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  1. Links Section In This Post, below the intro. I have been recently asked to start a thread, to talk about weather teleconnections and similar topics. This is often a topic not very well discussed on other weather places, and places like Twitter. We have a number of experts, enthusiasts, and meteorologists, who are knowledgeable in this area. So this is a thread for technical discussion about the teleconnections, etc, as well as a place for questions about these topics. We need to start talking about these climate drivers more, as they are the key to unlocking medium-long term forecasts. We are making a place for technical discussion about these factors away from the main thread/s. So this thread is born. Teleconnections that could be up for discussion are: MJO, AAM/GWO, NAO, RRWT, NP jet, Mountain & Frictional Torques, AO/AAO, ENSO, IOD, AMO, SSTs in general, SOI, QBO, the Stratosphere, etc. Feel free to talk about related topics, but stick to this general topic. I encourage all posters to discuss and pose questions relating to the topic, and keep it a relaxed atmosphere. Any questions, just PM me or comment here. Hope we can make this work Links Section ERSL Link, Up to 24 hours behind. GWO 90 day Victor Gensini Site. Features Total AAM, Bias Corrected Rel AAM GEFS, CFS GWO Forecast. He stated he is soon to add torque products. Nick Schraldi GWO Site Non-Bias Corrected GEFS GWO forecast. Michael Ventrice Hovmoller from MV, to help spot AAM trends and patterns. GEFS. Carl Schreck More Hovmollers and other tropical charts to spot trends in the AAM. CFS forecast. NPJ Phase Diagrams/Albany Shows a GEFS forecast and observation of NP jetstream, which is largely controlled by the AAM. From @Bring Back 1962-63: Since the service provided through WDT was withdrawn there was a gap in this vitally important data. I've been in touch with Ed Berry, who along with Dr Klaus Weickmann (who retired 2 years ago) developed the GSDM (I posted on that on both the 33 and NetWx forums with Ed's excellent presentation earlier this year) and he told me that a friend of his still processes this data. He has kindly provided a link to that site plus the access user name and password: un = gsdm01 pw = gu3st#1 That will take you to this page where you'll find a lot more than just the ex WDT data: Monitoring of Global Atmospheric Angular Momentum (AAM) Budget NCEP-NCAR Reanalysis DAILY DATA Vertically-integrated 5-day running mean: 1968-1997 Climatology Plots show some of the features: MJO, SUB-MONTHLY, & RAPID TRANSITIONS. ( Plots contain data through (MM/DD/YYYY) = 09/26/2018 ) PLOTS LATEST 90 DAYS LATEST 90 DAYS w/ Seasonal Cycle CURRENT YEAR CURRENT YEAR w/ Seasonal Cycle Data Files AAM data file (Updated: Friday, 28-Sep-2018 08:35:48 CDT) AAM 1-21 data file (Updated: Friday, 28-Sep-2018 08:35:50 CDT) TAUC data file (Updated: Friday, 28-Sep-2018 08:35:57 CDT) TAUF data file (Updated: Friday, 28-Sep-2018 08:35:57 CDT) TAUG data file (Updated: Friday, 28-Sep-2018 08:35:57 CDT) TAUM data file (Updated: Friday, 28-Sep-2018 08:35:57 CDT) TEND data file (Updated: Friday, 28-Sep-2018 08:36:00 CDT) TEND (1-21) data file (Updated: Friday, 28-Sep-2018 08:36:02 CDT) TRANSP data file (Updated: Friday, 28-Sep-2018 08:36:00 CDT) MONTHLY DATA Vertically-integrated: 1968-1997 Climatology Plots show some of the features: ENSO, QBO, & TRENDS. ( Plots contain data through 08/31/2018 ) PLOTS 1958-PRESENT: Total Fields 1958-PRESENT: Anomaly Fields MJO Composites:
  2. How Predictable Are the Arctic and North Atlantic Oscillations? Exploring the Variability and Predictability of the Northern Hemisphere Authors: Daniela I. V. Domeisena, Gualtiero Badin and Inga M. Koszalka Published: 18th January, 2018 Abstract: The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) describe the dominant part of the variability in the Northern Hemisphere extratropical troposphere. Because of the strong connection of these patterns with surface climate, recent years have shown an increased interest and an increasing skill in forecasting them. However, it is unclear what the intrinsic limits of short-term predictability for the NAO and AO patterns are. This study compares the variability and predictability of both patterns, using a range of data and index computation methods for the daily NAO and AO indices. Small deviations from Gaussianity are found along with characteristic decorrelation time scales of around one week. In the analysis of the Lyapunov spectrum it is found that predictability is not significantly different between the AO and NAO or between reanalysis products. Differences exist, however, between the indices based on EOF analysis, which exhibit predictability time scales around 12–16 days, and the station-based indices, exhibiting a longer predictability of 18–20 days. Both of these time scales indicate predictability beyond that currently obtained in ensemble prediction models for short-term predictability. Additional longer-term predictability for these patterns may be gained through local feedbacks and remote forcing mechanisms for particular atmospheric conditions. Link to full paper (on the Researchgate website): Please note that there is also a fully downloadable pdf version on the Researchgate site but only directly to your own pc or device.
  3. Northern Hemisphere Stratospheric Pathway of Different El Niño Flavors in Stratosphere-Resolving CMIP5 Models Authors: N. Calvo, M. Iza, M. M. Hurwitz, E. Manzini, C. Peña-Ortiz, A. H. Butler, C. Cagnazzo, S. Ineson and C. I. Garfinkel Published: 10th May, 2017 Abstract: The Northern Hemisphere (NH) stratospheric signals of eastern Pacific (EP) and central Pacific (CP) El Niño events are investigated in stratosphere-resolving historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the role of the stratosphere in driving tropospheric El Niño teleconnections in NH climate. The large number of events in each composite addresses some of the previously reported concerns related to the short observational record. The results shown here highlight the importance of the seasonal evolution of the NH stratospheric signals for understanding the EP and CP surface impacts. CMIP5 models show a significantly warmer and weaker polar vortex during EP El Niño. No significant polar stratospheric response is found during CP El Niño. This is a result of differences in the timing of the intensification of the climatological wavenumber 1 through constructive interference, which occurs earlier in EP than CP events, related to the anomalous enhancement and earlier development of the Pacific–North American pattern in EP events. The northward extension of the Aleutian low and the stronger and eastward location of the high over eastern Canada during EP events are key in explaining the differences in upward wave propagation between the two types of El Niño. The influence of the polar stratosphere in driving tropospheric anomalies in the North Atlantic European region is clearly shown during EP El Niño events, facilitated by the occurrence of stratospheric summer warmings, the frequency of which is significantly higher in this case. In contrast, CMIP5 results do not support a stratospheric pathway for a remote influence of CP events on NH teleconnections. Link to full paper:
  4. Impact of the Stratosphere on the Winter Tropospheric Teleconnections between ENSO and the North Atlantic and European Region Authors: Chiara Cagnazzor and Elisa Manzini Published: 21st August, 2008 Abstract: The possible role of stratospheric variability on the tropospheric teleconnection between El Niño–Southern Oscillation (ENSO) and the North Atlantic and European (NAE) region is addressed by comparing results from two ensembles of simulations performed with an atmosphere general circulation model fully resolving the stratosphere (with the top at 0.01 hPa) and its low-top version (with the top at 10 hPa). Both ensembles of simulations consist of nine members, covering the 1980–99 period and are forced with prescribed observed sea surface temperatures. It is found that both models capture the sensitivity of the averaged polar winter lower stratosphere to ENSO in the Northern Hemisphere, although with a reduced amplitude for the low-top model. In late winter and spring, the ENSO response at the surface is instead different in the two models. A large-scale coherent pattern in sea level pressure, with high pressures over the Arctic and low pressures over western and central Europe and the North Pacific, is found in the February–March mean of the high-top model. In the low-top model, the Arctic high pressure and the western and central Europe low pressure are very much reduced. The high-top minus low-top model difference in the ENSO temperature and precipitation anomalies is that North Europe is colder and the Northern Atlantic storm track is shifted southward in the high-top model. In addition, it has been found that major sudden stratospheric warming events are virtually lacking in the low-top model, while their frequency of occurrence is broadly realistic in the high-top model. Given that this is a major difference in the dynamical behavior of the stratosphere of the two models and that these events are favored by ENSO, it is concluded that the occurrence of sudden stratospheric warming events affects the reported differences in the tropospheric ENSO–NAE teleconnection. Given that the essence of the high-top minus low-top model difference is a more annular (or zonal) pattern of the anomaly in sea level pressure, relatively larger over the Arctic and the NAE regions, this interpretation is consistent with the observational evidence that sudden stratospheric warmings play a role in giving rise to persistent Arctic Oscillation anomalies at the surface. Link to full paper:
  5. 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:
  6. 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:
  7. 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:
  8. 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:<3548%3AUWAFAA>2.0.CO%3B2
  9. Time-Lagged Response of the Antarctic and High-Latitude Atmosphere to Tropical MJO Convection Authors: Gina R. Henderson, Bradford S. Barrett, Ashley Lois, and Haadi Elsaawy Published: 22nd February, 2018 (online: 18th April, 2018) Abstract: Intraseasonal tropical variability has important implications for the mid- and high-latitude atmosphere, and in recent studies has been shown to modulate a number of weather processes in the Northern Hemisphere, such as snow depth, sea ice concentration, precipitation, atmospheric rivers, and air temperature. In such studies, the extratropical atmosphere has tended to respond to the tropical convection of the leading mode of intraseasonal variability, the Madden–Julian oscillation (MJO), with a time lag of approximately 7 days. However, the time lag between the MJO and the Antarctic atmosphere has been found to vary between less than 7 and greater than 20 days. This study builds on previous work by further examining the time-lagged response of Southern Hemisphere tropospheric circulation to tropical MJO forcing, with specific focus on the latitude belt associated with the Antarctic Oscillation, during the months of June (austral winter) and December (austral summer) using NCEP–DOE Reanalysis 2 data for the years 1979–2016. Principal findings indicate that the time lag with the strongest height anomalies depends on both the location of the MJO convection (e.g., the MJO phase) and the season, and that the lagged height anomalies in the Antarctic atmosphere are fairly consistent across different vertical levels and latitudinal bands. In addition, certain MJO phases in December displayed lagged height anomalies indicative of blocking-type atmospheric patterns, with an approximate wavenumber of 4, whereas in June most phases were associated with more progressive height anomaly centers resembling a wavenumber-3-type pattern. Link to full paper:
  10. Atmospheric summer teleconnections and Greenland Ice Sheet surface mass variations: insights from MERRA-2 Authors: Young-Kwon Lim, Siegfried D Schubert, Sophie M J Nowicki, Jae N Lee, Andrea M Molod, Richard I Cullather, Bin Zhao and Isabella Velicogna Published: 1st February, 2016 (IOP Publishing Ltd) Abstract: The relationship between leading atmospheric teleconnection patterns and Greenland Ice Sheet (GrIS) temperature, precipitation, and surface mass balance (SMB) are investigated for the last 36 summers (1979–2014) based on Modern-Era Retrospective analysis for Research and Applications version 2 reanalyses. The results indicate that the negative phase of both the North Atlantic Oscillation (NAO) and Arctic Oscillation, associated with warm and dry conditions for the GrIS, lead to SMB decreases within 0–1 months. Furthermore, the positive phase of the East Atlantic (EA) pattern often lags the negative NAO, reflecting a dynamical linkage between these modes that acts to further enhance the warm and dry conditions over the GrIS, leading to a favorable environment for enhanced surface mass loss. The development of a strong negative NAO in combination with a strong positive EA in recent years leads to significantly larger GrIS warming compared to when the negative NAO occurs in combination with a negative or weak positive EA (0.69 K versus 0.13 K anomaly). During 2009 and 2011, weakened (as compared to conditions during the severe surface melt cases of 2010 and 2012) local high pressure blocking produced colder northerly flow over the GrIS inhibiting warming despite the occurrence of a strong negative NAO, reflecting an important role for the EA during those years. In particular, the EA acts with the NAO to enhance warming in 2010 and 2012, and weaken high pressure blocking in 2009 and 2011. In general, high pressure blocking primarily impacts the western areas of the GrIS via advective temperature increases, while changes in net surface radiative fluxes account for both western and eastern GrIS temperature changes. Link to full paper:
  11. Effects of Arctic Sea Ice Decline on Weather and Climate: A Review Authors: Timo Vihma Published: 9th March, 2014 Abstract: The areal extent, concentration and thickness of sea ice in the Arctic Ocean and adjacent seas have strongly decreased during the recent decades, but cold, snow-rich winters have been common over mid-latitude land areas since 2005. A review is presented on studies addressing the local and remote effects of the sea ice decline on weather and climate. It is evident that the reduction in sea ice cover has increased the heat flux from the ocean to atmosphere in autumn and early winter. This has locally increased air temperature, moisture, and cloud cover and reduced the static stability in the lower troposphere. Several studies based on observations, atmospheric reanalyses, and model experiments suggest that the sea ice decline, together with increased snow cover in Eurasia, favours circulation patterns resembling the negative phase of the North Atlantic Oscillation and Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia, which favours cold winters in Europe and northeastern Eurasia. A high over the western and a low over the eastern North America have also been suggested, favouring advection of Arctic air masses to North America. Mid-latitude winter weather is, however, affected by several other factors, which generate a large inter-annual variability and often mask the effects of sea ice decline. In addition, the small sample of years with a large sea ice loss makes it difficult to distinguish the effects directly attributable to sea ice conditions. Several studies suggest that, with advancing global warming, cold winters in mid-latitude continents will no longer be common during the second half of the twenty-first century. Recent studies have also suggested causal links between the sea ice decline and summer precipitation in Europe, the Mediterranean, and East Asia. Link to full paper:
  12. Increased Variability in the Early Winter Subarctic North American Atmospheric Circulation Authors: James E. Overland (NOAA)and Muyin Wang (Joint Institute for the Study of the Atmosphere and Ocean, University of Washington) Published: 23rd July, 2015 (published online: 11th September, 2015) Abstract: The last decade shows increased variability in the Arctic Oscillation (AO) index for December. Over eastern North America such increased variability depended on amplification of the climatological longwave atmospheric circulation pattern. Recent negative magnitudes of the AO have increased geopotential thickness west of Greenland and cold weather in the central and eastern United States. Although the increased variance in the AO is statistically significant based on 9-yr running standard deviations from 1950 to 2014, one cannot necessarily robustly attribute the increase to steady changes in external sources (sea temperatures, sea ice) rather than a chaotic view of internal atmospheric variability; this is due to a relatively short record and a review of associated atmospheric dynamics. Although chaotic internal variability dominates the dynamics of atmospheric circulation, Arctic thermodynamic influence can reinforce the regional geopotential height pattern. Such reinforcement suggests a conditional or state dependence on whether an Arctic influence will impact subarctic severe weather, based on different circulation regimes. A key conclusion is the importance of recent variability over potential trends in Arctic and subarctic atmospheric circulation. Continued thermodynamic Arctic changes are suggested as a Bayesian prior leading to a probabilistic approach for potential subarctic weather linkages and the potential for improving seasonal forecasts. Link to full paper:
  13. Bring Back 1962-63

    Arctic Oscillation - A Simple Guide

    Arctic Oscillation (AO) - A Simple Guide Authors: NOAA Published: Current - updated monthly Summary: This is another one of the excellent NOAA guides with a simple definition. There is a table showing the monthly values of the AO from 1950 right up to the most recent month. There is also a chart showing these values from 1950 to 2011. Link to Web Page:
  14. Impact of Large-scale Climatic Oscillations on Snowfall-related Climate Parameters in the World's Major Downhill Ski Areas: A Review Authors: Christian Lehr, Philip J. Ward and Matti Kummu Published: November 2012 Abstract: Skiers are passionate about finding the best snow conditions. Snow conditions in thousands of ski resorts around the world depend mainly on natural snowfall, particularly in the case of backcountry skiing. In various mountain ranges popular among skiers, snowfall is strongly linked to large-scale climatic oscillations. This paper reviews existing information on the impacts of several of these phenomena, such as the El Niño–Southern Oscillation, North Atlantic Oscillation, and North Pacific Index, on snowfall-related climate parameters in the world's major ski areas. We found that in each of the studied areas, one or more large-scale climatic oscillations affected snowfall-related climate parameters. Understanding the predictability of such oscillations is high on the climate research agenda. If this research leads to improved predictability in the coming years, this could be combined with the knowledge summarized in our paper on the relationships between climatic oscillations and snow-related parameters to provide useful information for winter sports and other snow-related fields. Link to full paper:
  15. The stratospheric pathway for Arctic impacts on midlatitude climate Authors: Tetsu Nakamura, Koji Yamazaki, Katsushi Iwamoto, Meiji Honda, Yasunobu Miyoshi, Yasunobu Ogawa, Yoshihiro Tomikawa, Jinro Ukita Published: March 2016 Abstract: Recent evidence from both observations and model simulations suggests that an Arctic sea ice reduction tends to cause a negative Arctic Oscillation (AO) phase with severe winter weather in the Northern Hemisphere, which is often preceded by weakening of the stratospheric polar vortex. Although this evidence hints at a stratospheric involvement in the Arctic‐midlatitude climate linkage, the exact role of the stratosphere remains elusive. Here we show that tropospheric AO response to the Arctic sea ice reduction largely disappears when suppressing the stratospheric wave mean flow interactions in numerical experiments. The results confirm a crucial role of the stratosphere in the sea ice impacts on the midlatitudes by coupling between the stratospheric polar vortex and planetary‐scale waves. Those results and consistency with observation‐based evidence suggest that a recent Arctic sea ice loss is linked to midlatitudes extreme weather events associated with the negative AO phase. Link to full paper:
  16. On the atmospheric response experiment to a Blue Arctic Ocean: Climate Response to Blue Arctic Ocean Authors: Nakamura, Yamazaki, Honda, Dethloff. Published: Sept 2016 Abstract: Arctic warming is among the most remarkable of climate change signals undergoing global warming, and has resulted in continuous changes to the Arctic environment. Arctic sea ice loss is one of the most symbolic signatures of these rapid climate changes. Changes to Arctic sea ice are crucial to the coupled climate system, because ice melting is stimulated by atmospheric and oceanic heat transport into the Arctic. In turn, radiative and thermal feedback from the melting ice induces further warming in the Arctic. In this context, there is particular interest in the recent negative tendency of the winter Arctic Oscillation (AO) [Thompson and Wallace, 1998, 2001] observed in association with the loss of Arctic sea ice [Honda et al., 2009; Jaiser et al., 2012; Liu et al., 2012] and an increase in the extent of Eurasian snow cover [Cohen et al., 2014; Furtado et al., 2016]. The negative phase of the AO is a manifestation of the exchange of the Arctic cold air mass and mid-latitude warm air mass following a stronger meandering of the flow, which implies an increase in atmospheric heat transport into the Arctic. Such circulation changes associated with a negative AO enhance Arctic warming and local surface heat flux anomalies due to Arctic sea ice loss. Link to full paper:
  17. Changes in meandering of the Northern Hemisphere circulation Authors: Giorgia Di Capua and Dim Coumou Published: 22nd September, 2016 (© 2016 IOP Publishing Ltd) Abstract: Strong waves in the mid-latitude circulation have been linked to extreme surface weather and thus changes in waviness could have serious consequences for society. Several theories have been proposed which could alter waviness, including tropical sea surface temperature anomalies or rapid climate change in the Arctic. However, so far it remains unclear whether any changes in waviness have actually occurred. Here we propose a novel meandering index which captures the maximum waviness in geopotential height contours at any given day, using all information of the full spatial position of each contour. Data are analysed on different time scale (from daily to 11 day running means) and both on hemispheric and regional scales. Using quantile regressions, we analyse how seasonal distributions of this index have changed over 1979–2015. The most robust changes are detected for autumn which has seen a pronounced increase in strongly meandering patterns at the hemispheric level as well as over the Eurasian sector. In summer for both the hemisphere and the Eurasian sector, significant downward trends in meandering are detected on daily timescales which is consistent with the recently reported decrease in summer storm track activity. The American sector shows the strongest increase in meandering in the warm season: in particular for 11 day running mean data, indicating enhanced amplitudes of quasi-stationary waves. Our findings have implications for both the occurrence of recent cold spells and persistent heat waves in the mid-latitudes. Link to full paper:
  18. Evidence linking Arctic amplification to extreme weather in mid-latitudes Received 17 January 2012; revised 20 February 2012; accepted 21 February 2012; published 17 March 2012 Authors: Jennifer A. Francis and Stephen J. Vavrus First Published: 17th January, 2012 (revised 20th February 2012; published online 17th March, 2012) Abstract: Arctic amplification (AA) – the observed enhanced warming in high northern latitudes relative to the northern hemisphere – is evident in lower‐tropospheric temperatures and in 1000‐to‐500 hPa thicknesses. Daily fields of 500 hPa heights from the National Centers for Environmental Prediction Reanalysis are analyzed over N. America and the N. Atlantic to assess changes in north‐south (Rossby) wave characteristics associated with AA and the relaxation of poleward thickness gradients. Two effects are identified that each contribute to a slower eastward progression of Rossby waves in the upper‐level flow: 1) weakened zonal winds, and 2) increased wave amplitude. These effects are particularly evident in autumn and winter consistent with sea‐ice loss, but are also apparent in summer, possibly related to earlier snow melt on high‐latitude land. Slower progression of upper‐level waves would cause associated weather patterns in mid‐latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves. Link to full paper: Further Reading - Link to later paper (by the same authors): Evidence for a wavier jet stream in response to rapid Arctic warming
  19. Evidence for a wavier jet stream in response to rapid Arctic warming Authors: Jennifer A Francis and Stephen J Vavrus Published: 6th January, 2015 (© 2015 IOP Publishing Ltd) Abstract: New metrics and evidence are presented that support a linkage between rapid Arctic warming, relative to Northern hemisphere mid-latitudes, and more frequent high-amplitude (wavy) jet-stream configurations that favor persistent weather patterns. We find robust relationships among seasonal and regional patterns of weaker poleward thickness gradients, weaker zonal upper-level winds, and a more meridional flow direction. These results suggest that as the Arctic continues to warm faster than elsewhere in response to rising greenhouse-gas concentrations, the frequency of extreme weather events caused by persistent jet-stream patterns will increase. Link to full paper: Further Reading - Link to earlier paper (by the same authors): Evidence linking Arctic amplification to extreme weather in mid-latitudes
  20. Mountain Torques and Northern Hemisphere Low-Frequency Variability.Part II: Regional Aspects Authors: Francois Lott, Andrew W. Robertson and Michael Ghil First Published: 16th November 2001 (published online: 1st June , 2004) Abstract: Important aspects of low-frequency variability (LFV) are regional in character, while the mountain torques of the Rockies and the Himalayas evolve quite independently of each other. The hemispheric analysis of Part I is complemented therefore herein by an analysis of the relationships between individual mountain torques and sectorial LFV patterns in the NCEP–NCAR reanalysis. In the 20–30-day band, relationships are found between the Rockies (Himalayas) torque and the dominant patterns of LFV over the Pacific (Eurasia). The composites of the atmospheric flow fields that accompany the Rockies (Himalayas) torque in this band exhibit similarities with known low-frequency oscillations that dominate the Pacific and North American (European and North Atlantic) sectors during certain winters. The composites keyed to the 20–30-day Rockies torque affect the persistent North Pacific (PNP) pattern that controls the extension of the midlatitude jet stream over the eastern Pacific. Furthermore, the unfiltered torques for the Northern Hemisphere (NH) and Rockies anticipate the onset of the two dominant winter Pacific circulation regimes that correlate strongly with the PNP pattern. The composites keyed to the 20–30-day Himalayas torque affect the North Atlantic Oscillation (NAO) pattern, which controls the intensity of the North Atlantic jet stream. Furthermore, the unfiltered torques for the NH and the Himalayas anticipate the breaks of the two dominant winter Atlantic circulation regimes, which correlate strongly with the NAO pattern. These analyses also show that the 20–30-day Rockies (Himalayas) torques produce substantial atmospheric angular momentum (AAM) changes, which are nearly in phase with and larger in amplitude than the AAM changes associated with the midlatitude eastern Pacific (North Atlantic) jet stream variations seen in the composite maps. This result suggests that the Rockies (Himalayas) torque variations drive, at least partially, but actively the changes in the eastern Pacific (North Atlantic) jet stream. These results are consistent with the Himalayas and the Rockies torques contributing separately to changes in the two leading hemispheric EOFs that were described in Part I; the two are associated with a hemispheric index cycle and the Arctic Oscillation, respectively. Link to full paper:<1272%3AMTANHL>2.0.CO%3B2
  21. Sudden Stratospheric Warming: Causes & Effects Authors: Randall Gates Simpson Published: 22nd April, 2013 Abstract (none): ***I highly recommend this amateur article - clear explanations with many charts and diagrams*** In this post I reveal what I think is an original synthesis that gives the full picture of some of the main causes and effects of Northern Hemisphere Sudden Stratospheric Warming events based on my readings of many other papers along with hours of my own original research. For research material I relied heavily on the use of the amazing amount of satellite derived reanalysis data as well as ground based observations. Link to full paper:
  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. Hindcasting the January 2009 Arctic Sudden Stratospheric Warming and Its Influence on the Arctic Oscillation with Unified Parameterization of Orographic Drag in NOGAPS. Part I: Extended-Range Stand-Alone Forecast Abstract: A very strong Arctic major sudden stratospheric warming (SSW) event occurred in late January 2009. The stratospheric temperature climbed abruptly and the zonal winds reversed direction, completely splitting the polar stratospheric vortex. A hindcast of this event is attempted by using the Navy Operational Global Atmospheric Prediction System (NOGAPS), which includes the full stratosphere with its top at around 65 km. As Part I of this study, extended-range (3 week) forecast experiments are performed using NOGAPS without the aid of data assimilation. A unified parameterization of orographic drag is designed by combining two parameterization schemes; one by Webster et al., and the other by Kim and Arakawa and Kim and Doyle. With the new unified orographic drag scheme implemented, NOGAPS is able to reproduce the salient features of this Arctic SSW event owing to enhanced planetary wave activity induced by more comprehensive subgrid-scale orographic drag processes. The impact of the SSW on the tropospheric circulation is also investigated in view of the Arctic Oscillation (AO) index, which calculated using 1000-hPa geopotential height. The NOGAPS with upgraded orographic drag physics better simulates the trend of the AO index as verified by the Met Office analysis, demonstrating its improved stratosphere–troposphere coupling. It is argued that the new model is more suitable for forecasting SSW events in the future and can serve as a tool for studying various stratospheric phenomena. Link to full paper: