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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 From @Bring Back 1962-63: Since the service provided through WDT was withdrawn there was a gap in this vitally important data.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. Review of Tropical‐Extratropical Teleconnections on Intraseasonal Time Scales Authors: Cristiana Stan, David M. Straus, Jorgen S. Frederiksen, Hai Lin, Eric D. Maloney and Courtney Schumacher First Published: 12th September, 2017 Abstract: The interactions and teleconnections between the tropical and midlatitude regions on intraseasonal time scales are an important modulator of tropical and extratropical circulation anomalies and their associated weather patterns. These interactions arise due to the impact of the tropics on the extratropics, the impact of the midlatitudes on the tropics, and two‐way interactions between the regions. Observational evidence, as well as theoretical studies with models of complexity ranging from the linear barotropic framework to intricate Earth system models, suggest the involvement of a myriad of processes and mechanisms in generating and maintaining these interconnections. At this stage, our understanding of these teleconnections is primarily a collection of concepts; a comprehensive theoretical framework has yet to be established. These intraseasonal teleconnections are increasingly recognized as an untapped source of potential subseasonal predictability. However, the complexity and diversity of mechanisms associated with these teleconnections, along with the lack of a conceptual framework to relate them, prevent this potential predictability from being translated into realized forecast skill. This review synthesizes our progress in understanding the observed characteristics of intraseasonal tropical‐extratropical interactions and their associated mechanisms, identifies the significant gaps in this understanding, and recommends new research endeavors to address the remaining challenges. Link to full paper:
  3. Relationship between Tropical Pacific SST and global atmospheric angular momentum in coupled models Authors: Huei−Ping Huang, Matthew Newman, Richard Seager, Yochanan Kushnir and Participating CMIP2+ Modeling Groups First Published: January 2004 Abstract: The sensitivity parameter S1 = ∆AAM/∆SST, where ∆AAM and ∆SST represent the anomalies of global atmospheric angular momentum (AAM) and tropical Pacific sea surface temperature (SST) in the NINO3.4 region, is compared for the CMIP2+ coupled models. The parameter quantifies the strength of atmospheric zonal mean zonal wind response to SST anomaly in the equatorial Pacific, an important process for the climate system. Although the simulated ∆AAM and ∆SST are found to exhibit great disparity, their ratios agree better among the coupled models (and with observation) with no significant outliers. This indicates that the processes that connect the AAM anomaly to tropical SST anomaly are not sensitive to the base SST and the detail of convective heating and are relatively easy to reproduce by the coupled models. Through this robust ∆SST−∆AAM relationship, the model bias in tropical Pacific SST manifests itself in the bias in atmospheric angular momentum. The value of S1 for an atmospheric model forced by observed SST is close to that for a coupled model with a similar atmospheric component, suggesting that the ∆SST− ∆AAM relationship is dominated by a one−way influence of the former forcing the latter. The physical basis for the ∆SST−∆AAM relationship is explored using a statistical equilibrium argument that links ∆SST to the anomaly of tropical tropospheric temperature. The resulting meridional gradient of tropospheric temperature is then linked to the change in zonal wind in the subtropical jets, the main contributor to ∆AAM, by thermal wind balance. Link to Paper: Credit goes to Tom @Isotherm for finding this paper - thank you.
  4. Seasonal and Regional Variations of Long-Term Changes in Upper-Tropospheric Jets from Reanalyses Authors: Gloria L. Manney and Michaela I. Hegglin First Published: September 15th, 2017 Published on line: December 19th, 2017 Abstract: Long-term changes in upper-tropospheric jet latitude, altitude, and strength are assessed for 1980–2014 using five modern reanalyses: MERRA, MERRA-2, ERA-Interim, JRA-55, and NCEP CFSR. Changes are computed from jet locations evaluated daily at each longitude to analyze regional and seasonal variations. The changes in subtropical and polar (eddy driven) jets are evaluated separately. Good agreement among the reanalyses in many regions and seasons provides confidence in the robustness of the diagnosed trends. Jet shifts show strong regional and seasonal variations, resulting in changes that are not robust in zonal or annual means. Robust changes in the subtropical jet indicate tropical widening over Africa except during Northern Hemisphere (NH) spring, and tropical narrowing over the eastern Pacific in NH winter. The Southern Hemisphere (SH) polar jet shows a robust poleward shift, while the NH polar jet shifts equatorward in most regions/seasons. Both subtropical and polar jet altitudes typically increase; these changes are more robust in the NH than in the SH. Subtropical jet wind speeds have generally increased in winter and decreased in summer, whereas polar jet wind speeds have weakened (strengthened) over Africa and eastern Asia (elsewhere) during winter in both hemispheres. The Asian monsoon has increased in area and appears to have shifted slightly westward toward Africa. The results herein highlight the importance of understanding regional and seasonal variations when quantifying long-term changes in jet locations, the mechanisms for those changes, and their potential human impacts. Comparison of multiple reanalyses is a valuable tool for assessing the robustness of jet changes. Link to Paper:
  5. The key role of background sea surface temperature over the cold tongue in asymmetric responses of the Arctic stratosphere to El Niño–Southern Oscillation Authors: Fei Xie, Xin Zhou, Jianping Li, Cheng Sun, Juan Feng and Xuan Ma Published: Nov 2018 Abstract: The response of the Arctic stratosphere to El Niño activity is strong but the response to La Niña activity is relatively weak. The asymmetric responses of Arctic stratosphere to El Niño and La Niña events are thought to be caused by asymmetric El Niño–Southern Oscillation (ENSO) teleconnections. Here, we suggest that the background sea surface temperature (SST) over cold tongue of tropical eastern Pacific may be an important contributor to the asymmetric ENSO teleconnections. The atmosphere is very sensitive to tropical SST variations in the range of 26 °C–30 °C. During El Niño events, the background SST over cold tongue plus El Niño SST anomalies typically falls into the range. Under these conditions, the atmospheric response to El Niño SST anomalies is strong. During La Niña events, the background SST plus La Niña SST anomalies is typically below the range, which leads to a weak response of the atmosphere to SST anomalies. The proposed mechanism is well supported by simulations. Link to full paper:
  6. Global Warming and ENSO – A “Helter-Skelter” Atmosphere Authors: Daphne Thompson (for WDT) Published: 11th December, 2017 Abstract: (None but I extracted this from the text): The purpose of this article is to make an effort to illustrate circulation impacts due to climate change, and give some high-level observational evidence of a La Niña-like response due to global warming, including at present. From another perspective, the ongoing El Niño could be contributing to La Niña aspects of the current atmospheric circulation given that it may be amplifying the present increase in the global mean temperature. Emphasis is then placed on the relevancy of climate change to subseasonal forecasting, including the present 16-30 day outlooks issued by WDT. The “global warming-La Niña” connection has been gaining some recognition in a few very recent publications in the refereed literature, as well as in on-line blogs written by well-respected scientists. The reader is encouraged to do a search. No attempt will be made to get into the complicated issues of the causes of climate change (including global warming) other than described above Link to full paper: Credit goes to Tams @Tamara for finding this paper - thank you.
  7. Studies of atmospheric angular momentum Authors: NOAA, Climate Diagnostics Center, Science Review Published: 25th/26th July, 2001 Chapter 4: Empirical and Process Studies Introduction to chapter 4, part 3: Atmospheric angular momentum (AAM) provides a convenient framework to study the role of mountains, surface wind stresses and various transport mechanisms in variability ranging from intraseasonal to interdecadal and beyond. Quantitative studies are feasible with current global assimilated datasets which show a good budget balance for global integrals, intraseasonal variations and during northern winter/spring. The budgets get much worse when gravity wave drag is included, if zonal integrals are considered or during summer/fall seasons. AAM is useful as an index of the large scale zonal flow since it is highly correlated with independent length-of-day measurements and with phenomena such as the QBO, ENSO, the MJO and possibly global warming. CDC scientists have examined several aspects of AAM variability, including: the link to MJO tropical convection, a linear model of global AAM and its torques, the global AAM budget imbalances due to gravity wave drag, the forcing for the semiannual seasonal component of AAM and the AAM response to global warming in an ensemble of coupled ocean-atmosphere model runs. CDC also monitors in real time the complete vertically integrated budget as part of its web-based maproom activities and distributes AAM and torque data to other researchers. Link to full paper: Link to Introduction to Chapter 4: Link to full Science Review:
  8. 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:
  9. Observed Changes in the Lifetime and Amplitude of the MJO Associated with Interannual ENSO Sea Surface Temperature Anomalies Authors: Benjamin Pohl and Adrian J. Matthews Published: 1st June, 2007 Abstract: The Madden–Julian oscillation (MJO) is analyzed using the reanalysis zonal wind– and satellite outgoing longwave radiation–based indices of Wheeler and Hendon for the 1974–2005 period. The average lifetime of the MJO events varies with season (36 days for events whose central date occurs in December, and 48 days for events in September). The lifetime of the MJO in the equinoctial seasons (March–May and October–December) is also dependent on the state of El Niño–Southern Oscillation (ENSO). During October–December it is only 32 days under El Niño conditions, increasing to 48 days under La Niña conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Niño, consistent with theoretical arguments concerning equatorial wave speeds. The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is found in 1975, which is at the same time as the well-known rupture in the ENSO time series that has been associated with the Pacific decadal oscillation. The mean amplitude of the MJO is 16% larger in the postrupture (1976–2005) compared to the prerupture (1950–75) period. Before the 1975 rupture, the amplitude of the MJO is maximum (minimum) under El Niño (La Niña) conditions during northern winter, and minimum (maximum) under El Niño (La Niña) conditions during northern summer. After the rupture, this relationship disappears. When the MJO–ENSO relationship is analyzed using all-year-round data, or a shorter dataset (as in some previous studies), no relationship is found. Link to full paper: Credit goes to Eric @Webberweather for finding this paper - thank you.
