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Found 33 results

  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. http://atlas.niu.edu/gwo/ Features Total AAM, Bias Corrected Rel AAM GEFS, CFS GWO Forecast. He stated he is soon to add torque products. Nick Schraldi GWO Site http://www.atmos.albany.edu/student/nschiral/gwo.html Non-Bias Corrected GEFS GWO forecast. Michael Ventrice http://mikeventrice.weebly.com/hovmollers.html Hovmoller from MV, to help spot AAM trends and patterns. GEFS. Carl Schreck https://ncics.org/portfolio/monitor/mjo/ More Hovmollers and other tropical charts to spot trends in the AAM. CFS forecast. NPJ Phase Diagrams/Albany http://www.atmos.albany.edu/facstaff/awinters/realtime/Deterministic_NPJPD.php Shows a GEFS forecast and observation of NP jetstream, which is largely controlled by the AAM. MJO Composites: http://www.atmos.albany.edu/facstaff/roundy/waves/rmmcyc/index200reg.html
  2. Estimates of Atmospheric Angular Momentum, Friction, and Mountain Torques during 1987–1988 Authors: R. A. Madden and P. Speth First Published: 1st November, 1995 Abstract: Atmospheric angular momentum (M), friction (TF), and mountain torques (TM) are estimated from a 13-month period of European Centre for Medium-Range Weather Forecasts (ECMWF) data. Cross-spectrum analysis between M and total torques results in high coherence and one-quarter cycle phase angles (TF + TM leading M) for timescales between 5 and 66 days, suggesting that variations of the total torque are reasonably well estimated for these slower variations. However, cross spectra between M and TF, and TM separately reveal that the relatively high coherence is present between M and TF only at periods longer than 20 days. Also comparison with other published values and the considerable lack of balance between TF + TM and M over a full year implies that our estimates of TF, based on the parameterization of surface wind stress in short-term forecasts of the ECMWF, are negatively biased. For the 13-month period, the average bias is about −15.2 Hadleys (1018 kg m2 s−2). During the period there are a few near 50-day oscillations in the M. Similar variations have been reported before and related to tropical intraseasonal oscillations of the same timescale. Two oscillations in M that are coincident with eastward-propagating cloud complexes of tropical intraseasonal oscillations are examined more closely. It is found that TF and TM work together to alter the M on the 50-day timescale, but that TM's contribution is three times larger than that of TF. During the two oscillations TF, reaches maxima when cloud complexes of tropical intraseasonal oscillations are in the vicinity of 90°E. It then declines but maintains positive anomalies at least until the cloud complexes reach the Central Pacific. The M reaches its maxima shortly thereafter. TM has sharp minima shortly before the cloud complexes are strongly developed in the Indian Ocean. Contributors to these minima are strong cast to west pressure gradients primarily across the Rocky Mountains. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469(1995)052<3681%3AEOAAMF>2.0.CO%3B2
  3. Where is ENSO stress balanced? Authors: Matthias Münnich and David Neelin First Published: 20th November, 2003 Published online: 14th April, 2004 Abstract: The zonal surface torque budget associated with the tropical wind stress anomalies during El Niño/Southern Oscillation is analyzed. Mountain and surface stress torques over South America are found to play a prominent role. Local momentum change is negligible for 6 month averages allowing the balance among regional contributions to the torque anomalies to be compared. During El Niño, eastward torque anomalies over the central equatorial Pacific are largely compensated by westward anomalies elsewhere in the equatorial band, notably over South America. Torque anomalies over South America and the Pacific in latitude bands north and south of the equator are both westward and are not compensated within the band, implying an export of eastward momentum to higher latitudes. Copyright © 2003 Royal Meteorological Society. Published by Elsevier Ltd. All rights reserved. Link to full paper: https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1016/j.atmoscilet.2003.11.001
  4. Improving weather forecasts via impacts of turbulent orographic form & small-scale orographic gravity wave drag on boundary layers Authors: Maarten Minkman (Master thesis) Published: November, 2017 Abstract: Numerical Weather Prediction (NWP) models have difficulties with representing stable boundary layer conditions. During these conditions NWP models need more drag than physically observed to increase the representation of cyclonics. Therefore, NWP models use a non-physical enhanced momentum drag (long-tail formulation), which negatively affects the skill for the near-surface wind speeds and boundary layer depth. To describe the extra needed drag for NWP models in a more physical way, this study investigates whether the already developed parameterizations of turbulent orographic form drag (TOFD) and small-scale orographic wave drag (GWDSBL) could describe the needed drag instead