratospheric Warming Authors: Hannah E. Attard, Rosimar Rios-Berrios, Corey T. Guastini and Andrea L. Lang. Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York Published: March 2016 Abstract: This paper investigates the tropospheric and stratospheric precursors to a major sudden stratospheric warming (SSW) that began on 6 January 2013. Using the Climate Forecast System Reanalysis dataset, the analysis identified two distinct decelerations of the 10-hPa zonal mean zonal wind at 65°N in December in addition to the major SSW, which occurred on 6 January 2013 when the 10-hPa zonal mean zonal wind at 65°N reversed from westerly to easterly. The analysis shows that the two precursor events preconditioned the stratosphere for the SSW. Analysis of the tropospheric state in the days leading to the precursor events and the major SSW suggests that high-latitude tropospheric blocks occurred in the days prior to the two December deceleration events, but not in the days prior to the SSW. A detailed wave activity flux (WAF) analysis suggests that the tropospheric blocking prior to the two December deceleration events contributed to an anomalously positive 40-day-average 100-hPa zonal mean meridional eddy heat flux prior to the SSW. Analysis of the stratospheric structure in the days prior to the SSW reveals that the SSW was associated with enhanced WAF in the upper stratosphere, planetary wave breaking, and an upper-stratospheric/lower-mesospheric disturbance. These results suggest that preconditioning of the stratosphere occurred as a result of WAF initiated by tropospheric blocking associated with the two December deceleration events. The two December deceleration events occurred in the 40 days prior to the SSW and led to the amplification of wave activity in the upper stratosphere and wave resonance that caused the January 2013 SSW. Link to full paper:
  16. Defining Sudden Stratospheric Warming in Climate Models: Accounting for Biases in Model Climatologies Authors: Junsu Kima et al Published: 4th March, 2017 Abstract: A sudden stratospheric warming (SSW) is often defined as zonal-mean zonal wind reversal at 10 hPa and 60°N. This simple definition has been applied not only to the reanalysis data but also to climate model output. In the present study, it is shown that the application of this definition to models can be significantly influenced by model mean biases (i.e., more frequent SSWs appear to occur in models with a weaker climatological polar vortex). To overcome this deficiency, a tendency-based definition is proposed and applied to the multimodel datasets archived for phase 5 of the Coupled Model Intercomparison Project (CMIP5). In this definition, SSW-like events are defined by sufficiently strong vortex deceleration. This approach removes a linear relationship between SSW frequency and intensity of the climatological polar vortex in the CMIP5 models. The models’ SSW frequency instead becomes significantly correlated with the climatological upward wave flux at 100 hPa, a measure of interaction between the troposphere and stratosphere. Lower stratospheric wave activity and downward propagation of stratospheric anomalies to the troposphere are also reasonably well captured. However, in both definitions, the high-top models generally exhibit more frequent SSWs than the low-top models. Moreover, a hint of more frequent SSWs in a warm climate is found in both definitions. Link to full paper:
  17. Planetary‐scale wave activity as a source of varying tropospheric response to stratospheric sudden warming events: A case study Authors: Patrick Martineau and Seok‐Woo Son Published: 3rd October, 2013 Abstract: Stratospheric Sudden Warming (SSW) events are typically, but not always, accompanied by negative Northern Annular Mode anomalies in the troposphere. However, large uncertainties remain as to which dynamical processes are responsible for those anomalies. In order to highlight sources of variability in stratosphere‐troposphere coupling among SSW events, we present a case study of three selected events and show detailed Transformed Eulerian Mean diagnostics for momentum changes in the stratosphere and troposphere in the course of those events. Our results suggest that planetary‐scale waves, especially the zonal wave number 2 component, may play an important role not only for the onset of tropospheric anomalies in response to SSW events but also for introducing variability in the vertical coupling, i.e., whether the tropospheric circulation anomalies lag, lead, or occur simultaneous to the weakening of the vortex. Particularly, the meridional propagation of those waves in the upper troposphere could be an important factor that determines whether SSW events lag or lead tropospheric anomalies. Link to full paper:
  18. Revisiting the ENSO–SSW Relationship Authors: Kanghyun Song and Seok-Woo Son Published: 24th, December 2017 Abstract: Stratospheric sudden warming (SSW) events exhibit pronounced interannual variability. Based on zonal wind reversals at 60°N and 10 hPa, it has been suggested that SSW events occur more preferentially during El Niño–Southern Oscillation (ENSO) winters (both El Niño and La Niña winters) than during ENSO-neutral winters. This relationship is reevaluated here by considering seven different SSW definitions. For all definitions, SSW events are detected more frequently during El Niño winters than during ENSO-neutral winters, in agreement with a strengthened planetary-scale wave activity. However, such a systematic relationship is not found during La Niña winters. While three SSW definitions, including the wind-reversal definition, show a higher SSW frequency during La Niña winters than during ENSO-neutral winters, other definitions show no difference or even lower SSW frequency during La Niña winters. This result, which is qualitatively insensitive to the choice of reanalysis datasets, ENSO indices, and SST datasets, indicates that the reported ENSO–SSW relationship is dependent on the details of the SSW definition. This result is interpreted in terms of different background wind, latitudinal extent of wind reversal, and planetary-scale wave activity during El Niño and La Niña winter SSW events. Link to full paper: Not yet available from a free-to-view source - if anyone can provide a link directly to this paper, please can you notify me by pm (or through the portal "reply to topic" or in the Interactive Area) Link to AMS Online Library for those with subscriptions:
  19. The association between stratospheric weak polar vortex events and cold air outbreaks in the Northern Hemisphere Authors: Erik W. Kolstad, Tarjei Breiteig and Adam A. Scaife Published: April 2010 Abstract: Previous studies have identified an association between temperature anomalies in the Northern Hemisphere and the strength of stratospheric polar westerlies. Large regions in northern Asia, Europe and North America have been found to cool during the mature and late stages of weak vortex events in the stratosphere. A substantial part of the temperature changes are associated with changes in the Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) pressure patterns in the troposphere. The apparent coupling between the stratosphere and the troposphere may be of relevance for weather forecasting, but only if the temporal and spatial nature of the coupling is known. Using 51 winters of re-analysis data, we show that the development of the lower-tropospheric temperature relative to stratospheric weak polar vortex events goes through a series of well-defined stages, including the formation of geographically distinct cold air outbreaks. At the inception of weak vortex events, a precursor signal in the form of a strong high-pressure anomaly over northwest Eurasia is associated with long-lived and robust cold anomalies over Asia and Europe. A few weeks later, near the mature stage of the weak vortex events, a shorter-lived cold anomaly emerges off the east coast of North America. The probability of cold air outbreaks increases by more than 50% in one or more of these regions during all phases of the w eak vortex events. This shows that the stratospheric polar vortex contains information that can be used to enhance forecasts of cold air outbreaks. As large changes in the frequency of extremes are involved, this process is important for the medium-range and seasonal prediction of extreme cold winter days. Three-hundred-year pre-industrial control simulations by 13 coupled climate models corroborate our results. Link to full paper:
  20. MORE-PERSISTENT WEAK STRATOSPHERIC POLAR VORTEX STATES LINKED TO COLD EXTREMES Authors: Kretschmer, Coumou, Agel, Barlow, Tziperman, Cohen. Published: May 2017 Abstract: (No abstract. Here is the Conclusion): Using cluster analysis, we identified dominant patterns of the stratospheric polar vortex in boreal winter. We showed that the polar vortex weakening over the last four decades was a result of more persistent weak polar vortex states (cluster 7) and less frequent strong polar vortex events (cluster 1) rather than an overall weakening. This shift in polar vortex states can account for most of the recent winter cooling trends over Eurasian mid latitudes via stratosphere–troposphere coupling. The observed sea level pressure and heat flux precursors are in agreement with proposed physical mechanisms and can explain the weakening of the polar vortex via a dynamical troposphere–stratosphere coupling. Our analysis shows that the Eurasian cooling trend in the era of Arctic amplification can largely be explained by polar vortex variability. Understanding the two-way link between stratospheric and tropospheric circulation is thus essential for understanding winter teleconnections in the Northern Hemisphere. Link to full paper:
  21. The Life Cycle of Northern Hemisphere Downward Wave Coupling between the Stratosphere and Troposphere Authors: Shaw, Perlwitz Published: Feb 2013 Abstract: The life cycle of Northern Hemisphere downward wave coupling between the stratosphere and troposphere via wave reflection is analyzed. Downward wave coupling events are defined by extreme negative values of a wave coupling index based on the leading principal component of the daily wave-1 heat flux at 30 hPa. The life cycle occurs over a 28-day period. In the stratosphere there is a transition from positive to negative total wave-1 heat flux and westward to eastward phase tilt with height of the wave-1 geopotential height field. In addition, the zonal-mean zonal wind in the upper stratosphere weakens leading to negative vertical shear. Following the evolution in the stratosphere there is a shift toward the positive phase of the North Atlantic Oscillation (NAO) in the troposphere. The pattern develops from a large westward-propagating wave-1 anomaly in the high-latitude North Atlantic sector. The subsequent equatorward propagation leads to a positive anomaly in midlatitudes. The near-surface temperature and circulation anomalies are consistent with a positive NAO phase. The results suggest that wave reflection events can directly influence tropospheric weather. Finally, winter seasons dominated by extreme wave coupling and stratospheric vortex events are compared. The largest impacts in the troposphere occur during the extreme negative seasons for both indices, namely seasons with multiple wave reflection events leading to a positive NAO phase or seasons with major sudden stratospheric warmings (weak vortex) leading to a negative NAO phase. The results reveal that the dynamical coupling between the stratosphere and NAO involves distinct dynamical mechanisms that can only be characterized by separate wave coupling and vortex indices. Link to full paper:
  22. Stratosphere‐troposphere exchange Authors: Holton J.R., Haynes P.H., McIntyre M.E., Douglass A.R., Rood R.B. and Pfister L. First Published: November, 1995 Abstract: In the past, studies of stratosphere-troposphere exchange of mass and chemical species have mainly emphasized the synoptic- and small-scale mechanisms of exchange. This review, however, includes also the global-scale aspects of exchange, such as the transport across an isentropic surface (potential temperature about 380 K) that in the tropics lies just above the tropopause, near the 100-hPa pressure level. Such a surface divides the stratosphere into an ''overworld'' and an extratropical ''lowermost stratosphere'' that for transport purposes need to be sharply distinguished. This approach places stratosphere-troposphere exchange in the framework of the general circulation and helps to clarify the roles of the different mechanisms involved and the interplay between large and small scales. The role of waves and eddies in the extratropical overworld is emphasized. There, wave-induced forces drive a kind of global-scale extratropical ''fluid-dynamical suction pump,'' which with draws air upward and poleward from the tropical lower stratosphere and pushes it poleward and downward into the extratropical troposphere. The resulting global-scale circulation drives the stratosphere away from radiative equilibrium conditions. Wave-induced forces may be considered to exert a nonlocal control, mainly downward in the extratropics but reaching laterally into the tropics, over the transport of mass across lower stratospheric isentropic surfaces. This mass transport is for many purposes a useful measure of global-scale stratosphere-troposphere exchange, especially on seasonal or longer timescales. Because the strongest wave-induced forces occur in the northern hemisphere winter season, the exchange rate is also a maximum at that season. The global exchange rate is not determined by details of near-tropopause phenomena such as penetrative cumulus convection or small-scale mixing associated with upper level fronts and cyclones. These smaller-scale processes must be considered, however, in order to understand the finer details of exchange. Moist convection appears to play an important role in the tropics in accounting for the extreme dehydration of air entering the stratosphere. Stratospheric air finds its way back into the troposphere through a vast variety of irreversible eddy exchange phenomena, including tropopause folding and the formation of so-called tropical upper tropospheric troughs and consequent irreversible exchange. General circulation models are able to simulate the mean global-scale mass exchange and its seasonal cycle but are not able to properly resolve the tropical dehydration process. Two-dimensional (height-latitude) models commonly used for assessment of human impact on the ozone layer include representation of stratosphere-troposphere exchange that is adequate to allow reasonable simulation of photochemical processes occurring inthe overworld. However, for assessing changes in the lowermost stratosphere, the strong longitudinal asymmetries in stratosphere-troposphere exchange render current two-dimensional models inadequate. Either current transport parameterizations must be improved, or else, more likely, such changes can be adequately assessed only by three-dimensional models. Link to Paper: Further Reading - Direct Link to a 2005 Presentation Endorsing this paper: The Brewer-Dobson Circulation
  23. Dynamic coupling of the stratosphere with the troposphere and sudden stratospheric warmings Author: Kevin E. Trenberth First Published: 1966 (revised 5th July, 1972 and published 1st April ,1973) Abstract: Numerical time integrations of a 9-layer quasigeostrophic highly truncated spectral model of the atmosphere are used to study tropospheric-stratospheric interaction with particular regard to sudden stratospheric warmings. The model is global and extends to 0.05 mb (71 km) with roughly 10 km resolution in the stratosphere, and includes an annual heating cycle. A linear baroclinic analysis of a similar model shows that the inclusion of spherical geometry allows significant growth rates in the long wave region of instability. Preliminary integrations without eddies reveal the seasonal variation of a thermally driven circulation. Model integrations simulating the months of December and January were made (i) without nonzonal forcing, and (ii) with nonzonal heating and orography included, to represent southern and northern hemisphere winters. The overall features of the atmosphere were very well simulated. With the inclusion of the annual heating cycle, the model successfully reproduced a more intense circulation in January than existed in December. This caused a maximum tropospheric meridional temperature gradient in the winter hemisphere to occur some weeks prior to the maximum in the external heating field. The presence of nonzonal heating in the winter hemisphere brought about an increase in circulation intensity and produced a stationary perturbation with a strong westward slope with height extending high into the stratosphere. These are features somewhat similar to those of the Aleutian system. Associated with this were considerably warmer temperatures in the polar night stratosphere and a weaker stratospheric westerly jet. The winter mesosphere of the model was driven in the manner of the lower stratosphere and a temperature maximum was produced in mid latitudes. Link to Paper:
  24. Propagation of planetary‐scale disturbances from the lower into the upper atmosphere Authors: J. G. Charney P. G. Drazin Published: January, 1961 Abstract: The possibility that a significant part of the energy of the planetary‐wave disturbances of the troposphere may propagate into the upper atmosphere is investigated. The propagation is analogous to the transmission of electromagnetic radiation in heterogeneous media. It is found that the effective index of refraction for the planetary waves depends primarily on the distribution of the mean zonal wind with height. Energy is trapped (reflected) in regions where the zonal winds are easterly or are large and westerly. As a consequence, the summer circumpolar anticyclone and the winter circumpolar cyclone in the upper stratosphere and mesosphere are little influenced by lower atmosphere motions. Energy may escape into the mesosphere near the equinoxes, when the upper‐atmosphere zonal flow reverses. At these times tunneling of the energy through a reflecting barrier is also possible. Most of the time, however, there appears to be little mechanical coupling on a planetary scale between the upper and lower atmospheres. Tropospheric sources of wave disturbances in the zonal flow are baroclinic instability and the forcing action of zonally asymmetric heating and topography. The transmissivity of the upper atmosphere increases with wavelength and is greater for the forced perturbations than for the unstable tropospheric waves, whose lengths must be smaller than the critical length for instability. The analysis indicates that baroclinically unstable wave disturbances originating in the troposphere probably do not propagate energy vertically at all. When energy is propagated to great heights, nonlinear vertical eddy transports of heat and momentum associated with the vertically propagating waves should modify the basic zonal flow. However, when the wave disturbance is a small stationary perturbation on a zonal flow that varies vertically but not horizontally, the second‐order effect of the eddies on the zonal flow is zero. Link to full paper (only a hand annotated version is available from a free-to-view source):[1961].pdf
  25. Some Observed Features of Stratosphere-Troposphere Coupling - Lecture Presenters: Mark P. Baldwin, David B. Stephenson, David W.J. Thompson, Timothy J. Dunkerton, Andrew J. Charlton and Alan O’Neill Date: 1st May, 2003 Conclusions: - The lower stratosphere during winter changes slowly, and affects the troposphere through wave propagation. The details are not well understood. - Persistence and predictability of the the troposphere involves 1) waves and 2) the long timescale found in the lowermost stratosphere. - Extended-range (>10 days) predictability of the AO using the stratosphere represents a new source of forecasting information. - Forecast models that do not simulate stratospheric dynamics will not be able to exploit this effect. Link to full lecture presentation slides:
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