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  1. 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.
  2. Blessed Weather

    Are there two types of La Nina?

    Are there two types of La Nina? Authors: Jong‐Seong Kug and Yoo‐Geun Ham Published: August 2011 Abstract: In this study, the existence of two types of La Nina events is examined using observations and model output. We find that cold events in the central and eastern Pacific SST, are highly correlated unlike the corresponding warm events. When two types of La Nina are defined based on the same criteria for the types of warm events, the SST and precipitation patterns between the two types of La Nina are much less distinctive or less independent. In other words, there is a strong asymmetric character between warm and cold events. This asymmetric character is also examined in 20 climate models that participate in the CMIP3. Most climate models have difficulty in simulating independently the two types of El Nino and La Nina events; however, they simulate the two types of El Nino more independently than they simulate the two types of La Nina, supporting our observational arguments to some degree. Link to full paper:
  3. 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:
  4. 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:
  5. 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:
  6. Causes and Predictability of the Negative Indian Ocean Dipole and Its Impact on La Niña During 2016 Authors: Eun-Pa Lim and Harry H. Hendon Published: 3rd October, 2017 Abstract: In the latter half of 2016 Indonesia and Australia experienced extreme wet conditions and East Africa suffered devastating drought, which have largely been attributed to the occurrence of strong negative Indian Ocean Dipole (IOD) and weak La Niña. Here we examine the causes and predictability of the strong negative IOD and its impact on the development of La Niña in 2016. Analysis on atmosphere and ocean reanalyses and forecast sensitivity experiments using the Australian Bureau of Meteorology’s dynamical seasonal forecast system reveals that this strong negative IOD, which peaked in July-September, developed primarily by the Indian Ocean surface and subsurface conditions. The long-term trend over the last 55 years in sea surface and subsurface temperatures, which is characterised by warming of the tropical Indian and western Pacific and cooling in the equatorial eastern Pacific, contributed positively to the extraordinary strength of this IOD. We further show that the strong negative IOD was a key promoter of the weak La Niña of 2016. Without the remote forcing from the IOD, this weak La Niña may have been substantially weaker because of the extraordinarily long-lasting warm surface condition over the dateline from the tail end of strong El Niño of 2015–16. Link to full paper:
  7. El Nino, El Nino Modoki, La Nina, Indian Ocean Dipole - YouTube Presentation Presentation Team from: PMF IAS Presentation Date: 29th October, 2015 Link to YouTube presentation (24 minutes): or click on the chart below: Link to Written Notes and Slides:
  8. Seasonal predictability of winter ENSO types in operational dynamical model predictions Authors: Hong-Li Ren, Adam A. Scaife, Nick Dunstone, Ben Tian, Ying Liu, Sarah Ineson, June-Yi Lee, Doug Smith, Changzheng Liu, Vikki Thompson, Michael Vellinga and Craig MacLachlan Published: 25th July, 2018 Abstract: The El Niño-Southern Oscillation (ENSO) events of recent decades have been divided into the two different types based on their spatial patterns, the Eastern Pacific (EP) type and Central Pacific (CP) type. Their most significant difference is the distinguished zonal center locations of sea surface temperature (SST) anomalies in the equatorial Pacific. In this study, based on six operational climate models, we evaluate predictability of the two types of ENSO events in winter to examine whether dynamical predictions can distinguish between the two spatial patterns at lead time of 1 month and tell us more than simply whether an event is on the way. We show that winter EP and CP El Niño and La Niña events can only be distinguished in a minority of these models at 1-month lead, and the EP type tends to has a more realistic zonal positions of SST pattern centers than the CP type. Compared to the SST patterns, the differences between the two types are less apparent in precipitation especially for the two La Niña types in the models. Examinations of the extratropical teleconnections to the two ENSO types show that some of the models can reproduce the differences between EP and CP teleconnections. Evaluations of model predictions show that the EP El Niño event has the same level hit rate with the CP El Niño and the CP La Niña event has much higher hit rate than the EP La Niña. While the multi-model ensemble increases Niño index prediction skill, it does not help to improve forecast skill of center longitude index of the SST patterns and distinguish the two types of ENSO events. Although ENSO skill is very high at this lead time, the rapid loss of the initialized information on the different ENSO types in most of the models severely limits the predictability of the two types of winter ENSO events and more research is needed to improve the performance of climate models in forecasting the two ENSO types. Link to full paper:
  9. The Distinct Contributions of the Seasonal Footprinting and Charged‐Discharged Mechanisms to ENSO Complexity Authors: Jin‐Yi Yu and Shih‐Wei Fang Published: 28th June, 2018 Abstract: This study finds the seasonal footprinting (SF) mechanism to be a key source of El Niño–Southern Oscillation (ENSO) complexity, whereas the charged‐discharged (CD) mechanism acts to reduce complexity. The CD mechanism forces El Niño and La Niña to follow each other, resulting in a more cyclic and less complex ENSO evolution, while the SF mechanism involves subtropical forcing and results in an ENSO evolution that is more episodic and irregular. The SF mechanism also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño‐La Niña asymmetries. The strength of CD mechanism has been steady, but SF mechanism has intensified during the past two decades, making ENSO more complicated. Most Climate Model Intercomparison Project version 5 models overestimate the strength of the CD mechanism but underestimate the strength of the SF mechanism, causing their simulated ENSOs to be too regular and symmetric. Plain Language Summary El Niño–Southern Oscillation (ENSO) is known to have profound climate impacts worldwide, but the causes of its complex behaviors are still not fully understood. In this study, we show that the subtropical atmosphere‐ocean coupled forcing is a key source of ENSO complexity, whereas the tropical ocean heat content variation acts to reduce ENSO complexity. The subtropical forcing also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño‐La Niña asymmetries. In contrast to the steady strength of the tropical variation throughout the past six decades, the strength of the subtropical forcing has increased since the early 1990s. This may have made ENSO more complex recently and, if this trend does not reverse, possibly into the coming decades. Contemporary climate models overestimate the strength of the tropical ocean heat content variation but underestimate the strength of the subtropical forcing, which may be a reason why contemporary models produce ENSO behavior that is too regular. Link to full paper: complexity.GRL-2018.pdf
  10. Timing of subsurface heat magnitude for the growth of El Niño events Authors: Joan Ballester, Desislava Petrova, Simona Bordoni, Ben Cash and Xavier Rodó Published: 3rd August, 2017 Abstract: The subsurface heat buildup in the western tropical Pacific and the recharge phase in equatorial heat content are intrinsic elements of El Niño–Southern Oscillation, leading to changes in zonal wind stress, sea surface temperature, and thermocline tilt that characterize the growing and mature phases of El Niño (EN) events. Here we use numerical simulations to study the impact on subsequent EN episodes of a sudden increase or decrease in ocean heat content during the recharge phase and compare results with previous studies in which this perturbation is prescribed earlier during the tilting mode. We found that while not substantially affected by the phase at which a sudden rise in heat content is prescribed, the timing and magnitude of the events are very sensitive to the phase at which a major decrease is imposed. The different response to the phase of increases and decreases substantiates the importance of nonlinear subsurface ocean dynamics to the onset and growth of EN episodes and provides insight into the irreversibility of the events at different stages of the oscillation. Link to full paper:
  11. Both air-sea components are crucial for El Niño forecast from boreal spring Authors: Xiang-Hui Fang and Mu Mu Published: 12th July, 2018 Abstract: The spring predictability barrier severely limits our ability to forecast the El Niño-Southern Oscillation (ENSO) from and across the boreal spring. Our observational analysis shows that the spring predictability barrier (SPB) can be largely reduced when information from both the ocean and atmosphere are effectively taken into account during the boreal spring. The correlation coefficient between the predicted and observed sea surface temperature anomalies over the equatorial central–eastern Pacific determined by a simple quaternary linear regression model is >0.81 for the period 1980–2016. The frame structure of the ENSO evolution is mostly controlled by variations in the oceanic heat content along the equatorial Pacific and the zonal wind stress over the tropical western Pacific during the boreal spring. These results indicate that to predict ENSO events with a long lead time, i.e., largely reducing the SPB, variations in both the ocean and atmosphere during the boreal spring should be well predicted first. While the oceanic information is mainly located in the equatorial Pacific and well characterized by the delayed oscillator and recharging oscillator models, variations in the atmosphere may contain information beyond this area and are more difficult to deal with. Link to full paper:
  12. The Northern Hemisphere Extratropical Atmospheric Circulation Response to ENSO: How Well Do We Know It and How Do We Evaluate Models Accordingly? Authors: Clara Deser, Isla R. Simpson, Karen A. McKinnon and Adam S. Phillips Published: 14th March, 2017 Abstract: Application of random sampling techniques to composite differences between 18 El Niño and 14 La Niña events observed since 1920 reveals considerable uncertainty in both the pattern and amplitude of the Northern Hemisphere extratropical winter sea level pressure (SLP) response to ENSO. While the SLP responses over the North Pacific and North America are robust to sampling variability, their magnitudes can vary by a factor of 2; other regions, such as the Arctic, North Atlantic, and Europe are less robust in their SLP patterns, amplitudes, and statistical significance. The uncertainties on the observed ENSO composite are shown to arise mainly from atmospheric internal variability as opposed to ENSO diversity. These observational findings pose considerable challenges for the evaluation of ENSO teleconnections in models. An approach is proposed that incorporates both pattern and amplitude uncertainty in the observational target, allowing for discrimination between true model biases in the forced ENSO response and apparent model biases that arise from limited sampling of non-ENSO-related internal variability. Large initial-condition coupled model ensembles with realistic tropical Pacific sea surface temperature anomaly evolution during 1920–2013 show similar levels of uncertainty in their ENSO teleconnections as found in observations. Because the set of ENSO events in each of the model composites is the same (and identical to that in observations), these uncertainties are entirely attributable to sampling fluctuations arising from internal variability, which is shown to originate from atmospheric processes. The initial-condition model ensembles thus inform the interpretation of the single observed ENSO composite and vice versa. Link to full paper:
  13. Is There Evidence of Changes in Tropical Atlantic Variability Modes under AMO Phases in the Observational Record? Authors: Marta Martín-Rey, Irene Polo, Belen Rodriguez-Fonseca, Teresa Losada and Alban Lazar Published: 9th June, 2017 Abstract: The Atlantic multidecadal oscillation (AMO) is the leading mode of Atlantic sea surface temperature (SST) variability at multidecadal time scales. Previous studies have shown that the AMO could modulate El Niño–Southern Oscillation (ENSO) variance. However, the role played by the AMO in the tropical Atlantic variability (TAV) is still uncertain. Here, it is demonstrated that during negative AMO phases, associated with a shallower thermocline, the eastern equatorial Atlantic SST variability is enhanced by more than 150% in boreal summer. Consequently, the interannual TAV modes are modified. During negative AMO, the Atlantic Niño displays larger amplitude and a westward extension and it is preceded by a simultaneous weakening of both subtropical highs in winter and spring. In contrast, a meridional seesaw SLP pattern evolving into a zonal gradient leads the Atlantic Niño during positive AMO. The north tropical Atlantic (NTA) mode is related to a Scandinavian blocking pattern during winter and spring in negative AMO, while under positive AMO it is part of the SST tripole associated with the North Atlantic Oscillation. Interestingly, the emergence of an overlooked variability mode, here called the horseshoe (HS) pattern on account of its shape, is favored during negative AMO. This anomalous warm (cool) HS surrounding an eastern equatorial cooling (warming) is remotely forced by an ENSO phenomenon. During negative AMO, the tropical–extratropical teleconnections are enhanced and the Walker circulation is altered. This, together with the increased equatorial SST variability, could promote the ENSO impacts on TAV. The results herein give a step forward in the better understanding of TAV, which is essential to improving its modeling, impacts, and predictability. Link to full paper:
  14. The Influence of El Niño–Southern Oscillation and the Atlantic Multidecadal Oscillation on Caribbean Tropical Cyclone Activity Authors: Philip J. Klotzbach Published: 7th June, 2010 Abstract: Caribbean basin tropical cyclone activity shows significant variability on interannual as well as multidecadal time scales. Comprehensive statistics for Caribbean hurricane activity are tabulated, and then large-scale climate features are examined for their impacts on this activity. The primary interannual driver of variability is found to be El Niño–Southern Oscillation, which alters levels of activity due to changes in levels of vertical wind shear as well as through column stability. Much more activity occurs in the Caribbean with La Niña conditions than with El Niño conditions. On the multidecadal time scale, the Atlantic multidecadal oscillation is shown to play a significant role in Caribbean hurricane activity, likely linked to its close relationship with multidecadal alterations in the size of the Atlantic warm pool and the phase of the Atlantic meridional mode. When El Niño–Southern Oscillation and the Atlantic multidecadal oscillation are examined in combination, even stronger relationships are found due to a combination of either favorable or unfavorable dynamic and thermodynamic factors. For example, 29 hurricanes tracked into the Caribbean in the 10 strongest La Niña years in a positive Atlantic multidecadal oscillation period compared with only two hurricanes tracking through the Caribbean in the 10 strongest El Niño years in a negative Atlantic multidecadal oscillation period. Link to full paper:
  15. The Defining Characteristics of ENSO Extremes and the Strong 2015/2016 El Niño Authors: Agus Santoso, Michael J. Mcphaden and Wenju Cai Published: 14th November, 2017 Abstract: The year 2015 was special for climate scientists, particularly for the El Niño Southern Oscillation (ENSO) research community, as a major El Niño finally materialized after a long pause since the 1997/1998 extreme El Niño. It was scientifically exciting since, due to the short observational record, our knowledge of an extreme El Niño has been based only on the 1982/1983 and 1997/1998 events. The 2015/2016 El Niño was marked by many environmental disasters that are consistent with what is expected for an extreme El Niño. Considering the dramatic impacts of extreme El Niño, and the risk of a potential increase in frequency of ENSO extremes under greenhouse warming, it is timely to evaluate how the recent event fits into our understanding of ENSO extremes. Here we provide a review of ENSO, its nature and dynamics, and through analysis of various observed key variables, we outline the processes that characterize its extremes. The 2015/2016 El Niño brings a useful perspective into the state of understanding of these events and highlights areas for future research. While the 2015/2016 El Niño is characteristically distinct from the 1982/1983 and 1997/1998 events, it still can be considered as the first extreme El Niño of the 21st century. Its extremity can be attributed in part to unusually warm condition in 2014 and to long‐term background warming. In effect, this study provides a list of physically meaningful indices that are straightforward to compute for identifying and tracking extreme ENSO events in observations and climate models. Plain Language Summary The El Niño Southern Oscillation (ENSO) continues to boast its prominence as Earth's strongest source of year‐to‐year climate variability with the appearance of a remarkable El Niño event in 2015–2016. The 2015/2016 El Niño was indeed a strong event with dramatic impact on a global scale. However, it exhibited distinct characteristics from those of past extreme El Niños in modern instrumental record. This challenges our previous understanding of an extreme El Niño which is important for ENSO prediction, monitoring, and future projections. The 2015/2016 El Niño has diversified the small sample of ENSO events in our short instrumental record. It has facilitated important discussions on our progress in understanding the nature of ENSO and its extremes, how they respond to greenhouse warming, and what the climate science community should do next in their quest to fully grasp the complexity of ENSO behavior. These are covered in this review paper which establishes the 2015/2016 El Niño as the first extreme El Niño of the 21st century. Link to full paper:
  16. ENSO Extremes and Diversity: Dynamics, Teleconnection, and Impacts - Workshop There were a series of excellent presentations and discussions from leading authors at the “ENSO Extremes and Diversity: Dynamics, Teleconnection, and Impacts” Workshop held in Sydney, New South Wales, Australia from 4th to 6th February, 2015. Link to full Workshop agenda: Link to the "Meeting Summaries": Link to list of workshop presentation abstracts: A selection of the full papers will appear in this portal shortly.
  17. Eastern Pacific tropical cyclones intensified by El Niño delivery of subsurface ocean heat Authors: Jin FF, Boucharel J and Lin II Published: 4th December, 2014 Abstract: The El Niño Southern Oscillation (ENSO) creates strong variations in sea surface temperature in the eastern equatorial Pacific, leading to major climatic and societal impacts. In particular, ENSO influences the yearly variations of tropical cyclone (TC) activities in both the Pacific and Atlantic basins through atmospheric dynamical factors such as vertical wind shear and stability. Until recently, however, the direct ocean thermal control of ENSO on TCs has not been taken into consideration because of an apparent mismatch in both timing and location: ENSO peaks in winter and its surface warming occurs mostly along the Equator, a region without TC activity. Here we show that El Niño--the warm phase of an ENSO cycle--effectively discharges heat into the eastern North Pacific basin two to three seasons after its wintertime peak, leading to intensified TCs. This basin is characterized by abundant TC activity and is the second most active TC region in the world. As a result of the time involved in ocean transport, El Niño's equatorial subsurface 'heat reservoir', built up in boreal winter, appears in the eastern North Pacific several months later during peak TC season (boreal summer and autumn). By means of this delayed ocean transport mechanism, ENSO provides an additional heat supply favourable for the formation of strong hurricanes. This thermal control on intense TC variability has significant implications for seasonal predictions and long-term projections of TC activity over the eastern North Pacific. Link to full paper: Not available as behind the Nature Research Journal paywall (see below for video presentation). Link to the Nature Research Journal website: The authors presented their paper at the “ENSO Extremes and Diversity: Dynamics, Teleconnection, and Impacts” Workshop held in Sydney, New South Wales, Australia from 4th to 6th February, 2015. Link to workshop presentation (slides and charts): - Boucharel_Sydney_WS_Feb2015_v2.pdf Link to full Workshop agenda:
  18. Intrabasin Variability of East Pacific Tropical Cyclones During ENSO Regulated by Central American Gap Winds Authors: Dan Fu, Ping Chang and Christina M. Patricola Published: 10th May, 2017 Abstract: Hurricane Patricia in 2015 was the strongest Pacific hurricane to make landfall in Mexico. Although Patricia fortuitously spared major cities, it reminded us of the threat tropical cyclones (TCs) pose in the eastern North Pacific (ENP) and the importance of improving our understanding and prediction of ENP TCs. Patricia’s intensity and the active 2015 ENP hurricane season have been partially attributed to the strong El Niño in 2015, however there is still a lack of fundamental understanding of the relationship between El Niño-Southern Oscillation (ENSO) and ENP TCs. Here, we demonstrate that ENSO drives intrabasin variability of ENP TCs, with enhanced (reduced) TC frequency in the western portion of the ENP during El Niño (La Niña), but reduced (enhanced) TC frequency in the eastern nearshore area, where landfalling TCs preferentially form. This intrabasin difference is primarily driven by the Central American Gap Winds (CAGW), which intensify (weaken) during El Niño (La Niña), producing low-level anticyclonic (cyclonic) relative vorticity anomalies and thus an unfavorable (favorable) environment for TC genesis. These findings shed new light on the dynamics linking ENP TC activity to ENSO, and highlight the importance of improving CAGW representation in models to make skillful seasonal forecasts of ENP TCs. Link to full paper:
  19. ENSO Amplitude Modulation Associated with the Mean SST Changes in the Tropical Central Pacific Induced by AMO Authors: In-Sik Kang, Hyun-ho No and Fred Kucharski Published: 22nd July, 2014 Abstract: The mechanism associated with the modulation of the El Niño–Southern Oscillation (ENSO) amplitude caused by the Atlantic multidecadal oscillation (AMO) is investigated by using long-term historical observational data and various types of models. The observational data for the period 1900–2013 show that the ENSO variability weakened during the positive phase of the AMO and strengthened in the negative phase. Such a relationship between the AMO and ENSO amplitude has been reported by a number of previous studies. In the present study the authors demonstrate that the weakening of the ENSO amplitude during the positive phase of the AMO is related to changes of the SST cooling in the eastern and central Pacific accompanied by the easterly wind stress anomalies in the equatorial central Pacific, which were reproduced reasonably well by coupled general circulation model (CGCM) simulations performed with the Atlantic Ocean SST nudged perpetually with the observed SST representing the positive phase of the AMO and the free integration in the other ocean basins. Using a hybrid coupled model, it was determined that the mechanism associated with the weakening of the ENSO amplitude is related to the westward shift and weakening of the ENSO zonal wind stress anomalies accompanied by the westward shift of precipitation anomalies associated with the relatively cold background mean SST over the central Pacific. Link to full paper:
  20. Sea surface temperature in the north tropical Atlantic as a trigger for El Niño/Southern Oscillation events Authors: Yoo-Geun Ham, Jong-Seong Kug, Jong-Yeon Park and Fei-Fei Jin Published: 6th January, 2013 Abstract: El Niño events, the warm phase of the El Niño/Southern Oscillation (ENSO), are known to affect other tropical ocean basins through teleconnections. Conversely, mounting evidence suggests that temperature variability in the Atlantic Ocean may also influence ENSO variability. Here we use reanalysis data and general circulation models to show that sea surface temperature anomalies in the north tropical Atlantic during the boreal spring can serve as a trigger for ENSO events. We identify a subtropical teleconnection in which spring warming in the north tropical Atlantic can induce a low-level cyclonic atmospheric flow over the eastern Pacific Ocean that in turn produces a low-level anticyclonic flow over the western Pacific during the following months. This flow generates easterly winds over the western equatorial Pacific that cool the equatorial Pacific and may trigger a La Niña event the following winter. In addition, El Niño events led by cold anomalies in the north tropical Atlantic tend to be warm-pool El Niño events, with a centre of action located in the central Pacific, rather than canonical El Niño events. We suggest that the identification of temperature anomalies in the north tropical Atlantic could help to forecast the development of different types of El Niño event. Link to full paper (you need to copy and open this file in separate tab): file:///C:/Users/David/Downloads/Hametal.2013NG_main.pdf You may need to go to the "Researchgate" site for a personal download. Just go this link below and click on "Download full text pdf" on that page: Supplementary Information: This paper was presented at the AMS 25th Conference on "Climate Variability and Change" held at the Austin Convention Center, Austin, Texas, USA between 5th and 10th January, 2013. Presentation date: 8th, January, 2013 Link to conference video presentation (14 minutes): Link to full conference agenda: There was an updated and revised paper published by the authors on 1st August, 2013 entitled: "Two distinct roles of Atlantic SSTs in ENSO variability: North Tropical Atlantic SST and Atlantic Niño" - Link to full paper, pdf:
  21. Modulation of Western North Pacific Tropical Cyclone Activity by the Atlantic Meridional Mode Article in Climate Dynamics · Authors: Wei Zhang, Gabriel A. Vecchi, Gabriele Villarini, Hiroyuki Murakami, Anthony Rosati, Xiaosong Yang, Liwei Jia and Fanrong Zeng Published: 19th March, 2016 Abstract: This study examines the year-to-year modulation of the western North Pacific (WNP) tropical cyclones (TC) activity by the Atlantic Meridional Mode (AMM) using both observations and the Geophysical Fluid Dynamics Laboratory Forecast-oriented Low Ocean Resolution Version of CM2.5 (FLOR) global coupled model. 1. The positive (negative) AMM phase suppresses (enhances) WNP TC activity in observations. The anomalous occurrence of WNP TCs results mainly from changes in TC genesis in the southeastern part of the WNP. 2. The observed responses of WNP TC activity to the AMM are connected to the anomalous zonal vertical wind shear (ZVWS) caused by AMM-induced changes to the Walker circulation. During the positive AMM phase, the warming in the North Atlantic induces strong descending flow in the tropical eastern and central Pacific, which intensifies the Walker cell in the WNP. The intensified Walker cell is responsible for the suppressed (enhanced) TC genesis in the eastern (western) part of the WNP by strengthening (weakening) ZVWS. 3. The observed WNPTC–AMM linkage is examined by the long-term control and idealized perturbations experiment with FLOR-FA. A suite of sensitivity experiments strongly corroborate the observed WNPTC–AMM linkage and underlying physical mechanisms Link to full paper (may need to copy to browser): file:///C:/Users/David/Downloads/ModulationofwesternNorthPacifictropicalcycloneactivitybytheAtlanticMeridionalMode.pdf
  22. A Theory for Polar Amplification from a General Circulation Perspective Authors: Sukyoung Lee Published: January, 2014 (Asia-Pacific Journal of Atmospheric Sciences, Volume 50, Issue 1, pp 31–43) Abstract: Records of the past climates show a wide range of values of the equator-to-pole temperature gradient with an apparent universal relationship between the temperature gradient and the global-mean temperature: relative to a reference climate, if the global mean temperature is higher (lower), the greatest warming (cooling) occurs at the polar regions. This phenomenon is known as polar amplification. Understanding this equator-to-pole temperature gradient is fundamental to climate and general circulation, yet there is no established theory from a perspective of the general circulation. Here, a general-circulation based theory for polar amplification is presented. Recognizing the fact that most of the available potential energy (APE) in the atmosphere is untapped, this theory invokes that La Niña-like tropical heating can help tap APE and warm the Arctic by exciting poleward and upward propagating Rossby waves. Link to full paper:
  23. On the Relationship between ENSO, Stratospheric Sudden Warmings and Blocking Authors: David Barriopedro and Natalia Calvo Published: 8th March, 2014 Abstract: This paper examines the influence of El Niño–Southern Oscillation (ENSO) on different aspects of major stratospheric sudden warmings (SSWs), focusing on the precursor role of blocking events. The results reveal an ENSO modulation of the blocking precursors of SSWs. European and Atlantic blocks tend to precede SSWs during El Niño (EN), whereas eastern Pacific and Siberian blocks are the preferred precursors of SSWs during La Niña (LN) winters. This ENSO preference for different blocking precursors seems to occur through an ENSO effect on regional blocking persistence, which in turn favors the occurrence of SSWs. The regional blocking precursors of SSWs during each ENSO phase also have different impacts on the upward propagation of planetary-scale wavenumbers 1 and 2; hence, they determine ENSO differences in the wavenumber signatures of SSWs. SSWs occurring during EN are preceded by amplification of wavenumber 1, whereas LN SSWs are predominantly associated to wavenumber-2 amplification. However, there is not a strong preference for splitting or displacement SSWs during any ENSO phase. This is mainly because during EN, splitting SSWs do not show a wavenumber-2 pattern. Link to full paper:
  24. ENSO‐based probabilistic forecasts of March–May U.S. tornado and hail activity Authors: Chiara Lepore, Michael K. Tippett and John T. Allen Published: 25th August, 2017 Abstract: Extended logistic regression is used to predict March–May severe convective storm (SCS) activity based on the preceding December–February (DJF) El Niño–Southern Oscillation (ENSO) state. The spatially resolved probabilistic forecasts are verified against U.S. tornado counts, hail events, and two environmental indices for severe convection. The cross‐validated skill is positive for roughly a quarter of the U.S. Overall, indices are predicted with more skill than are storm reports, and hail events are predicted with more skill than tornado counts. Skill is higher in the cool phase of ENSO (La Niña like) when overall SCS activity is higher. SCS forecasts based on the predicted DJF ENSO state from coupled dynamical models initialized in October of the previous year extend the lead time with only a modest reduction in skill compared to forecasts based on the observed DJF ENSO state. Link to full paper:
  25. Influence of the El Niño/Southern Oscillation on tornado and hail frequency in the United States Authors: John T. Allen, Michael K. Tippett and Adam H. Sobel Published: 16th March, 2015 Abstract: The El Niño-Southern Oscillation (ENSO) is characterized by changes in sea surface temperature (SST) and atmospheric convection in the tropical Pacific and modulates global weather and climate. The phase of ENSO influences United States (US) temperature and precipitation and has long been hypothesized to influence severe thunderstorm occurrence over the US. However, limitations of the severe thunderstorm observational record, combined with large year-to-year variability have made it difficult to demonstrate an ENSO influence during the peak spring season. Here we use environmental indices that are correlated with tornado and hail activity, and show that ENSO modulates tornado and hail occurrence during the winter and spring by altering the large-scale environment. We show that fewer tornadoes and hail events occur over the central US during El Niño and conversely more occur during La Niña conditions. Moreover, winter ENSO conditions often persist into early spring, and consequently the winter ENSO state can be used to predict changes in tornado and hail frequency during the following spring. Combined with our current ability to predict ENSO several months in advance our findings provide a basis for long-range seasonal prediction of severe thunderstorm activity. Link to full paper: Sobel_Research2.pdf