of the longtail formulation. On top of the short-tail formulation TOFD and GWDSBL were added individually and combined. The short-tail formulation is supported by observations and LES simulations. Polar WRF (version 3.7.1.) was used in a 10-day case study during the northern hemispheric winter over North America. Individual GWDSBL and TOFD both reduced the wind speeds with a bias reduction of 2% and 13% respectively compared to the short-tail formulation. As a result the 2-meter temperatures reduced locally and increased the general cold bias within WRF by 1-4◦C where the wind speeds reduced due to GWDSBL and TOFD. Besides the boundary layer the synoptic scale was affected to by GWDSBL and TOFD. The cyclonic core pressure was improved and TOFD and the combination of GWDSBL and TOFD even outperformed the long-tail formulation by reducing the biases with approx. 5%. The 500 hPa geopotential heigth was raised with more drag added to short-tail and decreased the bias. However, the Arctic and number of model levels showed a significant impact in the representation of the 500 hpa geopotential height. TOFD also reduced on average the jet stream by 1-4 m s−1 and resulted in a northward shift. In the end the combination of GWDSBL and TOFD is seen as the best option to describe the extra needed drag instead of the long-tail formulation. Link to full paper: http://edepot.wur.nl/427452
  5. Effect of Yunnan–Guizhou Topography at the Southeastern Tibetan Plateau on the Indian Monsoon Authors: Zhengguo Shi (Center for Excellence in Tibetan Plateau Earth Sciences, et al), Yingying Sha and Xiaodong Liu Published: 27th October, 2016 Abstract: Topographic insulation is one of the primary origins for the influence of the Tibetan Plateau (TP) on Asian climate. The Yunnan–Guizhou (YG) Plateau, at the southeastern margin of the TP, is known to block the northern branch of the Indian monsoon circulation in summer. However, it is an open question whether this blocking feeds back to the monsoon. In this study, the effect of the YG topography on the Indian monsoon and its comparison with that of the TP were evaluated using general circulation model experiments. The results showed that the TP strengthens the monsoon precipitation, especially during the onset. However, the YG topography significantly weakens the monsoon. With the YG topography, strengthened low-level airflow around the YG Plateau induces anomalous anticyclonic winds to the southwest, and the changes remodulate the whole circulation structure over Asia. As a result, the Indian monsoon becomes weakened from the Bay of Bengal to the Indian subcontinent and Arabian Sea, as does the associated precipitation. In addition, the YG topography affects the anomalous warming center over the TP and the precipitation during the monsoon onset. The YG-reduced summer precipitation occupied approximately one-third of the total increment compared to the entire TP. The Indian monsoon weakened by YG topography distinctly opposes the traditional paleoclimatic viewpoint that all of the TP topography contributes to the monsoon strengthening. In fact, the climatic effect of the TP depends closely upon both its central and marginal topography, and the topography of its subterrains does not necessarily play a similar role. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-16-0105.1
  6. Impacts of the uplift of four mountain ranges on the arid climate and dust cycle of inland Asia Authors: Hui Suna and Xiaodong Liua Published: 6th June, 2018 Abstract: Although some studies have shown that the uplift of the northern Tibetan Plateau (NTP) has an important influence on the climate of inland Asia, the respective roles played by different parts of the NTP in controlling regional aridification and dust cycling remains unclear. In this study, based on the geological facts that the NTP and its surrounding mountains have uplifted to a certain altitude since the Miocene, we used the dust-coupled regional climate model RegCM4.1 to explore the different impacts of the uplift of four mountain ranges (the Altai Mountains–Mongolian Plateau, the Tian Shan Mountains, the Pamir Mountains, and the Qilian Mountains) on the arid climate and dust cycle of inland Asia. The results showed distinct contributions of these different mountain ranges. With respect to aridity, the uplift of the Pamirs played a leading role in the aridification of inland Asia, causing the annual mean precipitation to decrease by 200% across a wide area of Northwest China owing to the “rain shadow” effect (a rain shadow is a dry area on the leeward side of a mountainous area). In terms of dust cycling, dust emissions and deposition increased in the Taklimakan Desert mainly due to the uplift of the Tian Shan Mountains and the Pamirs, the increase in magnitude of which was 10–20 times larger than that induced by the uplift of the Altai Mountains–Mongolian Plateau or Qilian Mountains. Besides, a narrow passage formed between the uplifted Altai Mountains–Mongolian Plateau and Tian Shan Mountains that accelerated the northwesterly air flow between them, thus increasing dust emissions in northern Xinjiang and the Gobi Desert. The increase in dust emissions in these two regions induced by the uplift of the Altai Mountains–Mongolian Plateau was 7–20 times greater than that induced by the uplift of the three other mountain ranges. Results also showed that the uplift of the Qilian Mountains not only controlled the dust emissions and deposition on the Loess Plateau, but also blocked the transportation of dust from Northwest China to the middle and lower reaches of the Yangtze River, leading to the dust loading being reduced by 10–20% in South China. The findings of this study improve our understanding of the influence of NTP tectonic uplift on change in the arid climate and dust cycle of inland Asia. Link to full paper: Please note that the full paper is currently behind a paywall on the "Science Direct" website (link below): https://www.sciencedirect.com/science/article/pii/S0031018218300968 Highlights • The uplift of the Pamirs played a leading role in the aridification of inland Asia. • The uplift of the Tian Shan Mountains and the Altai Mountains-Mongolian Plateau was the main influence on climate change in the Gobi Desert. • The uplift of the Qilian Mountains obstructed the transportation of dust from Northwest China to the middle and lower reaches of the Yangtze River.
  7. Annual atmospheric torques: Processes and regional contributions Authors: Olivier de Viron, Jean O. Dickey and Steven L. Marcus Published: 12th April, 2002 Abstract: All three components of annual atmospheric torque are analyzed with a focus on understanding the contributions from various sources and the physical interactions involved. The annual variations of the equatorial component are dominated by the torque on Earth's ellipticity, with the X component mainly due to an anomaly over the Himalayas, and the Y component associated with pressure anomalies over the North Pacific Ocean. The axial annual component is due to the combined effect of friction and mountain torque, whose amplitudes are at the same order of magnitude with the friction term being larger. Partial cancellation of the mountain torque over Asia and North America is effected by the out‐of phase contribution of the Andes (South America having the opposite seasonal cycle to Asia and North America). Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001GL013859
  8. Atmospheric torques and Earth’s rotation: what drove the millisecond-level length-of-day response to the 2015–2016 El Niño? Authors: Sébastien B. Lambert, Steven L. Marcus and Olivier de Viron Published: 14th November, 2017 Abstract El Ninõ-Southern Oscillation (ENSO) events are classically associated with a significant increase in the length of day (LOD), with positive mountain torques arising from an east-west pressure dipole in the Pacific driving a rise of atmospheric angular momentum (AAM) and consequent slowing of the Earth's rotation. The large 1982-1983 event produced a lengthening of the day of about 0.9 ms, while a major ENSO event during the 2015-2016 winter season produced an LOD excursion reaching 0.81 ms in January 2016. By evaluating the anomaly in mountain and friction torques, we found that (i) as a mixed eastern-central Pacific event, the 2015-2016 mountain torque was smaller than for the 1982-1983 and 1997-1998 events, which were pure eastern Pacific events, and (ii) the smaller mountain torque was compensated for by positive friction torques arising from an enhanced Hadley-type circulation in the eastern Pacific, leading to similar AAM-LOD signatures for all three extreme ENSO events. The 2015-2016 event thus contradicts the existing paradigm that mountain torques cause the Earth rotation response for extreme El Ninõ events. Link to full paper: https://www.earth-syst-dynam.net/8/1009/2017/esd-8-1009-2017.pdf
  9. 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: https://journals.ametsoc.org/doi/pdf/10.1175/2008JAS2725.1
  10. Torques and the Related Meridional and Vertical Fluxes of Axial Angular Momentum Authors: Joseph Egger and Klaus-Peter Hoinka Published: 21st July, 2004 Abstract: The budget equation of the zonally averaged angular momentum is analyzed by introducing belts of 1000-km width to cover the meridional plane from pole to pole up to an altitude of 28 km. Using ECMWF Re-Analysis (ERA) data the fluxes of angular momentum are evaluated as well as the mountain and friction torques per belt. Generalized streamfunctions and velocity potentials are introduced to better depict the fluxes related to the angular momentum transferred at the ground during an event of mountain or friction torque. The variance of the total flux divergence per belt is one order of magnitude larger than those of the torques. All variances peak at midlatitudes. As a rule, the structure of the generalized streamfunctions changes little during an event; that is, the structure of the nondivergent part of the fluxes is stable. That of the divergent part, as represented by the velocity potential, undergoes a rapid change near the peak of a torque event. Positive friction torque events in midlatitude belts are preceded by a divergence of angular momentum fluxes in that belt, which is linked to the anticyclonic mass circulation needed to induce the positive torque. The divergence in the belt breaks down shortly before the torque is strongest. Angular momentum is transported upward from the ground after that. Much of the angular momentum generated in a midlatitude belt by positive mountain torques is transported out of the domain, but there is also a short burst of upward transports. Angular momentum anomalies linked to torque events near the equator tend to be symmetric with respect to the equator. Related fluxes affect the midlatitudes of both hemispheres. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/MWR-2870.1 https://journals.ametsoc.org/doi/pdf/10.1175/MWR-2870.1