  10. Impact of interactive westerly wind bursts on CCSM3 Authors: Hosmay Lopez, Ben P. Kirtmana, Eli Tzipermanb and Geoffrey Gebbie Publication: "Dynamics of Atmospheres and Oceans" no. 59 (2013) pages 24–51 (SciVerse ScienceDirect) Journal Home Page: Published: 7th December, 2012 Abstract: Westerly wind bursts or events (WWBs or WWEs) are commonly viewed as stochastic processes, independent of any oceanic forcing. Some recent work and observations have suggested that these events can be viewed as state-dependent noise in that they are modulated by the SST variability. This potentially affects the predictability of the El Niño Southern Oscillation (ENSO). In this study, we examine the impact of parameterized WWBs on ENSO variability in the Community Climate System Model version 3.0 and 4.0 (CCSM3 and CCSM4). The WWBs parameterization is derived based on 50 years of atmospheric reanalysis data and observed estimates of tropical Pacific SST. To study the impact of WWBs three experiments are performed. In the first experiment, the model is integrated for several hundred years with no prescribed WWBs events (i.e. the control). In the second case, state-independent WWBs events are introduced. In other words, the occurrence, location, duration, and scale of the WWBs are determined (within bounds) randomly. These wind events are always positive (eastward) without a westward counterpart and are totally independent of the anomalies in the state variables, and can be thought of as additive noise. For the third case, the WWBs are introduced but as multiplicative noise or state-dependent forcing, modulated by SST anomalies. The statistical moments for the Niño 3.4 index shows that the state-dependent case produced larger El Niño Southern Oscillation (ENSO) events and the bias toward stronger cold events is reduced as compared to the control and the state-independent runs. There is very little difference between the control and the state-independent WWB simulations suggesting that the deterministic component of the burst is responsible for reshaping the ENSO events. Lag-lead correlation of ocean variables with Niño 3.4 index suggests larger temporal coherence of the ENSO events. This, along with SSTA composites, also suggest a shift toward a more self sustained mechanism as the experiments progress from the control to the state dependent WWBs. Overall, the parameterized WWBs have the capability to modify the ENSO regime in the CGCM, demonstrating the importance of sub-seasonal variability on interannual time scales. The fast varying (stochastic) component of WWB is of little importance, whereas the slow (SST dependent) component has a significant impact overall. The results are consistent between CCSM3 and CCSM4. Highlights: State dependent and state independent WWB parameterizations are incorporated into coupled general circulation models, CCSM3 and CCSM4. ► CCSM4 is more sensitive to state independent forcing than CCSM3. ► State dependent forcing produces more ENSO events in both extremes, and the model cold phase bias is reduced. ► Lag coherence is degraded (enhanced) in the state-independent (state-dependent) case. ► ENSO characteristics shifted from an event type to an oscillator type as the experiment progress from the state-independent to the state-dependent case. Link to full paper: The full journal is still behind a paywall but the authors have made their personal copy of this part available in pdf format:
  11. Modulation of equatorial Pacific westerly/easterly wind events by the MJO and convectively‑coupled Rossby waves Authors: Martin Puy, J. Vialard, M. Lengaigne and E. Guilyardi Published: 16th June, 2015 Abstract: Synoptic wind events in the equatorial Pacific strongly influence the El Niño/Southern Oscillation (ENSO) evolution. This paper characterizes the spatio-temporal distribution of Easterly (EWEs) and Westerly Wind Events (WWEs) and quantifies their relationship with intraseasonal and interannual large-scale climate variability. We unambiguously demonstrate that the Madden–Julian Oscillation (MJO) and Convectively-coupled Rossby Waves (CRW) modulate both WWEs and EWEs occurrence probability. 86 % of WWEs occur within convective MJO and/or CRW phases and 83 % of EWEs occur within the suppressed phase of MJO and/or CRW. 41 % of WWEs and 26 % of EWEs are in particular associated with the combined occurrence of a CRW/MJO, far more than what would be expected from a random distribution (3 %). Wind events embedded within MJO phases also have a stronger impact on the ocean, due to a tendency to have a larger amplitude, zonal extent and longer duration. These findings are robust irrespective of the wind events and MJO/CRW detection methods. While WWEs and EWEs behave rather symmetrically with respect to MJO/CRW activity, the impact of ENSO on wind events is asymmetrical. The WWEs occurrence probability indeed increases when the warm pool is displaced eastward during El Niño events, an increase that can partly be related to interannual modulation of the MJO/CRW activity in the western Pacific. On the other hand, the EWEs modulation by ENSO is less robust, and strongly depends on the wind event detection method. The consequences of these results for ENSO predictability are discussed. Link to full guide:
  12. Predictability of SST-Modulated Westerly Wind Bursts Authors: Geoffrey Gebbie and Eli Tziperman Published: 15th July, 2009 Abstract: Westerly wind bursts (WWBs), a significant player in ENSO dynamics, are modeled using an observationally motivated statistical approach that relates the characteristics of WWBs to the large-scale sea surface temperature. Although the WWB wind stress at a given location may be a nonlinear function of SST, the characteristics of WWBs are well described as a linear function of SST. Over 50% of the interannual variance in the WWB likelihood, zonal location, duration, and fetch is explained by changes in SST. The model captures what is seen in a 17-yr record of satellite-derived winds: the eastward migration and increased occurrence of wind bursts as the western Pacific warm pool extends. The WWB model shows significant skill in predicting the interannual variability of the characteristics of WWBs, while the prediction skill of the WWB seasonal cycle is limited by the record length of available data. The novel formulation of the WWB model can be implemented in a stochastic or deterministic mode, where the deterministic mode predicts the ensemble-mean WWB characteristics. Therefore, the WWB model is especially appropriate for ensemble prediction experiments with existing ENSO models that are not capable of simulating realistic WWBs on their own. Should only the slowly varying component of WWBs be important for ENSO prediction, this WWB model allows a shortcut to directly compute the slowly varying ensemble-mean wind field without performing many realizations. Link to full paper:
  13. Quantifying the Dependence of Westerly Wind Bursts on the Large-Scale Tropical Pacific SST Authors: Eli Tziperman and Lisan Yu Published: 15th June, 2007 Abstract: The correlation between parameters characterizing observed westerly wind bursts (WWBs) in the equatorial Pacific and the large-scale SST is analyzed using singular value decomposition. The WWB parameters include the amplitude, location, scale, and probability of occurrence for a given SST distribution rather than the wind stress itself. This approach therefore allows for a nonlinear relationship between the SST and the wind signal of the WWBs. It is found that about half of the variance of the WWB parameters is explained by only two large-scale SST modes. The first mode represents a developed El Niño event, while the second mode represents the seasonal cycle. More specifically, the central longitude of WWBs, their longitudinal extent, and their probability seem to be determined to a significant degree by the ENSO-driven signal. The amplitude of the WWBs is found to be strongly influenced by the phase of the seasonal cycle. It is concluded that the WWBs, while partially stochastic, seem an inherent part of the large-scale deterministic ENSO dynamics. Implications for ENSO predictability and prediction are discussed. Link to full paper:
  14. The South Pacific Meridional Mode as a Thermally Driven Source of ENSO Amplitude Modulation and Uncertainty Authors: Sarah M. Larson, Kathy V. Pegion and Ben P. Kirtman Published: 5th June, 2018 Abstract: This study seeks to identify thermally driven sources of ENSO amplitude and uncertainty, as they are relatively unexplored compared to wind-driven sources. Pacific meridional modes are argued to be wind triggers for ENSO events. This study offers an alternative role for the South Pacific meridional mode (SPMM) in ENSO dynamics, not as an ENSO trigger, but as a coincident source of latent heat flux (LHF) forcing of ENSO SSTA that, if correctly (incorrectly) predicted, could reduce (increase) ENSO prediction errors. We utilize a coupled model simulation in which ENSO variability is perfectly periodic and each El Niño experiences identical wind stress forcing. Differences in El Niño amplitude are primarily thermally driven via the SPMM. When El Niño occurs coincidentally with positive phase SPMM, the positive SPMM LHF anomaly counteracts a fraction of the LHF damping of El Niño, allowing for a more intense El Niño. If the SPMM phase is instead negative, the SPMM LHF amplifies the LHF damping of El Niño, reducing the event’s amplitude. Therefore, SPMM LHF anomalies may either constructively or destructively interfere with coincident ENSO events, thus modulating the amplitude of ENSO. The ocean also plays a role, as the thermally forced SSTA is then advected westward by the mean zonal velocity, generating a warming or cooling in the ENSO SSTA tendency in addition to the wind-forced component. Results suggest that in addition to wind-driven sources, there exists a thermally driven piece to ENSO amplitude and uncertainty that is generally overlooked. Links between the SPMM and Pacific decadal variability are discussed. Link to full paper: This recent paper is still behind a paywall on the AMS website but I found the authors' pre-submission pdf version on the Researchgate site:
  15. Westerly Wind Events in the Tropical Pacific and their Influence on the Coupled Ocean‐Atmosphere System: A Review Authors: Matthieu Lengaigne, Jean‐Philippe Boulanger, Christophe Menkes, Pascale Delecluse, Julia Slingo, C. Wang, S.P. Xie and J.A. Carton Published: 19th March, 2013 Abstract: Observational and modeling aspects about Westerly Wind Events (WWEs) and their influence on the tropical Pacific Ocean-atmosphere system are reviewed. WWEs are a large part of the intraseasonal zonal wind activity over the warm pool. They have typical amplitudes of 7 m s-1, zonal width of 20° longitude and duration of about 8 days. Their root causes are often a combination of various factors including the Madden-Julian Oscillation, cold surges from mid-latitudes, tropical cyclones and other mesoscale phenomena. The relationship between WWEs and the ENSO cycle is complex, involving among others the equatorial characteristics of the WWEs, the oceanic background state and the internal atmospheric variability. Both observational and modeling studies demonstrate that WWEs tend to cool the far western Pacific, shift the warm pool eastward and warm the central-eastern Pacific through the generation of Kelvin waves. They are therefore important processes for the central and eastern Pacific warming during the onset and development phase of El Niño. The strong atmospheric feedbacks that are likely to be generated by the ocean response to WWEs even suggest that a single WWE is capable of establishing the conditions under which El Niño can occur. The important role played by WWEs in the evolution and amplitude of recent El Niño events may therefore strongly limit the predictability of El Niño. Link to full paper: t
  16. A Review of ENSO Theories Authors: Chunzai Wang Published: 10th October, 2018 Abstract: The ENSO occurrence can be usually explained by two views of (1) a self-sustained oscillatory mode and (2) a stable mode interacting with high-frequency forcing such as westerly wind bursts and Madden-Julian Oscillation events. The positive ocean-atmosphere feedback in the tropical Pacific hypothesized by Bjerknes leads ENSO event to a mature phase. After ENSO event matures, negative feedbacks are needed to cease ENSO anomaly growth. Four negative feedbacks have been proposed: (1) reflected Kelvin waves at the ocean western boundary, (2) a discharge process due to Sverdrup transport, (3) western Pacific wind-forced Kelvin waves, and (4) anomalous zonal advections and wave reflection at the ocean eastern boundary. These four ENSO mechanisms are respectively called as the delayed oscillator, the recharge-discharge oscillator, the western Pacific oscillator and the advective-reflective oscillator. The unified oscillator is developed by including all ENSO mechanisms, i.e., all of four ENSO oscillators are special cases of the unified oscillator. The tropical Pacific Ocean and atmosphere interaction can also induce coupled slow westward and eastward propagating modes. An advantage of the coupled slow modes is that they can be used to explain the propagating property of interannual anomalies, whereas the oscillatory modes produce a standing oscillation. The research community has recently paid attention to different types of ENSO events by focusing on the central Pacific El Niño. All of the ENSO mechanisms may work for the central Pacific El Niño events, with an addition that the central Pacific El Niño may be related to forcing or processes in the extratropical Pacific. Link to full paper: This very recent paper is behind a paywall but I found this link to the pre-submission version: Once in the Researchgate site, you'll find a personally downloadable to your own browser pdf file.
  17. Atmospheric and surface variations during westerly wind bursts in the tropical western Pacific Authors: John Fasullo and Peter J. Webster Published: 6th April 1999 Abstract: An analysis is made of variations in both the surface energy balance and the regional atmospheric dynamic and thermal structure during 44 westerly wind bursts (WWBs) in the western equatorial Pacific Ocean from 1979 to 1995. The study assesses winds, convective available potential energy, cloud properties, precipitation, surface temperature, and surface heat flux while distinguishing between brief (5–25 day periodicity) and sustained (30–90 day) WWBs. Datasets used in the study include fields from the NCEP/NCAR and ECMWF re‐analyses, and satellite retrievals of clouds (ISCPP), precipitation (MSU), moisture (TOVS), and surface solar flux. Both brief and sustained WWBs, by definition, experience strong low‐level westerly winds that typically induce an increased surface latent‐heat flux of approximately 30 W m−2. Enhanced cloud thickness, precipitation, and upper tropospheric easterly wind anomalies accompany surface westerly winds, though maxima in winds lag those in clouds and precipitation by about one day for brief WWBs and four days for sustained events. WWBs of both types experience strong seasonality, occurring frequently in all seasons except boreal summer. Important distinctions between brief and sustained WWBs can also be made. Westerly anomalies typically extend above 200 hPa during brief WWBs but are generally confined to the lower troposphere (below 400 hPa) for sustained events. Sustained WWBs are also preceded by a quiescent period of reduced cloud thickness and surface winds that is accompanied by strong incident solar flux. Convective instability, as judged by a variety of techniques, increases by approximately 30% during this quiescent period. Brief WWBs do not include the precursory surface warming or convective destabilization of sustained WWBs. Notwithstanding the warming episodes before the events, sustained WWBs are associated with a net surface cooling approximately 40% larger than brief WWBs. The relationship between brief and sustained WWBs and the phase of the Madden‐Julian Oscillation (MJO) (as judged from outgoing long‐wave radiation) is also examined. Results support the classification of events as ‘brief’ and ‘sustained’ as used in this study, with brief WWBs occurring frequently during both wet and dry phases of the MJO, while sustained WWBs occur uniquely during the MJO wet phase. The association of brief and sustained WWBs with the MJO is shown to be independent of the El Niiio Southern Oscillation phase. It is therefore proposed that some, but not all, WWBs may be viewed as the surface signature of the MJO and that the mechanisms responsible for the MJO play an integral role in the formation and sustenance of sustained WWBs. Link to full paper:
  18. Assessing the Relationship between MJO and Equatorial Pacific WWBs in Observations and CMIP5 Models Authors: Jie Feng and Tao Lian Published: 13th July, 2018 Abstract: This study evaluates the relationship between the Madden–Julian Oscillation (MJO) and the occurrence of equatorial Pacific Westerly Wind Bursts (WWBs). During the convective MJO phase, anomalous surface westerlies prevail in and west of the convective MJO center, providing favorable conditions for WWBs. Compared with the probability of WWBs expected under a null hypothesis that WWBs occur randomly, the convective MJO phase almost doubles the probability of a WWB occurring. Nevertheless, only 34.46% of WWBs co-occur with the convective MJO, which is much less than that reported in previous studies. We show that when the MJO and WWBs are defined using the same field with overlapping frequencies, the percentage of WWBs co-occurred with the convective MJO shows a significant increase. However, the higher percentage is simply caused by the fact that the strong WWBs during a convective MJO are more likely to be identified than those during the suppressed and neutral MJO phases. 45.80% of WWBs are found occurred in the full MJO phase (both the convective and suppressed MJO phases), which is slightly higher than that expected based on randomness. Although the full MJO has statistically significant impact on WWBs likelihood, the influence from the full MJO on the tropical Pacific sea surface temperature anomaly is much weaker as compared to that from the WWBs. The relationships between the MJO and WWBs simulated in CMIP5 models are also assessed, and the percentage of WWBs co-occurred with MJO simulated in models is in general less than that in observations. Link to full paper: This recent paper is still behind a paywall on the AMS website but I found the Preliminary Accept pdf version on the Researchgate site (you will need to to click on the download option at the top left of the page and it will appear in your browser or on your device):
  19. Westerly Wind Bursts and Their Relationship with Intraseasonal Variations and ENSO. Part I: Statistics Authors: Ayako Seiki, Yukari N. Takayabu Published: Oct 2007 Abstract: Statistical features of the relationship among westerly wind bursts (WWBs), the El Niño–Southern Oscillation (ENSO), and intraseasonal variations (ISVs) were examined using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis data (ERA-40) for the period of January 1979–August 2002. WWBs were detected over the Indian Ocean and the Pacific Ocean, but not over the Atlantic Ocean. WWB frequencies for each region were lag correlated with a sea surface temperature anomaly over the Niño-3 region. WWBs tended to occur in sequence, from the western to eastern Pacific, leading the El Niño peak by 9 months to 1 month, respectively, and after around 11 months, over the Indian Ocean. These results suggest that WWB occurrences are not random, but interactive with ENSO. Composite analysis revealed that most WWBs were associated with slowdowns of eastward-propagating convective regions like the Madden–Julian oscillation (MJO), with the intensified Rossby wave response. However, seasonal and interannual variations in MJO amplitude were not correlated with WWB frequency, while a strong MJO event tended to bear WWBs. It is suggested that the strong MJO amplitude promotes favorable conditions, but it is not the only factor influencing WWB frequency. An environment common to WWB generation in all regions was the existence of background westerlies around the WWB center near the equator. It is inferred that ENSO prepares a favorable environment for the structural transformation of an MJO, that is, the intensified Rossby wave response, that results in WWB generations. The role of the background wind fields on WWB generations will be discussed in a companion paper from the perspective of energetics. Link to full paper:
  20. Westerly Wind Bursts: ENSO’s Tail Rather than the Dog? Authors: EISENMAN, YU, TZIPERMAN Published: Jan 2005 Abstract: Westerly wind bursts (WWBs) in the equatorial Pacific occur during the development of most El Niño events and are believed to be a major factor in ENSO’s dynamics. Because of their short time scale, WWBs are normally considered part of a stochastic forcing of ENSO, completely external to the interannual ENSO variability. Recent observational studies, however, suggest that the occurrence and characteristics of WWBs may depend to some extent on the state of ENSO components, implying that WWBs, which force ENSO, are modulated by ENSO itself. Satellite and in situ observations are used here to show that WWBs are significantly more likely to occur when the warm pool is extended eastward. Based on these observations, WWBs are added to an intermediate complexity coupled ocean–atmosphere ENSO model. The representation of WWBs is idealized such that their occurrence is modulated by the warm pool extent. The resulting model run is compared with a run in which the WWBs are stochastically applied. The modulation of WWBs by ENSO results in an enhancement of the slow frequency component of the WWBs. This causes the amplitude of ENSO events forced by modulated WWBs to be twice as large as the amplitude of ENSO events forced by stochastic WWBs with the same amplitude and average frequency. Based on this result, it is suggested that the modulation of WWBs by the equatorial Pacific SST is a critical element of ENSO’s dynamics, and that WWBs should not be regarded as purely stochastic forcing. In the paradigm proposed here, WWBs are still an important aspect of ENSO’s dynamics, but they are treated as being partially stochastic and partially affected by the large-scale ENSO dynamics, rather than being completely external to ENSO. It is further shown that WWB modulation by the large-scale equatorial SST field is roughly equivalent to an increase in the ocean–atmosphere coupling strength, making the coupled equatorial Pacific effectively self-sustained. Link to full paper:
  21. Stratospheric Control of the Madden–Julian Oscillation Author: Lesley J. Gray Published: Nov 2016 Abstract: Interannual variation of seasonal-mean tropical convection over the Indo-Pacific region is primarily controlled by El Niño–Southern Oscillation (ENSO). For example, during El Niño winters, seasonal-mean convection around the Maritime Continent becomes weaker than normal, while that over the central to eastern Pacific is strengthened. Similarly, subseasonal convective activity, which is associated with the Madden–Julian oscillation (MJO), is influenced by ENSO. The MJO activity tends to extend farther eastward to the date line during El Niño winters and contract toward the western Pacific during La Niña winters. However, the overall level of MJO activity across the Maritime Continent does not change much in response to the ENSO. It is shown that the boreal winter MJO amplitude is closely linked with the stratospheric quasi-biennial oscillation (QBO) rather than with ENSO. The MJO activity around the Maritime Continent becomes stronger and more organized during the easterly QBO winters. The QBO-related MJO change explains up to 40% of interannual variation of the boreal winter MJO amplitude. This result suggests that variability of the MJO and the related tropical–extratropical teleconnections can be better understood and predicted by taking not only the tropospheric circulation but also the stratospheric mean state into account. The seasonality of the QBO–MJO link and the possible mechanism are also discussed. Link to full paper:
  22. Tropospheric QBO–ENSO Interactions and Differences between the Atlantic and Pacific Authors: Felicitas Hansen GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany Katja Matthes GEOMAR Helmholtz Centre for Ocean Research Kiel, and Christian-Albrechts-Universität zu Kiel, Kiel, Germany Sebastian Wahl GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany Date: 20th November 2015 Abstract: This study investigates the interaction of the quasi-biennial oscillation (QBO) and the El Niño–Southern Oscillation (ENSO) in the troposphere separately for the North Pacific and North Atlantic region. Three 145-yr model simulations with NCAR’s Community Earth System Model Whole Atmosphere Community Climate Model (CESM-WACCM) are analyzed where only natural (no anthropogenic) forcings are considered. These long simulations allow the authors to obtain statistically reliable results from an exceptional large number of cases for each combination of the QBO (westerly and easterly) and ENSO phases (El Niño and La Niña). Two different analysis methods were applied to investigate where nonlinearity might play a role in QBO–ENSO interactions. The analyses reveal that the stratospheric equatorial QBO anomalies extend down to the troposphere over the North Pacific during Northern Hemisphere winter only during La Niña and not during El Niño events. The Aleutian low is deepened during QBO westerly (QBOW) as compared to QBO easterly (QBOE) conditions, and the North Pacific subtropical jet is shifted northward during La Niña. In the North Atlantic, the interaction of QBOW with La Niña conditions (QBOE with El Niño) results in a positive (negative) North Atlantic Oscillation (NAO) pattern. For both regions, nonlinear interactions between the QBO and ENSO might play a role. The results provide the potential to enhance the skill of tropospheric seasonal predictions in the North Atlantic and North Pacific region. Link to original text:
  23. Multivariate ENSO Index - NOAA homepage Author/Contact: NOAA/ESRL Contents: El Niño/Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause global climate variability on interannual time scales. Here we attempt to monitor ENSO by basing the Multivariate ENSO Index (MEI) on the six main observed variables over the tropical Pacific. These six variables are: sea-level pressure (P), zonal (U) and meridional (V) components of the surface wind, sea surface temperature (S), surface air temperature (A), and total cloudiness fraction of the sky (C). These observations have been collected and published in ICOADS for many years. The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb,..., Nov/Dec). After spatially filtering the individual fields into clusters (Wolter, 1987), the MEI is calculated as the first unrotated Principal Component (PC) of all six observed fields combined. This is accomplished by normalizing the total variance of each field first, and then performing the extraction of the first PC on the co-variance matrix of the combined fields (Wolter and Timlin, 1993). In order to keep the MEI comparable, all seasonal values are standardized with respect to each season and to the 1950-93 reference period. This webpage consists of seven main parts, three of which are updated every month: 1. A short description of the Multivariate ENSO Index (MEI); 2. Historic La Niña events since 1950; 3. Historic El Niño events since 1950; 4. MONTHLY UPDATED MEI loading maps for the latest season; 5. MONTHLY UPDATED MEI anomaly maps for the latest season; 6. MONTHLY UPDATED Discussion of recent conditions; 7. Publications and MEI data access. Link to NOAA MEI webpage:
  24. El Niño/Southern Oscillation behaviour since 1871 as diagnosed in an extended multivariate ENSO index (MEI.ext) Authors: Klaus Wolter, Michael S. Timlin Published: April 2011 Abstract: El Niño/Southern Oscillation (ENSO) remains the most important coupled ocean–atmosphere phenomenon to cause global climate variability on seasonal to interannual time scales. This paper addresses the need for a reliable ENSO index that allows for the historical definition of ENSO events in the instrumental record back to 1871. The Multivariate ENSO Index (MEI) was originally defined as the first seasonally varying principal component of six atmosphere–ocean (COADS) variable fields in the tropical Pacific basin. It provides for a more complete and flexible description of the ENSO phenomenon than single variable ENSO indices such as the SOI or Niño 3.4 SST. Here we describe our effort to boil the MEI concept down to its most essential components (based on SLP, SST) to enable historical analyses that more than double its period of record to 1871–2005. The new MEI.ext confirms that ENSO activity went through a lull in the early‐ to mid‐20th century, but was just about as prevalent one century ago as in recent decades. We diagnose strong relationships between peak amplitudes of ENSO events and their duration, as well as between their peak amplitudes and their spacing (periodicity). Our effort is designed to help with the assessment of ENSO conditions through as long a record as possible to be able to differentiate between ‘natural’ ENSO behaviour in all its rich facets, and the ‘Brave New World’ of this phenomenon under evolving GHG‐related climate conditions. So far, none of the behaviour of recent ENSO events appears unprecedented, including duration, onset timing, and spacing in the last few decades compared to a full century before then. Link to full paper: Link to NOAA MEI Analysis and Charts pages:
  25. On the 60-month cycle of multivariate ENSO index Authors: Adriano Mazzarella, Andrea Giuliacci, Ioannis Liritzis Published: March 2010 Abstract: Many point indices have been developed to describe El Niño/Southern Oscillation, but the multivariate El Niño Southern Oscillation (ENSO) index (MEI) is considered the most representative since it links six different meteorological parameters measured over the tropical Pacific. Spectral analysis with appropriate data reduction techniques of monthly values of MEI (1950– 2008) has allowed the identification of a large 60-month cycle, statistically confident at a level larger than 99%. The highest values of MEI (typical of El Niño events) and the lowest values of MEI (typical of La Niña events) are concordant with respective maxima and minima values of the identified 60-month cycle. Link to full paper: Link to NOAA MEI Analysis and Charts pages:
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