ENSO Extremes and Diversity: Dynamics, Teleconnections, and Impacts Workshop Report
ENSO Workshop Australia
(4-6 February 2015, University of New South Wales, Sydney, Australia)
by Agus Santoso
With input from the scientific committee members: Wenju Cai, Mat Collins, Mike McPhaden, Fei-Fei Jin, Eric Guilyardi, Gabe Vecchi, Dietmar Dommenget, and Guojian Wang
*Access via https://www.climatescience.org.au/content/806-enso-workshop-australia-2015
#A summary is provided at the Bulletin of the American Meteorological Society: http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-15-00141.1
How to cite this report:
Santoso, A., W. Cai, M. Collins, M. McPhaden, F.-F. Jin, E. Guilyardi, G. Vecchi, D. Dommenget, and G. Wang, 2015: ENSO Extremes and Diversity: Dynamics, Teleconnections, and Impacts. Bull. Amer. Meteor. Soc., in press. doi: http://dx.doi.org/10.1175/BAMS-D-15-00141.1
ENSO at a glance
Originating in the tropical Pacific Ocean, the El Niño Southern Oscillation (ENSO) is Earth’s most dominant source of year-to-year climate variability that exerts a profound worldwide impact. The western Pacific is home to Earth’s largest pool of warm water, an abundant source of moisture that fuels the ascending branch of the Walker circulation. The raising air descends over the cold and dry eastern Pacific where atmospheric pressure is high. This surface pressure difference supports the westward blowing Trade Winds, balanced by a tilted oceanic thermocline - from the deep western end to the nutrient-rich shallow side off the American coasts.
Due to Coriolis force, the Trade Winds transport heat and water poleward either side of the equator, driving equatorial upwelling. Equatorward return flows occur below the wind-driven Ekman layer, in effect deepening the basin-wide equatorial thermocline to increase the volume of warm water above the thermocline. The winds, thermocline slope and circulations slacken and strengthen in concert with warming and cooling of the eastern Pacific during ENSO warm phase (El Niño) and cold phase (La Niña). These disturbed phases of the tropical Pacific climate, which are achieved through the Bjerknes positive coupled air-sea feedback, peak in boreal winter and vacillate irregularly on interannual time scales, sometimes manifesting in diverse spatial patterns. Sea surface temperature (SST) anomalies can peak in the Central Pacific rather than in the usual eastern Pacific, in an event known as ‘ENSO Modoki’ or simply ‘Central Pacific ENSO’.
Significant oceanic and atmospheric changes occur not just in the tropical Pacific but also beyond through planetary waves, influencing weather and climatic patterns worldwide. During an El Niño, western Pacific countries such as Australia and Indonesia often endure droughts, forest fires, and food shortages. On the other hand, countries in the far eastern Pacific such as Peru, Ecuador, and southwestern states of the US can experience heightened risk of severe storms that lead to flooding and landslides. Ability to predict ENSO events well in advance is a necessity to guard the welfare of the global community, especially in a climate that is undergoing significant changes under the greenhouse effect.
Underlying a predictive capacity of a climate system is a grounded understanding of its dynamics. Our knowledge of ENSO has significantly advanced over the last 30 years with increasing availability and quality of observational data and paleo proxies, development of ENSO theories and formulas, and improvements in climate models. Over this time, much has been learnt about the general behaviour of ENSO as demonstrated by the applicability of conceptual frameworks such as the recharge-discharge and delayed oscillators. At the heart of these theories is the propagation of Kelvin and Rossby waves that act to increase the equatorial warm water volume to precondition an El Niño, and vice versa for La Niña. Such linear theories however hold an in-built presumption that El Niño and La Niña can be considered as opposite but broadly symmetrical in properties, and oscillatory in occurrences.
The 1982/83 and 1997/98 ‘super El Niño’ events pose a challenge to the validity of this linearity assumption, and our understanding of ENSO. The 1997/98 event was dubbed ‘the climate event of the 20th Century’ for its extraordinary magnitude and global-scale destructive effects which caused multi-billion dollars in economic losses and claimed thousands of human lives. These are the only two strong El Niño events in the modern record that exhibited markedly different characteristics from other ENSO events. Apart from their exceptional magnitude, they are also known as ‘eastward propagating El Niño’ due to their unique west-to-east propagation of sea surface temperature (SST) anomalies, a behaviour not seen during any other ENSO events.
The rarity of such non-linear events means that their mechanisms have not been fully understood. Climate models can help bridge this gap, but their accuracy in simulating ENSO, despite their continued improvements, is still questionable, although this is also partly due to the relatively short observations to compare them against. However, current state of the art climate models remain the best tool to tease out the intricate dynamics of ENSO, predict its occurrences and make future projections.
Modelling studies over the last two years have shed further insights on the characteristics and mechanisms of extreme El Niño, and found that the frequency of extreme El Niño is projected to increase significantly under greenhouse warming. This finding stresses urgency in the ability to predict their occurrences.
ENSO Workshop Australia
In boreal spring of 2014, the tropical Pacific was primed for an El Niño, with most forecast agencies such as NOAA and the Australian Bureau of Meteorology elevated their El Niño probability to more than 60%. A remarkable increase in warm water volume with a series of westerly wind bursts in boreal spring alerted ENSO experts to the possibility of a strong event, one which some thought could be the first extreme El Niño since 1997, generating news headlines worldwide. However, while the equatorial Pacific remained anomalously warm, the expected super El Niño did not materialise. That failed expectation may in part be a reflection of our incomplete knowledge of extreme El Niño and its predictability, or perhaps the very nature of the ENSO system itself. Indeed new discoveries continue to be made. In fact, it is only recently that the notion of an extreme La Niña has been proposed and its occurrences, along with extreme El Niño, are expected to increase as the globe continues to warm.
Against this backdrop of progress, uncertainties, and ensuing greenhouse warming, it is timely to ask - what is the current state of understanding of ENSO in terms of its diverse behaviour, extremity, impacts and teleconnections? It is for this reason that about 60 ENSO researchers gathered in Sydney, Australia earlier this year for a 3-day workshop at the University of New South Wales. The 60 participants consisted of leading ENSO experts and senior scientists, including 20 postdocs and graduate students from Australia, US, China, UK, France, Germany, Korea, Japan, India, Peru, and Indonesia, as well as additional 20 participants from academia and industry. 39 oral and 24 poster presentations were delivered over the 3 days, highlighting recent advances and ongoing research in various aspects of ENSO, to facilitate the discussions in the context of ENSO extremes and diversity. The science presented covered the use of modelling, observations, theories, and paleo reconstructions. The programs, abstracts, and most presentations can be found in https://www.climatescience.org.au/content/806-enso-workshop-australia-2015.
Some key variables during the evolution of past El Niño events since 1982. Each evolution starts from January up to December the following year. The most recent developments starting in January 2014 is shown by the dashed green line. The El Niño event that some experts expected it to peak as an extreme El Niño in December 2014 failed to materialise, despite the large amount of subsurface heat in March-May 2014 that came only second after the 1997 extreme El Niño. As discussed at the workshop, a large warm water volume is a necessary but not sufficient condition for an extreme El Niño. The event however appears to be still evolving.
The workshop was opened by a stimulating presentation titled ‘Who Killed the Big 2014 El Niño’ by Mike McPhaden which outlined the possible factors that may hold the key to understanding this event. The general perception is that the atmospheric feedback failed to occur, despite the significant ocean subsurface warming, and this appears to be due to various factors such as the state
of the Indian Ocean, the cold phase of the Interdecadal Pacific Oscillation (IPO) or Pacific Decadal Oscillation (PDO), expansion of the Indo-Pacific warm pool, muted westerly wind bursts, and damping processes that may have occurred too early in the season. All of these factors reflect the main complexities of ENSO dynamics which today remain an active topic of investigation. These are outlined below as discussed by several presenters.
Role of the background state on ENSO variability: The mean climate upon which ENSO evolves varies on multi-decadal time scales, manifesting in a global-scale phenomenon in what commonly known as the IPO or PDO (see presentation by Henley on a new index of the IPO). Disentangling the link between ENSO and IPO has been one challenging research topic as the two are intertwined. Is the IPO an ENSO rectification effect on the mean climate, for instance through ENSO skewness as discussed by a speaker (Yu Y.-Q.)? Or is the IPO an independent mode of low-frequency variability which impacts ENSO (see, e.g., Tatebe on the South Pacific source of decadal variability)? A clear example of this was demonstrated in a presentation that ENSO predictability is weaker during negative phase of the IPO than during positive IPO (Hendon). Our current theoretical understanding, as mentioned by Fei-Fei Jin, a keynote presenter, is that modest changes in the basic state can lead to different types of El Niño emerging during certain epochs. As such, the presence of the IPO would complicate the detection of greenhouse warming effects, and other external forcing, on ENSO changes (see also Sullivan).
Precursors: Positive warm water volume anomaly was mentioned to be a necessary condition for a strong El Niño, and so were strong westerly wind anomalies in boreal summer. Warm water volume increase is achieved through Kelvin waves which are often initiated by westerly wind bursts. However, it was shown that the different spatial and temporal characteristics of the wind bursts mean that the effect of the generated equatorial waves on development of El Niño events can be very different (McGregor S.; Hayashi). There was also a highlight on the potential link between variations in the warm pool spatial structure and development of strong El Niño (Hu). Other precursors of extreme El Niño mentioned include specific vacillations in surface pressure in the southern and northern hemispheres, respectively referred to as the ‘Southern Hemisphere Booster’ and the ‘Pacific Meridional Mode’ (Jin; see also An S.-I. on this topic). Such insights have direct implications on seasonal prediction of El Niño.
Interbasin interactions: While ENSO influences air-sea processes outside the tropical Pacific, these remote forcing also affects ENSO evolution. The interactions between climate variability in the Indian (e.g., Indian Ocean Dipole), Atlantic, and extratropics (e.g., North Pacific Oscillation) were presented (Terray; Kajtar; Yeh; Yu Y.-S.), with apparent impacts on ENSO evolution. The results, which can be model dependent, suggest that understanding ENSO evolution should be approached from a global perspective rather than fixating on the tropical Pacific.
Coupled feedback processes: Representing the Bjerknes feedback in climate models remains a challenging issue and an apparent source of uncertainties in ENSO simulation. An important contribution to this challenge is inaccuracies in cloud simulation. As shown in one presentation (Raedel), cloud feedback accounts for substantial portion of ENSO variability in a climate model. This issue needs to be resolved, not least because the growth of extreme El Niños was suggested to involve convectively nonlinear Bjerknes feedback (Takahashi). On a closely related topic, it was shown that characteristics of coupled feedback processes are notably different between canonical ENSO and ENSO Modoki (Marathe), contributing to the debate as to whether ENSO Modoki is an independent mode of variability or part of the ENSO continuum. In any case, as demonstrated in several presentations (Holbrook; Brown J. N.; Li T.; Kim S.-T.; Vijayeta; Ferrett), quantification of coupled feedback processes, as well as devising more complete ENSO metrics (Brown J. N.), continues to be useful for understanding ENSO characteristics across climate models and in response to greenhouse forcing, especially when it also takes into account spatial structures.