  11. Isentropic Pressure and Mountain Torques Authors: Joseph Egger and Klaus-Peter Hoinka Published: 12th March, 2009 Abstract: The relation of pressure torques and mountain torques is investigated on the basis of observations for the polar caps, two midlatitude and two subtropical belts, and a tropical belt by evaluating the lagged covariances of these torques for various isentropic surfaces. It is only in the polar domains and the northern midlatitude belts that the transfer of angular momentum to and from the earth at the mountains is associated with pressure torques acting in the same sense. The situation is more complicated in all other belts. The covariances decline with increasing potential temperature (height). The role of both torques in the angular momentum budget of a belt is discussed. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/2009MWR2848.1
  12. Axial Angular Momentum: Vertical Fluxes and Response to Torques Authors: Joseph Egger and Klaus-Peter Hoinka Published: 4th November, 2003 Abstract: The horizontally averaged global angular momentum μ at a certain height reacts only to the vertical divergence of the angular momentum flux at least above the crest height of the earth's orography. The flux is tied to the torques at the surface. Data are used to evaluate the flux and the response of μ to the torques. It is shown that the accuracy of the data is sufficient for an investigation of this interaction. It is found that the horizontally averaged angular momentum in the upper troposphere and lower stratosphere tends to be negative before an event of positive friction torque. Downward transports of negative angular momentum from these layers allow the angular momentum to further decrease near the ground, even shortly before the event although the friction torque is positive at that time. The impact of the mountains during this process is demonstrated. The ensuing positive response to the friction torque is felt throughout the troposphere. The final decay of this reaction involves downward transports of μ with typical velocities of ∼1–2 km day−1. The angular momentum in the lower troposphere tends to be negative before an event of positive mountain torque. There is a short burst of rapid upward transport of positive angular momentum during the event itself, which reaches the stratosphere within 1–2 days. A phase of decay follows with slow downward transport of positive angular momentum. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(2004)132<1294%3AAAMVFA>2.0.CO%3B2
  13. The Dynamics of Intraseasonal Atmospheric Angular Momentum Oscillations Authors: Dr Klaus M. Weickmann, George N. Kiladis and Prashant D. Sardeshmuykh Published: 17th October, 1996 Abstract: The global and zonal atmospheric angular momentum (AAM) budget is computed from seven years of National Centers for Environmental Prediction data and a composite budget of intraseasonal (30–70 day) variations during northern winter is constructed. Regressions on the global AAM tendency are used to produce maps of outgoing longwave radiation, 200-hPa wind, surface stress, and sea level pressure during the composite AAM cycle. The primary synoptic features and surface torques that contribute to the AAM changes are described. In the global budget, the friction and mountain torques contribute about equally to the AAM tendency. The friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres. The mountain torque peaks when anomalies in the midlatitude Northern Hemisphere and subtropical Southern Hemisphere are weak but of the same sign. The picture is different for the zonal mean budget, in which the meridional convergence of the northward relative angular momentum transport and the friction torque are the dominant terms. During the global AAM cycle, zonal AAM anomalies move poleward from the equator to the subtropics primarily in response to momentum transports. These transports are associated with the spatial covariance of the filtered (30–70 day) perturbations with the climatological upper-tropospheric flow. The zonally asymmetric portion of these perturbations develop when convection begins over the Indian Ocean and maximize when convection weakens over the western Pacific Ocean. The 30–70-day zonal mean friction torque results from 1) the surface winds induced by the upper-tropospheric momentum sources and sinks and 2) the direct surface wind response to warm pool convection anomalies. The signal in relative AAM is complemented by one in “Earth” AAM associated with meridional redistributions of atmospheric mass. This meridional redistribution occurs preferentially over the Asian land mass and is linked with the 30–70-day eastward moving convective signal. It is preceded by a surface Kelvin-like wave in the equatorial Pacific atmosphere that propagates eastward from the western Pacific region to the South American topography and then moves poleward as an edge wave along the Andes. This produces a mountain torque on the Andes, which also causes the regional and global AAM to change. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469(1997)054<1445%3ATDOIAA>2.0.CO%3B2
  14. What is the GSDM and how does it help with subseasonal weather forecasts? A YouTube Presentation Presentation By: Edward K Berry (Senior Weather-Climate Scientist) Presentation Event: American Meteorology Society - Student Chapter, College of DuPage, Chicago Presentation Date: 28th March, 2018 Summary: Leading meteorological scientists Ed Berry and Dr Klaus Weickmann jointly developed their GSDM (Global Synoptic Dynamic Model) while they were working at NOAA in the late 1990s and earlier years of this century. They also devised the GWO (Global Wind Oscillation) as a way of plotting and measuring the amounts of relative global AAM (Atmospheric Angular Momentum), frictional torque and mountain torque at different phases of the cycle. They became leaders in this specialist research which has been used to assist in understanding impacts on global weather patterns and upcoming changes up to a few weeks ahead. They left NOAA several years ago and Klaus Weickmann has retired. Ed Berry continues his excellent work on the GSDM and retains his lifelong passion to develop the model and its meteorological applications further. He recently gave a brilliant presentation about the model at an AMS meeting in Chicago. This is a one hour seminar with clear charts and explanations, ending with a question and answer session. I have watched it three times already and understand a little more about the GSDM from each viewing. For anyone wishing to learn more about AAM, the torques, the GWO and how they interact with other major teleconnections like phases of the ENSO (El Nino Southern Oscillation) and the MJO (Madden Julian Oscillation) then this is absolutely essential viewing. I also strongly recommend this for more advanced viewers as well. The presentation is right up-to-date and includes the 2018 SSW (Sudden Stratospheric Warming) event and links to key issues like climate change. Much of the presentation is slanted towards the North American climate and US weather patterns but it has a global significance and includes impacts on both hemispheres. Link to full presentation (1 hour and 4 minutes): https://youtu.be/Cv5CblXbYuQ I also reviewed this presentation on the main "Telconnections: A More technical Discussion" thread. This includes some examples of the charts used in the presentation. Just click on this direct link: What is the GSDM and how does it help with subseasonal weather forecasts? - A Review of This Presentation
  15. Mountains, the Global Frictional Torque, and the Circulation over the Pacific–North American Region Authors: Klaus Weickmann Published: 2nd April, 2003 Abstract: The global mountain (τM ) and frictional (τF ) torques are lag correlated within the intraseasonal band, with τF leading τM. The correlation accounts for 20%–45% of their variance. Two basic feedbacks contribute to the relationship. First, the mountain torque forces global atmospheric angular momentum (AAM) anomalies and the frictional torque damps them; thus, dτF/dt ∝ −τM. Second, frictional torque anomalies are associated with high-latitude sea level pressure (SLP) anomalies, which contribute to subsequent mountain torque anomalies; thus, dτM/dt ∝ τF. These feedbacks help determine the growth and decay of global AAM anomalies on intraseasonal timescales. The low-frequency intraseasonal aspect of the relationship is studied for northern winter through lag regressions on τF. The linear Madden–Julian oscillation signal is first removed from τF to focus the analysis on midlatitude dynamical processes. The decorrelation timescale of τF is similar to that of teleconnection patterns and zonal index cycles, and these familiar circulation features play a prominent role in the regressed circulation anomalies. The results show that an episode of interaction between the torques is initiated by an amplified transport of zonal mean–zonal momentum across 35°N. This drives a dipole pattern of zonal mean–zonal wind anomalies near 25° and 50°N, and associated SLP anomalies. The SLP anomalies at higher latitudes play an important role in the subsequent evolution. Regionally, the momentum transport is linked with large-scale eddies over the east Pacific and Atlantic Oceans that have an equivalent barotropic vertical structure. As these eddies persist/amplify, baroclinic wave trains disperse downstream over North American and east Asian topography. The wave trains interact with the preexisting, high-latitude SLP anomalies and drive them southward, east of the mountains. This initiates a large monopole mountain torque anomaly in the 20°–50°N latitude band. The wave trains associated with the mountain torque produce additional momentum flux convergence anomalies that 1) maintain the zonal wind anomalies forced by the original momentum transport anomalies and 2) help drive a global frictional torque anomaly that counteracts the mountain torque. Global AAM anomalies grow and decay over a 2-week period, on average. Over the Pacific–North American region, the wave trains evolve into the Pacific–North American (PNA) pattern whose surface wind anomalies produce a large portion of the compensating frictional torque anomaly. Case studies from two recent northern winters illustrate the interaction. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(2003)131<2608%3AMTGFTA>2.0.CO%3B2
  16. Mountain Torque Events at the Tibetan Plateau Authors: Joseph Egger and Klause-Peter Hoinka First Published: 19th December 2006 Abstract: The interaction of large-scale wave systems with the Tibetan Plateau (TP) is investigated by regressing pressure, potential temperature, winds, precipitation, and selected fluxes in winter onto the three components Toi of this massif’s mountain torque on the basis of the 40-yr ECMWF reanalysis (ERA-40) data. Events with respect to the equatorial “Greenwich” axis of the global angular momentum exhibit by far the largest torques (To1,), which essentially represent north–south pressure differences across the TP. The axial torque To3 peaks when the surface pressure is high at the eastern slope of the TP. The torque To2 with respect to the 90°E axis is closely related to To3 with To2 To3. The maximum (minimum) of To1 tends to occur about 1 day earlier than the minimum (maximum) of To2. All torque events are initiated by equivalent barotropic perturbations moving eastward along the northern rim of the TP. In general, the initial depression, for example, forms a southward-protruding extension at the eastern slope of the TP and a new high grows near Japan. Later, the perturbation near Japan moves eastward in To2 events but extends northward in To1 events. These flow developments cannot be explained by theories of topographic instability. The observed vertical motion at the lee slope is at best partly consistent with theories of linear quasigeostrophic wave motion along mountain slopes. These findings lead the authors to test the eventual usefulness of linear theories by fitting the linear terms of a novel statistical equation for the potential temperature to the observed changes of and the torque to the observations. This test indicates that the evolving regression patterns of can be explained by linear terms at least in specific domains. In turn, pressure tendency regressions at a selected level can be calculated on the basis of the linear tendencies above that level. The formation of the lee trough appears to be mainly caused by horizontal warm-air advection along the slopes, but changes of the potential temperature above the height of the TP also contribute significantly to the pressure changes in the lee. Cold-air advection aloft strengthens the Japan high. “Turbulent” transports appear to be mainly responsible for the decay of the perturbations but data accuracy problems impede the analysis. In particular, the noisiness of the vertical motion fields affects the skill of the linear calculation Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/2007MWR2126.1
  17. Mountain Waves and Downslope Winds - A Special Presentation Produced By: The Comet Program, University Corporation for Atmospheric Research Presentation date; 2004 Summary: There is no abstract and so I have provide this brief review. This presentation is the best that I've seen in defining and describing all the different types of atmospheric and mountain waves (many are shown in the tags above). There are some very clear illustrations, charts and diagrams with concise explanations below each one. The same principles apply to almost all the waves and it helps one to understand how the vertically propagating waves can reach the mesosphere, up to the critical level before being deflected downwards and then breaking in the stratosphere creating significant disturbances which can lead to triggering sudden stratospheric warming events. I most highly recommend the presentation. Link to Full Illustrated Presentation: http://stream1.cmatc.cn/pub/comet/MountainMeteorology/mwav/comet/mesoprim/mtnwave/print.htm#2
  18. The physics of orographic gravity wave drag Authors: Miguel A. C. Teixeira First Published: 11th July, 2014 Abstract: The drag and momentum fluxes produced by gravity waves generated in flow over orography are reviewed, focusing on adiabatic conditions without phase transitions or radiation effects, and steady mean incoming flow. The orographic gravity wave drag is first introduced in its simplest possible form, for inviscid, linearized, non-rotating flow with the Boussinesq and hydrostatic approximations, and constant wind and static stability. Subsequently, the contributions made by previous authors (primarily using theory and numerical simulations) to elucidate how the drag is affected by additional physical processes are surveyed. These include the effect of orography anisotropy, vertical wind shear, total and partial critical levels, vertical wave reflection and resonance, non-hydrostatic effects and trapped lee waves, rotation and nonlinearity. Frictional and boundary layer effects are also briefly mentioned. A better understanding of all of these aspects is important for guiding the improvement of drag parametrization schemes. Link to Full Paper: https://www.frontiersin.org/articles/10.3389/fphy.2014.00043/full
  19. Uncertainty analysis of atmospheric friction torque on the solid Earth Authors: Haoming Yan and Yong Huang First Published: 11th December, 2015 Abstract: The wind stress acquired from European Centre for Medium-Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction (NCEP) climate models and QSCAT satellite observations are analyzed by using frequency-wavenumber spectrum method. The spectrum of two climate models, i.e., ECMWF and NCEP, is similar for both 10 m wind data and model output wind stress data, which indicates that both the climate models capture the key feature of wind stress. While the QSCAT wind stress data shows the similar characteristics with the two climate models in both spectrum domain and the spatial distribution, but with a factor of approximately 1.25 times larger than that of climate models in energy. These differences show the uncertainty in the different wind stress products, which inevitably cause the atmospheric friction torque uncertainties on solid Earth with a 60% departure in annual amplitude, and furtherly affect the precise estimation of the Earth's rotation. Link to Paper: https://ac.els-cdn.com/S1674984716300301/1-s2.0-S1674984716300301-main.pdf?_tid=c9f4e787-e0d8-4df2-9487-e526cbdd54a5&acdnat=1523641140_b6c42688efd8a36f8864cb62251c7c61
  20. Dynamic coupling of the stratosphere with the troposphere and sudden stratospheric warmings Author: Kevin E. Trenberth First Published: 1966 (revised 5th July, 1972 and published 1st April ,1973) Abstract: Numerical time integrations of a 9-layer quasigeostrophic highly truncated spectral model of the atmosphere are used to study tropospheric-stratospheric interaction with particular regard to sudden stratospheric warmings. The model is global and extends to 0.05 mb (71 km) with roughly 10 km resolution in the stratosphere, and includes an annual heating cycle. A linear baroclinic analysis of a similar model shows that the inclusion of spherical geometry allows significant growth rates in the long wave region of instability. Preliminary integrations without eddies reveal the seasonal variation of a thermally driven circulation. Model integrations simulating the months of December and January were made (i) without nonzonal forcing, and (ii) with nonzonal heating and orography included, to represent southern and northern hemisphere winters. The overall features of the atmosphere were very well simulated. With the inclusion of the annual heating cycle, the model successfully reproduced a more intense circulation in January than existed in December. This caused a maximum tropospheric meridional temperature gradient in the winter hemisphere to occur some weeks prior to the maximum in the external heating field. The presence of nonzonal heating in the winter hemisphere brought about an increase in circulation intensity and produced a stationary perturbation with a strong westward slope with height extending high into the stratosphere. These are features somewhat similar to those of the Aleutian system. Associated with this were considerably warmer temperatures in the polar night stratosphere and a weaker stratospheric westerly jet. The winter mesosphere of the model was driven in the manner of the lower stratosphere and a temperature maximum was produced in mid latitudes. Link to Paper: ftp://ftp.library.noaa.gov/docs.lib/htdocs/rescue/mwr/101/mwr-101-04-0306.pdf
  21. Stochastic and oscillatory forcing of global atmospheric angular momentum Authors: Weickmann, K.M., W.A. Robinson, and C. Penland First Published: 27th June, 2000 Abstract: The temporal variability and forcing of global atmospheric angular momentum (AAM) is studied using a three‐component Markov model derived from observed statistics of global AAM and the global torques. The model consists of stochastic forcing by the mountain (τM) and friction (τ *F) torque plus a pervasive negative feedback on AAM by the friction torque. AAM anomalies are damped at a 30‐day timescale and forced by torques having 1.5‐day ( τM) and 6‐day (τ *F) decorrelation timescales. A large portion of the intraseasonal variance and covariance of AAM, τM, and τF is accounted for by the Markov model. Differences between the modeled and the observed covariances are maximized in the 10‐ to 90‐day band and account for 10–30% of the variance when using data not stratified by season. An especially prominent deviation from the Markov model is the oscillatory forcing of AAM by the frictional torque at 30‐ to 60‐day periods. Additionally, there is greater coherent variance between τF and τM across the entire 10‐ to 90‐day band, with the frictional torque leading the mountain torque. This “feedback” between the global torques results from physical processes not represented in the Markov model. The synoptic characteristics of the stochastic mountain and frictional torques and of the oscillatory Madden‐Julian Oscillation are described. Link to Paper: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2000JD900198
  22. The atmospheric angular momentum cycle during the tropical Madden-Julian Oscillation Authors: Weickmann, K. M., S.J.S. Khalsa and J. Eischeid First Published: 8th October, 1992 Abstract: The period 1 December 1984 to 3 February 1985 was associated with strong intraseasonal fluctuations in both the global atmospheric angular momentum (AAM) and tropical convection. Consistent changes were observed in the length of day. The AAM budget for the 65-day period is examined here using circulation data from the National Meteorological Center. Surprisingly well-balanced global and zonal budgets are obtained for the vertically integrated AAM. This enables a closer examination of regional changes, to assess how they might be responsible for the changes in the global AAM. Both friction and mountain torques are important in the global AAM budget. The increase of AAM is associated first with a positive friction torque, then with a positive mountain torque. The subsequent decrease of AAM results from a negative friction torque. The accompanying regional changes are mostly confined to the Northern Hemisphere, with high global AAM associated with a stronger and southward-displaced subtropical jet. In the zonal budget, meridional AAM fluxes by the zonally asymmetric eddies are important and appear to lead the torques by a few days. The increase of AAM begins with a shift of the tropical convection from the east Indian to the west Pacific Ocean. The consequent enhancement of the trades east of the Philippines gives a positive friction torque. The friction torque also has a contribution from enhanced trades over Central America and the tropical Atlantic Ocean, which appear to be linked to an equatorward propagating upper-tropospheric wave over the region. A persistent high pressure anomaly subsequently develops to the east of the Himalayas, giving a positive mountain torque. The global AAM rises in response to these torques, but as the circumpolar vortex expands the trades are weakened, causing a negative friction torque and the final reduction of the AAM. Interestingly, no coherent signals are seen in the weak zonal-mean convection anomalies accompanying these AAM changes. Rather, the AAM budget suggests that the tropical Madden–Julian oscillation and the global AAM are linked through the interaction of Rossby waves generated by the tropical heating with a zonally varying ambient flow and with mountains. The surface stresses have both a local component related to the convection and a remote component induced by upper-tropospheric AAM fluxes. Link to Paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469(1994)051<3194%3ATAAMCA>2.0.CO%3B2
  23. The tropical Madden-Julian oscillation and the global wind oscillation Authors: Klaus Weickmann and Edward Berry First Published: June 12th , 2008 Abstract: The global wind oscillation (GWO) is a subseasonal phenomenon encompassing the Madden-Julian Oscillation (MJO) and mid-latitude processes like meridional momentum transports and mountain torques. A phase space is defined for the GWO following the approach of Wheeler and Hendon (2004) for the MJO. In contrast to the oscillatory behavior of the MJO, two red noise processes define the GWO. The red noise spectra have variance at periods that bracket the 30-60 day band generally used to define the MJO. The MJO and GWO correlation accounts for 25% of their variance and crossspectra show well-defined phase relations. However, considerable independent variance still exists in the GWO. During MJO and GWO episodes, key events in the circulation and tropical convection derived from composites can be used for monitoring and for evaluating prediction model forecasts, especially for weeks 1-3. A case study during April-May 2007 focuses on the GWO and two ~30 day duration orbits with extreme anomalies in GWO phase space. The MJO phase space projections for the same time were partially driven by mountain torques and meridional transports. The case reveals the tropical-extratropical character of subseasonal events and its role in creating slowly evolving planetary-scale circulation and tropical convection anomalies Link to Paper: https://www.esrl.noaa.gov/psd/map/clim/wb08_final.pdf
  24. 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: https://cims.nyu.edu/~obuhler/Oliver_Buhler/Publications_files/EPFluxThyEJFM14.pdf Direct Link Across to the Original "EP" Paper - "On the Transfer of Energy in Stationary Mountain Waves": Eliassen & Palm 1960 Paper
  25. Relation between variations in the intensity of the zonal circulation of the atmosphere and the displacements of the semi-permanent centers of action Authors: Carl G. Rossby and Collaborators (Journal of Marine Research) Published: 1939 Abstract (not produced - snipped copy of introductory paragraph shown): Link to full paper: https://pdfs.semanticscholar.org/7e2e/e1793eb9f4cd42d6fd78afd535eb071ed019.pdf
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