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Found 15 results

  1. 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
  2. Saharan Dust and the Nonlinear Evolution of the African Easterly Jet–African Easterly Wave System - Presentation and Paper A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: Dustin Grogan, T. Nathan and S. H. Chen  Presentation Date: 18th April, 2016 Presentation Summary: The direct radiative effects of Saharan mineral dust aerosols on the nonlinear evolution of African easterly waves (AEWs) and their interactions with the African easterly jet (AEJ) are examined using the Weather Research and Forecasting (WRF) model coupled to an online dust model. The model is initialized with zonal-mean distributions of wind, temperature and dust consistent with summertime conditions over North Africa. The dust modifies the lifecycle of the AEWs in the following way: the domain-averaged eddy kinetic energy (EKE) is enhanced during the linear and nonlinear growth phases, reaching a larger peak amplitude that subsequently decays more rapidly, eventually equilibrating at lower amplitude. The increase in EKE during the growth phases is due to local increases in barotropic energy conversions in the dust plume north of the AEJ. The dust-modified, rapidly decaying phase is primarily associated with enhanced barotropic decay that occurs near the top of the plume north of the AEJ. The timing of peak EKE depends on the initial dust concentration. Throughout the evolution of the AEJ-AEW system, the dust increases the maximum zonal-mean wind speeds. The increase is due to the dust-modified mean meridional circulation during the AEW growth phase. The dust-modified maximum wind speeds are also displaced farther southward and upward during the AEW growth phase, which is due to the enhanced wave fluxes decelerating the flow more efficiently north of the AEJ. The spatial-temporal evolution of the dust-modified critical surface is shown to play an important role in the evolution of the AEJ-AEW system. Link to conference video presentation: Please note that this presentation was withdrawn as the authors later published a full paper which was initially behind a paywall but it has just been made available on free to view basis (see below). Link to full conference agenda: Publication Date: 2nd September, 2016 Abstract (revised version for paper): The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume. The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position. Link to full paper:
  3. 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:
  4. 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):
  5. Snow–(N)AO Teleconnection and Its Modulation by the Quasi-Biennial Oscillation Authors: Y. Peings Published: 29th November, 2017 Abstract: This study explores the wintertime extratropical atmospheric response to Siberian snow anomalies in fall, using observations and two distinct atmospheric general circulation models. The role of the quasi-biennial oscillation (QBO) in modulating this response is discussed by differentiating easterly and westerly QBO years. The remote influence of Siberian snow anomalies is found to be weak in the models, especially in the stratosphere where the “Holton–Tan” effect of the QBO dominates the simulated snow influence on the polar vortex. At the surface, discrepancies between composite analyses from observations and model results question the causal relationship between snow and the atmospheric circulation, suggesting that the atmosphere might have driven snow anomalies rather than the other way around. When both forcings are combined, the simulations suggest destructive interference between the response to positive snow anomalies and easterly QBO (and vice versa), at odds with the hypothesis that the snow–North Atlantic Oscillation/Arctic Oscillation [(N)AO] teleconnection in recent decades has been promoted by the QBO. Although model limitations in capturing the relationship exist, altogether these results suggest that the snow–(N)AO teleconnection may be a stochastic artifact rather than a genuine atmospheric response to snow-cover variability. This study adds to a growing body of evidence suggesting that climate models do not capture a robust and stationary snow–(N)AO relationship. It also highlights the need for extending observations and/or improving models to progress on this matter. Link to full paper:
  6. The Role of Zonal Asymmetry in the Enhancement and Suppression of Sudden Stratospheric Warming Variability by the MJO Authors: Wanying Kang and Eli Tziperman Published: 20th February, 2018 Abstract: Sudden stratospheric warming (SSW) events influence the Arctic Oscillation and midlatitude extreme weather. Previous work showed the Arctic stratosphere to be influenced by the Madden–Julian oscillation (MJO) and that the SSW frequency increases with an increase of the MJO amplitude, expected in a warmer climate. It is shown here that the zonal asymmetry in both the background state and forcing plays a dominant role, leading to either enhancement or suppression of SSW events by MJO-like forcing. When applying a circumglobal MJO-like forcing in a dry dynamic core model, the MJO-forced waves can change the general circulation in three ways that affect the total vertical Eliassen–Palm flux in the Arctic stratosphere. First, weakening the zonal asymmetry of the tropospheric midlatitude jet, and therefore preventing the MJO-forced waves from propagating past the jet. Second, weakening the jet amplitude, reducing the waves generated in the midlatitudes, especially stationary waves, and therefore the upward-propagating planetary waves. Third, reducing the Arctic lower-stratospheric refractory index, which prevents waves from upward propagation. These effects stabilize the Arctic vortex and lower the SSW frequency. The longitudinal range to which the MJO-like forcing is limited plays an important role as well, and the strongest SSW frequency increase is seen when the MJO is located where it is observed in current climate. The SSW suppression effects are active when the MJO-like forcing is placed at different longitudinal locations. This study suggests that future trends in both the MJO amplitude and its longitudinal extent are important for predicting the Arctic stratosphere response. Link to full paper:
  7. Dynamics of 2013 Sudden Stratospheric Warming event & its impact on cold weather over Eurasia: Role of planetary wave reflection Authors: Debashis Nath, Wen Chen, Cai Zelin, Alexander Ivanovich Pogoreltsev and Ke Wei Published: 7th April, 2016 Abstract: In the present study, we investigate the impact of stratospheric planetary wave reflection on tropospheric weather over Central Eurasia during the 2013 Sudden Stratospheric Warming (SSW) event. We analyze EP fluxes and Plumb wave activity fluxes to study the two and three dimensional aspects of wave propagation, respectively. The 2013 SSW event is excited by the combined influence of wavenumber 1 (WN1) and wavenumber 2 (WN2) planetary waves, which makes the event an unusual one and seems to have significant impact on tropospheric weather regime. We observe an extraordinary development of a ridge over the Siberian Tundra and the North Pacific during first development stage (last week of December 2012) and later from the North Atlantic in the second development stage (first week of January 2013), and these waves appear to be responsible for the excitation of the WN2 pattern during the SSW. The wave packets propagated upward and were then reflected back down to central Eurasia due to strong negative wind shear in the upper stratospheric polar jet, caused by the SSW event. Waves that propagated downward led to the formation of a deep trough over Eurasia and brought extreme cold weather over Kazakhstan, the Southern part of Russia and the Northwestern part of China during mid-January 2013. Link to full paper:
  8. 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:
  9. 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:
  10. Vortex Preconditioning due to Planetary and Gravity Waves prior to Sudden Stratospheric Warmings Authors: John R. Albers and Thomas Birner Published: 14th June, 2014 Abstract: Reanalysis data are used to evaluate the evolution of polar vortex geometry, planetary wave drag, and gravity wave drag prior to split versus displacement sudden stratospheric warmings (SSWs). A composite analysis that extends upward to the lower mesosphere reveals that split SSWs are characterized by a transition from a wide, funnel-shaped vortex that is anomalously strong to a vortex that is constrained about the pole and has little vertical tilt. In contrast, displacement SSWs are characterized by a wide, funnel-shaped vortex that is anomalously weak throughout the prewarming period. Moreover, during split SSWs, gravity wave drag is enhanced in the polar night jet, while planetary wave drag is enhanced within the extratropical surf zone. During displacement SSWs, gravity wave drag is anomalously weak throughout the extratropical stratosphere. Using the composite analysis as a guide, a case study of the 2009 SSW is conducted in order to evaluate the roles of planetary and gravity waves for preconditioning the polar vortex in terms of two SSW-triggering scenarios: anomalous planetary wave forcing from the troposphere and resonance due to either internal or external Rossby waves. The results support the view that split SSWs are caused by resonance rather than anomalously large wave forcing. Given these findings, it is suggested that vortex preconditioning, which is traditionally defined in terms of vortex geometries that increase poleward wave focusing, may be better described by wave events (planetary and/or gravity) that “tune” the geometry of the vortex toward its resonant excitation points. Link to full paper:
  11. The MJO‐SSW Teleconnection: Interaction Between MJO‐Forced Waves and the Midlatitude Jet Authors: Wanying Kang and Eli Tziperman Published: 19th April, 2018 Abstract: The Madden‐Julian Oscillation (MJO) was shown to affect both present‐day sudden stratospheric warming (SSW) events in the Arctic and their future frequency under global warming scenarios, with implications to the Arctic Oscillation and midlatitude extreme weather. This work uses a dry dynamic core model to understand the dependence of SSW frequency on the amplitude and longitudinal range of the MJO, motivated by the prediction that the MJO will strengthen and broaden its longitudinal range in a warmer climate. We focus on the response of the midlatitude jets and the corresponding generated stationary waves, which are shown to dominate the response of SSW events to MJO forcing. Momentum budget analysis of a large ensemble of spinup simulations suggests that the climatological jet response is driven by the MJO‐forced meridional eddy momentum transport. The results suggest that the trends in both MJO amplitude and longitudinal range are important for the prediction of the midlatitude jet response and for the prediction of SSWs in a future climate. Plain Language Summary: Sudden stratospheric warming (SSW) events occur in the Arctic stratosphere during winter approximately every other year, featuring an abrupt warming and a breakdown of the polar vortex. These events affect midlatitude extreme weather events and are therefore of a societal relevance, making it important to be able to predict a change in their frequency in a future climate change scenario. In the present climate, these events seem to be only weakly influenced by the Madden‐Julian Oscillation (MJO), the dominant intraseasonal variability in the tropics. The authors have previously shown that a strengthening of the MJO, which is expected in a warmer future climate, may lead to more frequent SSW events. The mechanism behind such enhanced future SSW frequency is shown here to involve a nonlinear interaction of MJO‐forced atmospheric waves with the midlatitude tropospheric jet. The waves make the midlatitude jet more asymmetric in longitude, therefore causing it to emit stronger stationary waves that reach the polar Arctic stratosphere, therefore leading to the more frequent occurrence of SSW events. This motivates studying this teleconnection between the tropical MJO and the Arctic SSW events using more detailed models, to increase our confidence in the prediction of future climate and weather regimes. Link to full paper:
  12. 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:
  13. The Angular Momentum Budget of the Transformed Eulerian Mean Equations Authors: Joseph Egger and Klaus-Peter Hoinka Published: 24th April, 2008 Abstract: The axial angular momentum (AAM) budget of zonal atmospheric annuli extending from the surface to a given height and over meridional belts is discussed within the framework of conventional and transformed Eulerian mean (TEM) theory. Conventionally, it is only fluxes of AAM through the boundaries and/or torques at the surface that are able to change the AAM of an annulus. TEM theory introduces new torques in the budget related to the vertically integrated Eliassen–Palm flux divergence and also new AAM fluxes of the residual difference circulation. Some of these torques are displayed for various annuli. In particular, the application of TEM theory generates a large positive torque at tropospheric upper boundaries in the global case. This torque is much larger than the global mountain and friction torques but is cancelled exactly by the new vertical AAM fluxes through the upper boundary. It is concluded that the TEM approach complicates the analysis of AAM budgets but does not provide additional insight. Isentropic pressure torques are believed to be similar to the TEM torques at the upper boundary of an annulus. The isentropic pressure torques are evaluated from data and found to differ in several respects from the TEM torques. Link to full paper:
  14. A gentle stroll through EP (Eliassen & Palm) flux theory Author: Oliver Bühler Published: 7th February, 2014 Abstract: The celebrated 1960 paper by Eliassen & Palm (hereafter EP) put on record several brilliant discoveries in the theory of linear waves on shear flows for rotating stratified fluid systems. These discoveries opened up a new perspective on linear wave dynamics in the atmosphere and on the nascent theory of nonlinear interactions between the waves and the mean flow. Arguably, the most important discovery was that of their eponymous wave activity flux vector in the meridional plane and of the conditions under which this important flux was non-divergent. In this short paper we will retrace some of the steps of EP and explore how their path-breaking discoveries came to be understood in the light of subsequent theories. Of course, an endeavour like this runs the risk of looking patronizing, if only because of 50 years of hindsight, but this is not intended: it was the power of their original discoveries that inspired five decades of further research, with new results still coming out today. Link to full paper: Direct Link Across to the Original "EP" Paper - "On the Transfer of Energy in Stationary Mountain Waves": Eliassen & Palm 1960 Paper
  15. On the Three-Dimensional Propagation of Stationary Waves Authors: R. Alan Plumb Published: 1st February, 1985 Abstract: A locally applicable (nonzonally-averaged) conservation relation is derived for quasi-geostrophic stationary waves on a zonal flow, a generalization of the Eliassen-Palm relation. The flux which appears in this relation constitutes, it is argued, a useful diagnostic of the three-dimensional propagation of stationary wave activity. This is illustrated by application to a simple theoretical model of a forced Rossby wave train and to a Northern Hemisphere winter climatology. Results of the latter procedure suggest that the major forcing of the stationary wave field derives from the orographic effects of the Tibetan plateau and from nonorographic effects (diabatic heating and/or interaction with transient eddies) in the western North Atlantic and North Pacific Oceans and Siberia. No evidence is found in the data for wave trains of tropical origin; forcing by the orographic effects of the Rocky mountains seems to be of secondary importance. Link to full paper:<0217%3AOTTDPO>2.0.CO%3B2
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