Seasonal phase locking: The explanation for why ENSO variability tends to peak in boreal winter is not complete. Yet seasonal phase locking contributes to the non-linear behaviour of ENSO, such as the southward wind shift at the peak of strong El Niño that leads to the demise of the event and preconditioning for a La Niña in the following year through wave propagation (Timmermann; Jin; Abellan Villardon; Chakravorty; Stuecker). The role of wave propagation on ENSO evolution through western boundary reflection was further noted in a presentation (Zhang X.-B.). Seasonal feedback between cloud and SST was suggested to be a mechanism for ENSO seasonal phase locking (Dommenget). Other potential contributors suggested include the annual cycle of the warm pool, as well as feedback from the Atlantic and Indian Ocean variability (Terray).
ENSO asymmetry: El Niño and La Niña are not a mirror image to each other. For instance, El Niño events tend to be of shorter duration and can attain a much larger magnitude than La Niña. Asymmetry underpins the notion of ENSO extremes as extensively discussed in several presentations, from the characteristics of SST anomaly patterns and ocean mixed layer heat budget (e.g., Cai, Li T., Kim W.) to the non-linear response of rainfall to the SST anomalies (Chung). The role of reversing equatorial Pacific zonal currents was highlighted (Kim W.), which together with an eastward shift of atmospheric convection during an El Niño developing phase can push an event to extreme amplitudes. This was also proposed as a possible explanation for the differing skewness of ENSO SST anomalies across models (see also Choi on the link between non-linearity in rainfall and zonal winds).
The aforementioned factors are an integral component of ENSO dynamics. A better understanding in one will lead to further insights in another. Rapid gain of knowledge has been made possible by the breadth of data output from the latest generation of climate models made accessible to the science community through the Coupled Model Intercomparison Project (CMIP) initiative. The output from the 5th phase of CMIP (CMIP5) were utilised by several presenters. It was shown in some presentations that although CMIP5 models still have issues in simulating many aspects of ENSO including its extremity, they can exhibit some success in seasonal prediction of the 1997/98 event (Takahashi; Ineson). This is in part due to good quality observational data used to initialise the models, but the predictive capacity itself is hampered by numerical drifts and model bias (see also presentation by Luo on model bias and the negative IPO).
The challenge in simulating ENSO was highlighted in presentations that specifically focussed on modelling. One presentation (Brown J. N.) showed that atmosphere-ocean dynamics can be very different in an ocean-only model forced by air-sea fluxes from when it is coupled to an atmospheric model, illustrating the complex nature of a coupled phenomenon in ENSO. In earlier generations of models, ‘flux adjustment’ was used to more realistically capture seasonal cycle and minimise drifts. However, it has been viewed to be a rather artificial fix to a problem that is inherently complex. A finer model resolution has been generally thought to be the most appropriate solution (see also presentation by Brown J. N.). However, a speaker (Vecchi) put forth a compelling argument for the use of flux adjustment which appears to actually have overarching benefits along with increasing resolution. Nonetheless, for the mean time and in spite of these issues, the large diversity and accessibility of CMIP models, as well as being relatively economical for running experiments, makes them desirable for understanding the future projections of ENSO. In light of short modern observational record, paleo reconstructions are also a useful tool and here a speaker (McGregor H.) provided a paleo evidence that an increase in ENSO variance since the mid-Holocene is outside the range of internal variability, indicating the role of external forcing (see also presentation by Saint-Lu).
How ENSO responds to greenhouse warming has been a long contentious issue, with disagreement among models in the projection of ENSO amplitude, which is typically defined as the magnitude of eastern equatorial Pacific SST anomalies. The possible causes for the disagreement were discussed (Li; Rashid). One speaker (Kim S.-T.) showed that CMIP5 models with more realistic relative importance of the Bjerknes feedback components exhibit a time-varying response in ENSO amplitude, in contrast to the less realistic group of models exhibiting a static increase into the future. The time-varying response appears to be driven by surface warming difference between the Indian and the Pacific Ocean which in itself is an interesting feature that needs to be investigated further. As discussed in other presentations (Cai; Wang; Zheng), recent projection studies have instead utilised dynamical processes rather than SST anomalies confined in a fixed region. These include the propagation direction of the SST anomalies and rainfall response to ENSO SST patterns superimposed on the background warming. These studies found an increase in the frequency of events with unique features seen in the 1982/83 and 1997/98 extreme El Niños, as well as extreme La Niña events. A review paper on this topic is underway.
The workshop featured many presentations on ENSO teleconnection and impacts (e.g., Ashok on modulation by the Atlantic Multi-decadal Oscillation; Halpern on observational aspects), including effects on regional rainfall (Salamena; Tedeschi; Brown J. R.) and interannual variability of atmospheric CO2 concentrations (Kim J.-S.). Presentations on the atmospheric teleconnection of ENSO include effect on East Asian rainfall (Kug); effect on Arctic Oscillation via the stratosphere (Imada); and influence of La Niña on eastern Australian extreme rainfall in 2010 via SST warming trend and the positive phase of the Southern Annular Mode (Lim; see also Browning on paleo ENSO-SAM linkage). There were also many presentations on oceanic teleconnection (e.g., Santoso on the Indonesian Throughflow), including the impact of ENSO on Southern Ocean circulation (Langlais). There was a paleo-based presentation (Zinke) linking La Niña with ocean heat wave events off Southwest Australia dubbed the Ningaloo Niño, via ocean advection through the Indonesian Throughflow. A marked increase in the occurrence of Ningaloo Niño since the late 1990s was suggested by a speaker (Feng) to be related to the negative IPO, with a possible feedback onto ENSO. Another highlight was a talk (Boucharel) on the influence of El Niño heat discharge on eastern Pacific tropical cyclones, underscoring the importance of ENSO ocean teleconnection on extreme weather.
The workshop concluded with a discussion led by Mat Collins on coordinated multi-model experiments proposed for CMIP6 and the opportunities they present for understanding ENSO dynamics. It was agreed that, in addition to all the analysis that will be performed on the many CMIP6 multi-model experiments, some more targeted experiments should be conducted to investigate aspects such as the role of the pattern of mean SST change in driving uncertainties in rainfall response to ENSO events in the future. Further discussions on the details will be scheduled for future meetings.
The elusive 2014 El Niño development has left question marks in our understanding of ENSO. At the same time it has served as a strong motivation to sustain research efforts in this important area of climate science. The hype created by this enigmatic event has in some ways helped increase the awareness of the general public on ENSO (and climate science in general), as well as on operational forecasting and monitoring of ENSO (see presentation by Catherine Ganter, a representative from the Bureau of Meteorology). The task now rests on the scientific community to deliver further insights on this magnificent climate phenomenon called the El Niño Southern Oscillation.
Open research questions and opportunities
The workshop essentially highlights the following research questions that will keep the community busy in the coming years:
- How robust are ENSO projections amidst model uncertainties and relatively short instrumental records?
- How can the Bjerknes and other feedbacks be properly represented in climate models?
- Should we employ flux adjustment in climate models?
- How does the background state variations (e.g. IPO) link with ENSO?
- What are the dynamics and characteristics of extreme ENSO events?
- How predictable are extreme ENSO events?
- What are the mechanisms determining ENSO seasonal phase locking?
- What are the dynamics of ENSO diversity, or is ENSO Modoki a truly independent mode of variability?
- How do inter-basin feedback interactions affect ENSO?
- What are the oceanic and atmospheric teleconnection processes of extreme ENSO?
- What paleo proxies can be used to study the spatial evolution of ENSO?
With increasing availability of observations, improved models and intricate experiments, and more paleo proxies, some of these issues will be eventually resolved.
In summary, this workshop had outlined new insights and impediments in present ENSO research and highlighted the need to bring together theories, models, observations, and paleo reconstructions to make significant progress. Also importantly, the workshop had demonstrated the togetherness and inclusiveness of the ENSO research community, and that the community has the breadth of expertise, enthusiasm, and perseverance required for tackling the challenges that lie ahead.
This workshop was financially supported by CSIRO and ARC Centre of Excellence for Climate System Science. Logistic support provided by the Climate Change Research Centre and the University of New South Wales is gratefully acknowledged. This workshop would not have been meaningful without all of the participants who shared their science and contributed to the discussions. Workshop program, abstracts, list of participants, and most presentations can be found at https://www.climatescience.org.au/content/806-enso-workshop-australia-2015. We also thank Sarah Ineson, Harry Hendon, Neil Holbrook and Wonmoo Kim for proofreading the meeting summary version published at the Bulletin of the American Meteorological Society. The workshop contributes to the objective of CLIVAR of the World Climate Research Programme (WCRP) and to the interests of the International Commission on Climate (ICCL) of International Association of Meteorology and Atmospheric Sciences (IAMAS)/International Union of Geodesy and Geophysics (IUGG).
Some relevant further reading:
Barnston, A. G., M. K. Tippett, M. L. L'Heureux, S. Li, and D. G. DeWitt, 2012: Skill of Real-Time Seasonal ENSO Model Predictions during 2002–11: Is Our Capability Increasing? Bull. Amer. Meteor. Soc., 93, 631–651.
Cai, W., S. Borlace, M. Lengaigne, P. van Rensch, M. Collins, G. Vecchi, A. Timmermann, A. Santoso, M. McPhaden, L. Wu, M. H. England, G. Wang, E. Guilyardi, and F.-F. Jin, 2014: Increasing frequency of extreme El Niño events due to greenhouse warming. Nature Climate Change, 4, 111-116.
Cai, W., G. Wang, A. Santoso, M. J. McPhaden, L. Wu, F.-F. Jin, A. Timmermann, M. Collins, G. Vecchi, M. Lengaigne, M. H. England, D. Dommenget, K. Takahashi, and E. Guilyardi, 2015: Increased frequency of extreme La Niña events under greenhouse warming. Nature Climate Change, 5, 132-137.
Capotondi, A., A. T. Wittenberg, M. Newman, E. Di Lorenzo, J.-Y. Yu, P. Braconnot, J. Cole, B. Dewitte, B. Giese, E. Guilyardi, F.-F. Jin, K. Karnauskas, B. Kirtman, T. Lee, N. Schneider, Y. Xue, S.-W. Yeh, 2015: Understanding ENSO diversity. Bull. Amer. Meteor. Soc, in press. doi: http://dx.doi.org/10.1175/BAMS-D-13-00117.1
Guilyardi, G., W. Cai, M. Collins, A. Fedorov, F.-F. Jin, A. Kumar, D. –Z. Sun, and A. Wittenberg, 2012: New Strategies for Evaluating ENSO Processes in Climate Models. Bull. Amer. Meteor. Soc., 93, 235–238.
Holbrook, N. J., J. Li, M. Collins, E. Di Lorenzo, F.-F. Jin, T. Knutson, M. Latif, C. Li, S. B. Power, R. Huang, and G. Wu, 2014: Decadal Climate Variability and Cross-Scale Interactions: ICCL 2013 Expert Assessment Workshop. Bull. Amer. Meteor. Soc., 95, ES155–ES158.
McPhaden, M., J., A. Timmermann, M. J. Widlansky, M. A. Balmaseda, and T. N. Stockdale, 2015: The curious case of the El Niño that never happened: A perspective from 40 years of progress in climate research and forecasting. Bull. Amer. Meteor. Soc., in press. doi: http://dx.doi.org/10.1175/BAMS-D-14-00089.1.
Santoso, A., S. McGregor, F.-F. Jin, W. Cai, M. H. England, S.-I. An, M. McPhaden, E. Guilyardi, 2013: Late-twentieth-century emergence of the El Niño propagation asymmetry and future projections. Nature, 504, 126-130.
Reported in May 2015