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Found 25 results

  1. Seasonal and Regional Variations of Long-Term Changes in Upper-Tropospheric Jets from Reanalyses Authors: Gloria L. Manney and Michaela I. Hegglin First Published: September 15th, 2017 Published on line: December 19th, 2017 Abstract: Long-term changes in upper-tropospheric jet latitude, altitude, and strength are assessed for 1980–2014 using five modern reanalyses: MERRA, MERRA-2, ERA-Interim, JRA-55, and NCEP CFSR. Changes are computed from jet locations evaluated daily at each longitude to analyze regional and seasonal variations. The changes in subtropical and polar (eddy driven) jets are evaluated separately. Good agreement among the reanalyses in many regions and seasons provides confidence in the robustness of the diagnosed trends. Jet shifts show strong regional and seasonal variations, resulting in changes that are not robust in zonal or annual means. Robust changes in the subtropical jet indicate tropical widening over Africa except during Northern Hemisphere (NH) spring, and tropical narrowing over the eastern Pacific in NH winter. The Southern Hemisphere (SH) polar jet shows a robust poleward shift, while the NH polar jet shifts equatorward in most regions/seasons. Both subtropical and polar jet altitudes typically increase; these changes are more robust in the NH than in the SH. Subtropical jet wind speeds have generally increased in winter and decreased in summer, whereas polar jet wind speeds have weakened (strengthened) over Africa and eastern Asia (elsewhere) during winter in both hemispheres. The Asian monsoon has increased in area and appears to have shifted slightly westward toward Africa. The results herein highlight the importance of understanding regional and seasonal variations when quantifying long-term changes in jet locations, the mechanisms for those changes, and their potential human impacts. Comparison of multiple reanalyses is a valuable tool for assessing the robustness of jet changes. Link to Paper:
  2. 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:
  3. Bring Back 1962-63

    Drivers of Polar Warming - Presentation

    Drivers of Polar Warming - Presentation Workshop Presentation: Aspen Global Change Institute, Aspen, Colorado Workshop Programme: “Understanding the Causes and Consequences of Polar Amplification” - June 12th -16th, 2017 Presenter: John C Fyfe Presentation Date: 13th June, 2017 Abstract: None Link to full video presentation (28 minutes): Link to full agenda and presentations:
  4. Sudden Stratospheric Warmings and Anomalous Upward Wave Activity Flux Authors: Thomas Birner, John R. Albers Published: 28th, June, 2017 Abstract: Abrupt breakdowns of the polar winter stratospheric circulation such as sudden stratospheric warmings (SSWs) are a manifestation of strong two-way interactions between upward propagating planetary waves and the mean flow. The importance of sufficient upward wave activity fluxes from the troposphere and the preceding state of the stratospheric circulation in forcing SSW-like events have long been recognized. Past research based on idealized numerical simulations has suggested that the state of the stratosphere may be more important in generating extreme stratospheric events than anomalous upward wave fluxes from the troposphere. Other studies have emphasized the role of tropospheric precursor events. Here reanalysis data are used to define events of extreme stratospheric mean flow deceleration (SSWs being a subset) and events of extreme lower tropospheric upward planetary wave activity flux. While the wave fluxes leading to SSW-like events ultimately originate near the surface, the anomalous upward wave activity fluxes associated with these events primarily occur within the stratosphere. The crucial dynamics for forcing SSW-like events appear to take place in the communication layer just above the tropopause. Anomalous upward wave fluxes from the lower troposphere may play a role for some events, but seem less important for the majority of them. Link to full paper:
  5. 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:
  6. On the Relationship between ENSO, Stratospheric Sudden Warmings and Blocking Authors: David Barriopedro and Natalia Calvo Published: 8th March, 2014 Abstract: This paper examines the influence of El Niño–Southern Oscillation (ENSO) on different aspects of major stratospheric sudden warmings (SSWs), focusing on the precursor role of blocking events. The results reveal an ENSO modulation of the blocking precursors of SSWs. European and Atlantic blocks tend to precede SSWs during El Niño (EN), whereas eastern Pacific and Siberian blocks are the preferred precursors of SSWs during La Niña (LN) winters. This ENSO preference for different blocking precursors seems to occur through an ENSO effect on regional blocking persistence, which in turn favors the occurrence of SSWs. The regional blocking precursors of SSWs during each ENSO phase also have different impacts on the upward propagation of planetary-scale wavenumbers 1 and 2; hence, they determine ENSO differences in the wavenumber signatures of SSWs. SSWs occurring during EN are preceded by amplification of wavenumber 1, whereas LN SSWs are predominantly associated to wavenumber-2 amplification. However, there is not a strong preference for splitting or displacement SSWs during any ENSO phase. This is mainly because during EN, splitting SSWs do not show a wavenumber-2 pattern. Link to full paper:
  7. Extraordinary Features of Planetary Wave Propagation During the Boreal Winter 2013/2014 & Zonal Wave Number 2 Predominance Authors: Y. Harada and T. Hirooka Published: 12th October, 2017 Abstract: Observational features of the winter 2013/2014 are investigated using of the Japanese 55‐year Reanalysis data. This winter can be characterized by the continuous predominance of planetary waves of zonal wave number two (WN2) that did not cause major sudden stratospheric warming (SSW) events. It is found that the vertical component of the Eliassen‐Palm flux of WN2 for the winter 2013/2014 is almost equal to the highest value of the winter 2008/2009. The longitudinal distribution of vertical components of Plumb wave activity flux for this winter shows marked downward propagation around 100°W and upward propagation around 60°E, both of which are the strongest of their type among the 56 winters since 1958/1959. The convergence of wave packets propagating from around 60°E contributes to the development and continuance of the quasi‐barotropic Aleutian High, which is associated with the extension of negative extended refractive index (Ks) region. The extension of negative Ks region is related to the convergence or reflection of the wave packets emanating from tropospheric blocking highs developing in the North Pacific Ocean; the development and continuance of the quasi‐barotropic Aleutian High is considered to be one of plausible reasons for the lack of major SSWs in the winter 2013/2014. In addition to these results, we revealed the significant contribution of smaller scale waves (with a zonal wave number of three or more) to the structure of localized wave packet propagation in the stratosphere. Link to full paper (workshop presentation):
  8. The preconditioning of major sudden stratospheric warmings Authors: S. Bancalá, K. Krüger and M. Giorgetta Published: 16th February, 2012 Abstract: The preconditioning of major sudden stratospheric warmings (SSWs) is investigated with two long time series using reanalysis (ERA‐40) and model (MAECHAM5/MPI‐OM) data. Applying planetary wave analysis, we distinguish between wavenumber‐1 and wavenumber‐2 major SSWs based on the wave activity of zonal wavenumbers 1 and 2 during the prewarming phase. For this analysis an objective criterion to identify and classify the preconditioning of major SSWs is developed. Major SSWs are found to occur with a frequency of six and seven events per decade in the reanalysis and in the model, respectively, thus highlighting the ability of MAECHAM5/MPI‐OM to simulate the frequency of major SSWs realistically. However, from these events only one quarter are wavenumber‐2 major warmings, representing a low (∼0.25) wavenumber‐2 to wavenumber‐1 major SSW ratio. Composite analyses for both data sets reveal that the two warming types have different dynamics; while wavenumber‐1 major warmings are preceded only by an enhanced activity of the zonal wavenumber‐1, wavenumber‐2 events are either characterized by only the amplification of zonal wavenumber‐2 or by both zonal wavenumber‐1 and zonal wavenumber‐2, albeit at different time intervals. The role of tropospheric blocking events influencing these two categories of major SSWs is evaluated in the next step. Here, the composite analyses of both reanalysis and model data reveal that blocking events in the Euro‐Atlantic sector mostly lead to the development of wavenumber‐1 major warmings. The blocking–wavenumber‐2 major warming connection can only be statistical reliable analyzed with the model time series, demonstrating that blocking events in the Pacific region mostly precede wavenumber‐2 major SSWs. Link to full paper:
  9. Varying stratospheric responses to tropical Atlantic SST forcing from early to late winter Authors: Jian Rao and Rongcai Ren Published: 9th November, 2017 Abstract: Using multiple reanalysis datasets and model simulations, we begin in this study by isolating the tropical Atlantic Ocean (TAO) sea surface temperature (SST) signals that are independent from ENSO, and then investigate their influences on the northern winter stratosphere. It is revealed that TAO SST forcing does indeed have significant effects on the northern winter stratosphere, but these effects vary from early to late winter in a way that explains the overall insignificant effect when the seasonal average is considered. The stratospheric polar vortex is anomalously weaker/warmer in November–December, stronger/colder in January–March, and weaker/warmer again in April–May during warm TAO years. The varying impacts of the TAO forcing on the extratropical stratosphere are related to a three-stage response of the extratropical troposphere to the TAO forcing during cold season. The tropospheric circulation exhibits a negative North Atlantic Oscillation–like response during early winter, an eastward propagating Rossby wave pattern in mid-to-late winter, and a meridional dipole over North America in spring. Associated with this is varying planetary wave activity in the stratosphere, manifested as an increase in early winter, a decrease in mid-to-late winter, and an increase again in spring. The varying modulation of stratospheric circulation by TAO forcing is consistently confirmed in three reanalysis datasets, and model simulations (fully coupled model and its component AGCM). The exception to the robustness of this verification is that the circumpolar wind response in the fully coupled model is relatively weaker, and that in its component AGCM appears a month later than observed. Link to full paper:
  10. 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:
  11. Zonal Phase Speed, Wave Propagation into the Stratosphere, and Atmospheric Blocking Authors: Daniela I.V. Domeisen, O. Martius and B. Jiménez Esteve Presentation: 28th June, 2017 Abstract: Several criteria determine the upward propagation of tropospheric anomalies into the stratosphere: Upward propagation of Rossby waves critically depends on the stratospheric background flow, the horizontal wave number and strength and duration of the wave forcing, and – as will be the focus of this study – the zonal phase speed of the propagating wave. According to the Charney-Drazin criterion, an eastward phase speed enables the propagating wave to advance into stronger stratospheric winds. This may be critical for waves of shorter wavelengths, which tend to be inhibited from propagating into the Northern Hemisphere winter stratosphere, or for waves propagating into the even stronger winds in Southern Hemisphere winter. It is shown that the typical phase speeds observed in the extratropical atmosphere lie in the range that allows for a significant change in upward propagation, meaning that the stratosphere actively selects eastward propagating waves. This has a significant impact for the upward impact of a variety of tropospheric phenomena: Atmospheric blocking, here defined as the occurrence of geopotential height anomalies in the extratropical troposphere that tend to block the zonal flow, has long been suggested to have an influence on stratospheric variability, for example as precursors to Stratospheric Sudden Warming (SSW) events. While blocks tend to be of a rather stationary nature, they often exhibit finite zonal phase speeds, both in the downstream (advection by the mean flow) and the upstream directions. This study shows that the range of phase speeds that is typical of blocking highs can indeed alter the structure of upward wave propagation, as confirmed in a simplified model and in case studies. Differences can also be found in the phase speed of planetary wave numbers, depending on the type of SSW event that they are preceding. These findings may have implications for the understanding of the upward coupling by waves propagating into the stratosphere, especially ahead of extreme stratospheric events. In addition, this research indicates that a possible change in the occurrence and nature of blocking or a change in the stratospheric mean flow with climate change may further impact stratospheric variability. 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. Tropospheric Cooling as a Mechanism for Stratospheric Polar Vortex Disturbances Authors: Thomas S. Ehrmann and S. J. Colucci Presentation: 28th June, 2017 Abstract: Disturbances in the Northern Hemisphere wintertime stratospheric polar vortex and their connection with cold temperature anomalies in the mid-latitude troposphere are studied using MERRA Reanalysis data for the winter seasons of 1980-2014. By taking geometric moments of potential vorticity in the upper stratosphere, 55 disturbances of the polar vortex are identified during the 35 winter seasons either as splits or displacements. The position of the polar vortex during each disturbance event is averaged to generate an area averaging filter. A potential vorticity inversion method is used to show negative height tendencies in the stratosphere, driven by negative temperature tendencies in the troposphere and lower stratosphere, under the disturbed polar vortex preceding most disturbance events. This suggests that tropospheric cooling may help determine the orientation of the stratospheric polar vortex during at least some disturbance events. 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. 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:
  14. 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:
  15. 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:
  16. Mechanisms Governing Interannual Variability of Stratosphere‐to‐Troposphere Ozone Transport Authors: John R. Albers, Judith Perlwitz, Amy H. Butler et al Published: 16th January, 2018 Abstract: Factors governing the strength and frequency of stratospheric ozone intrusions over the Pacific‐North American region are considered for their role in modulating tropospheric ozone on interannual timescales. The strength of the association between two major modes of climate variability—the El Niño–Southern Oscillation (ENSO) and the Northern Annular Mode (NAM)—and the amount of ozone contained in stratospheric intrusions are tested in the context of two mechanisms that modulate stratosphere‐to‐troposphere transport (STT) of ozone: (StratVarO3) the winter season buildup of ozone abundances in the lowermost stratosphere (LMS) and (JetVar) Pacific jet and wave breaking variability during spring. In essence, StratVarO3 corresponds to variability in the amount of ozone per intrusion, while JetVar governs the frequency of intrusions. The resulting analysis, based on two different reanalysis products, suggests that StratVarO3 is more important than JetVar for driving interannual variations in STT of ozone over the Pacific‐North American region. In particular, the abundance of ozone in the LMS at the end of winter is shown to be a robust indicator of the amount of ozone that will be contained in stratospheric intrusions during the ensuing spring. Additionally, it is shown that the overall strength of the winter season stratospheric NAM is a useful predictor of ozone intrusion strength. The results also suggest a nuanced relationship between the phase of ENSO and STT of ozone. While ENSO‐related jet variability is associated with STT variability, it is wave breaking frequency rather than typical ENSO teleconnection patterns that is responsible for the ENSO‐STT relationship. Link to full paper: Please note that the full paper is currently behind a paywall – AGU 100 link:
  17. Separating the stratospheric and tropospheric pathways of El Niño–Southern Oscillation teleconnections Authors: Amy H Butler, Lorenzo M Polvani and Clara Deser Published: 26th February 2014 Abstract: The El Niño–Southern Oscillation (ENSO) is a major driver of Northern Hemisphere wintertime variability and, generally, the key ingredient used in seasonal forecasts of wintertime surface climate. Modeling studies have recently suggested that ENSO teleconnections might involve both a tropospheric pathway and a stratospheric one. Here, using reanalysis data, we carefully distinguish between the two. We first note that the temperature and circulation anomalies associated with the tropospheric pathway are nearly equal and opposite during the warm (El Niño) and cold (La Niña) phases of ENSO, whereas those associated with the stratospheric pathway are of the same sign, irrespective of the ENSO phase. We then exploit this fact to isolate the two pathways. Our decomposition reveals that ENSOs climate impacts over North America are largely associated with the tropospheric pathway, whereas ENSOs climate impacts over the North Atlantic and Eurasia are greatly affected by the stratospheric pathway. The stratospheric pathway, which we here define on the basis of the occurrence of one or more sudden stratospheric warmings in a given winter, and whose signature projects very strongly on the North Atlantic Oscillation, is found to be present 60% of the time during ENSO winters (of either phase): it therefore likely plays an important role in improving seasonal forecasts, notably over the North Atlantic and the Eurasian continent. Link to full paper:
  18. The whole atmosphere response to changes in the Earth’s magnetic field from 1900 to 2000: An example of “top-down” vertical coupling Authors: Ingrid Cnossen, Hanli Liu and Hua Lu Published: 2nd February, 2016 Abstract: We study the effects of changes in the Earth’s magnetic field between 1900 and 2000 on the whole atmosphere (0–500 km altitude), based on simulations with the Whole Atmosphere Community Climate Model eXtension. Magnetic field changes directly affect the temperature and wind in the upper atmosphere (> ~110 km) via Joule heating and the ion drag force. However, we also find significant responses in zonal mean temperature and zonal wind in the Southern Hemisphere (SH) middle- to high-latitude troposphere, stratosphere, and mesosphere of up to ±2 K and ±2 m/s, as well as regionally significant changes in Northern Hemisphere (NH) polar surface temperatures of up to ±1.3 K, in December-January-February. In the SH, changes in gravity wave filtering in the thermosphere induce a change in the residual circulation that extends down into the upper mesosphere, where further changes in the mean wind climatologyare generated, togetherwith changes in local planetarywave generationand/or amplification and gravity wave filtering. This induces further changes to a residual circulation cell extending down into the troposphere. However, inaccuracies in the simulated SH upper mesospheric wind climatology probably mean that the simulated temperature andwind responses in the SH lower andmiddle atmosphere are also inaccurate. TheNHmiddle atmosphere response is zonally asymmetric, consisting of a significant change in the positioning and shape of the upper stratospheric polar vortex, which is dynamically consistent with the surface temperature response. However, the downward coupling mechanism in the NH is generally less clear. Link to full paper:
  19. 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:
  20. Bring Back 1962-63

    Stratosphere‐troposphere exchange

    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
  21. 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
  22. Tropospheric Precursors and Stratospheric Warmings Authors: Dr Judah Cohen, Justin Jones Published: 26th October, 2010 Abstract: Many tropospheric Arctic Oscillation (AO) events are preceded by stratospheric AO events and even earlier in time by anomalous upward energy flux associated with Rossby waves in the troposphere. This study identifies lower-tropospheric circulation anomalies that precede large AO events in both the troposphere and stratosphere and the anomalous upward energy flux. Compositing analysis of stratospheric warming events identifies regional tropospheric precursors, which precede stratospheric warmings. The tropospheric precursor is found to vary when compositing over polar vortex displacements and splits separately. Prior to vortex displacements the main anomaly sea level pressure center of the tropospheric precursor is located across northwest Eurasia and is associated with the Siberian high. Prior to vortex splits a similar anomaly center is identified in the tropospheric precursor but is weaker and appears to be more strongly related to a shift in the storm tracks. Differences in the sea level pressure anomalies in the North Atlantic and the North Pacific are also observed when comparing the precursors prior to vortex displacements and splits. Identification of a unique tropospheric precursor to stratospheric warming and subsequent tropospheric AO events can help to improve understanding troposphere–stratosphere coupling. Furthermore, the observational evidence presented here can be compared with model simulations of winter climate variability and lead to potential model improvements. Link to full paper:
  23. 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:
  24. What kind of stratospheric sudden warming propagates to the troposphere? Authors: Ken I. Nakagawa and Koji Yamazaki Published: 16th February, 2006 Abstract An edited version of this paper was published by AGU. Copyright 2006, American Geophysical Union, Geophysical Research Letters, 33 The factors affecting the downward propagation of stratospheric sudden warming (SSW) events to the troposphere are studied through composite analysis of 45-year reanalysis data from the European Centre for Medium-Range Weather Forecasts. During the growth stage of SSW, events that propagate into the troposphere exhibit enhanced upward flux of the wavenumber 2 wave, while events that do not propagate downward display reduced wavenumber 2 flux. In both events, upward flux of the wavenumber-1 wave is enhanced, but the enhancement is stronger in the non-propagating event. The composite for propagating events reveals a negative Eurasian pattern of horizontal geopotential anomalies in the troposphere during the growth stage, and a negative Arctic Oscillation pattern following the event, while non-propagating events are preceded by a positive Eurasian pattern. In both types of event, the tropospheric anomalies are generated mainly by tropospheric planetary wave forcing prior to the emergence of SSW. Link to full paper:
  25. MJO‐Related Tropical Convection Anomalies Lead to More Accurate Stratospheric Vortex Variability in Sub-seasonal Forecast Models Abstract: The effect of the Madden‐Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex in the period preceding stratospheric sudden warmings is evaluated in operational subseasonal forecasting models. Reforecasts which simulate stronger MJO‐related convection in the Tropical West Pacific also simulate enhanced heat flux in the lowermost stratosphere and a more realistic vortex evolution. The time scale on which vortex predictability is enhanced lies between 2 and 4 weeks for nearly all cases. Those stratospheric sudden warmings that were preceded by a strong MJO event are more predictable at ∼20 day leads than stratospheric sudden warmings not preceded by a MJO event. Hence, knowledge of the MJO can contribute to enhanced predictability, at least in a probabilistic sense, of the Northern Hemisphere polar stratosphere. Link to full paper: