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Found 13 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: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-18-0574.1 https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018JD029368
  2. 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: https://www.mdpi.com/2073-4433/9/12/467/htm Credit goes to @sebastiaan1973 for finding this paper
  3. Role of Finite-Amplitude Eddies and Mixing in the Life Cycle of Stratospheric Sudden Warmings Authors: Sandro W. Lubis, Clare S. Y. Huang, Noboru Nakamura, Nour-Eddine Omrani and Martin Jucker Published: 26th October, 2018 Abstract: Despite the advances in theories and data availability since the first observation of stratospheric sudden warmings (SSWs) in the 1950s, some dynamical aspects of SSWs remain elusive, including the roles of wave transience at finite amplitude and irreversible wave dissipation due to mixing. This is likely due to a limitation of the traditional theory for SSWs that is tailored to small-amplitude waves and is unsuitable for large-scale wave events. To circumvent these difficulties, the authors utilized a novel approach based on finite-amplitude wave activity theory to quantify the roles of finite-amplitude wave transience and mixing in the life cycle of SSWs. In this framework, a departure from the exact nonacceleration relation can be directly attributed to irreversible mixing and diabatic forcings. The results show that prior to the warming event, an increase in pseudomomentum/wave activity largely compensates for the anomalous Eliassen–Palm flux convergence, while the total wave dissipation due to mixing (enstrophy dissipation) and radiative forcing only plays a secondary role. After the vortex breaks down, enhanced mixing increases irreversible wave dissipation and in turn slows down vortex recovery. It is shown that (i) a rapid recovery of the polar vortex is characterized by weak wave transience that follows a nonacceleration relation reversibly and (ii) a delayed recovery is attributed to stronger and more persistent irreversible wave dissipation due to mixing, a deviation from the classical nonacceleration relation. The results highlight the importance of mixing in the asymmetry between breakdown and recovery of the polar vortex during SSWs. Link to full paper: This is behind an AMS paywall but there is another link to the Researchgate site with the early online release with the full "preliminary" version from September 2018 (just scroll down that page) and there is also a personal pdf download version available from there. https://www.researchgate.net/publication/327854069_Role_of_Finite-Amplitude_Eddies_and_Mixing_in_the_Life_Cycle_of_Stratospheric_Sudden_Warmings
  4. Contributions of Ice Thickness to the Atmospheric Response From Projected Arctic Sea Ice Loss Authors: Zachary Labe, Yannick Peings and Gudrun Magnusdottir Published: 18th May, 2018 Abstract: A large ensemble of simulations from a high‐top atmospheric general circulation model are conducted to compare the atmospheric responses from loss of Arctic sea ice thickness and sea ice concentration. The response to projected sea ice thickness loss indicates substantial surface warming over the Arctic Ocean and up to 1°C of cooling in Eurasia. While the dynamic circulation response from sea ice thickness loss is smaller in magnitude, it has a similar spatial anomaly pattern as that due to sea ice concentration loss. This pattern resembles the negative phase of the Northern Annular Mode. The simulations reveal that sea ice thickness loss enhances the thermodynamic and large‐scale circulation response from sea ice anomalies. These results stress the importance of considering a realistic sea ice thickness distribution in future atmospheric general circulation model sea ice perturbation experiments. Plain Language Summary The atmospheric response to the loss of Arctic sea ice remains uncertain in current climate change assessments. Prior studies addressing the response have mostly focused on changes in sea ice concentration or the fraction of sea ice cover in a defined area. However, this neglects the role of sea ice thickness, which is projected to substantially thin during the 21st century. Using a series of climate model simulations, this study provides a comprehensive comparison of the atmospheric response to the loss of Arctic sea ice thickness and sea ice concentration. The experiments show a nonnegligible influence of sea ice thickness on near‐surface warming of the Arctic and the large‐scale atmospheric circulation. This study demonstrates the importance of accurately representing changes in Arctic sea ice thickness in future climate model studies. Link to full paper: Please note that the full paper is currently behind an AGU100 paywall. Link to AGU100 website: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018GL078158 Link to Supplementary information: https://agupubs.onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1029%2F2018GL078158&attachmentId=2216685271
  5. 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: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017JD026890
  6. Upward Wave Activity Flux as a Precursor to Extreme Stratospheric Events and Subsequent Anomalous Surface Weather Regimes Authors: Lorenzo M. Polvani and Darryn W. Waugh Published: 12th January, 2004 Abstract: It has recently been shown that extreme stratospheric events (ESEs) are followed by surface weather anomalies (for up to 60 days), suggesting that stratospheric variability might be used to extend weather prediction beyond current time scales. In this paper, attention is drawn away from the stratosphere to demonstrate that the originating point of ESEs is located in the troposphere. First, it is shown that anomalously strong eddy heat fluxes at 100 hPa nearly always precede weak vortex events, and conversely, anomalously weak eddy heat fluxes precede strong vortex events, consistent with wave–mean flow interaction theory. This finding clarifies the dynamical nature of ESEs and suggests that a major source of stratospheric variability (and thus predictability) is located in the troposphere below and not in the stratosphere itself. Second, it is shown that the daily time series of eddy heat flux found at 100 hPa and integrated over the prior 40 days, exhibit a remarkably high anticorrelation (−0.8) with the Arctic Oscillation (AO) index at 10 hPa. Following Baldwin and Dunkerton, it is then demonstrated that events with anomalously strong (weak) integrated eddy heat fluxes at 100 hPa are followed by anomalously large (small) surface values of the AO index up to 60 days following each event. This suggests that the stratosphere is unlikely to be the dominant source of the anomalous surface weather regimes discussed in Thompson et al. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0442(2004)017<3548%3AUWAFAA>2.0.CO%3B2
  7. 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: https://pdfs.semanticscholar.org/c68f/4d487633429caecdaba8eba41c5151557a6f.pdf?_ga=2.215728272.644931620.1527702945-1326629836.1527702945
  8. 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: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/jgrd.50871
  9. 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: https://rmets.onlinelibrary.wiley.com/doi/epdf/10.1002/qj.620
  10. 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: http://www.phys.ocean.dal.ca/people/po/Whistler/baldwin.pdf
  11. Several weeks ago, a few members of the 33andrain Dream Team saw a pattern unfolding in the long range. The long range became the mid range, and though the models diverged, our Weather Expert members did not; several remained committed to the potential for a Significant, if not Major, East Coast Snow Event. We had three radio shows with the NYMW crew & guests: They included @earthlight, @CCB!, @Superstorm93, @Weathergun, & @Dsnowx53 . There have been valuable thoughts shared by @BMC10, @Isotherm, @Wxoutlooksblog, @Frank_Wx, @DualJet, @donsutherland1, and many more. We are approaching the final hours: The Euro, GFS, CMC, UK, and additional foreign models have just a couple of runs left before the first flakes fly. The RGEM, SREFs, NAM, HRRR, and other close-range models are important, but the guidance from our resident experts is more important. As an increasingly serious situation unfolds, please let the conversation flow. During model run times, let those who know, share. And, Go. As @earthlight would say, wishing everyone feet and feet of snow.
  12. Several weeks ago, a few members of the 33andrain Dream Team saw a pattern unfolding in the long range. The long range became the mid range, and though the models diverged, our Weather Expert members did not; several remained committed to the potential for a Significant, if not Major, East Coast Snow Event. We had two radio shows with the NYMW crew & guests: They included @earthlight, @CCB!, @Superstorm93, @Weathergun, & @Dsnowx53 . There have been valuable thoughts shared by @BMC10, @Isotherm, @Wxoutlooksblog, @Frank_Wx, @DualJet, and many more. We are approaching the witching hour; The Euro, GFS, CMC, UK, and random foreign models have all now supported the threat initially laid out by our resident experts. The SREFs, NAM, and others now have some of the puzzle pieces within range. As an increasingly serious situation unfolds, please let the conversation flow. During model run times, let those who know, share. Ask questions. Let's keep this community the most interactive weather community around. Here are model times, for those who may be interested. And, Go. GFS (American Long-Range Global Model) 0z: 10:25pm-11:50pm 6z: 4:25am-5:50am 12z: 10:25am-11:50am 18z: 4:25pm-5:50pm GFS Ensembles 0z: 11:45pm-12:35am 6z: 5:45am-6:35am 12z: 11:45am-12:35pm 18z: 5:45pm-6:35pm NAM 12-km (American High-Resolution Medium Range Model) 0z: 8:35pm 6z: 2:35am 12z: 8:35am 18z: 2:35pm NAM 4-km (Downscaled NAM) 0z: 8:45pm 6z: 2:45am 12z: 8:45am 18z: 2:45pm HRRR 3-km (High-Resolution Local Model) Runs every hour, at the top of the hour (00-23z) CMC (Canadian Global Model) 0z: 11:00pm-12:05am 12z : 11:00am-12:05pm RGEM 10-km (Canadian Regional Model) 0z: 11:05pm-12:10am 6z: 2:50am-3:330m 12z: 11:05am-12:10pm 18z: CMC Ensembkes 0z: 12:30am-1:30am 12z : 12:30pm-1:30pm UKMET (English Global Model) 0z: 11:45am 12z: 11:45pm ECMWF (European High-Resolution Model) 0z: 1:40am-2:00am 12z: 1:40pm-2:00pm ECMWF Ensembles 0z: 2:00am-3:00am 12z: 2:00pm-3:00pm NOGAPS (American Navy Global Model) 00z = 11:30 am 12z = 11:30 pm DGEX (Soon to be discontinued) 06z = 4:50 AM 18z = 4:50 PM JMA (Japanese Global Model) 00z = Between 1:00 & 3:00 am 12z = Between 1:00 & 3:00 am
  13. Well, after 13 years the DGEX will finally be retired with the GFS assuming all of its "responsibilities". I have to say that even though the DGEX was one of the most useless models that I have ever worked with (just behind the CRAS), I still have great memories of the head-scratching and hilarious legacy this model had accumulated. For example in 2005 when a few runs were pulling Katrina through a 592dm ridge and putting it over NYC, or in 2006 when it did inexplicably well with the Feb 06 blizzard for NYC, and just all of the solutions during winter season that seemed to bend the laws of physics, but warmed the hearts of even the most hardened-meteorologists and made them chuckle. It's been an interesting ride with this model, but soon the servers will shut down and we'll just have stories to tell of the infamous DGEX. Thanks for the memories http://www.nws.noaa.gov/os/notification/tin16-41nam_updates.htm
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