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Review the Causes and Consequences of Cold Events
A United Nations University / NCAR / UNEP Activity
Workshop Executive Summary Executive Summary for the La Niña Summit Report Table of Contents
This is an executive summary of the deliberations and presentations of the participants to the Workshop held in Boulder, Colorado, from 15-17 July 1998 on "A review of the causes and consequences of cold events: A La Niña summit". The workshop was supported by the United Nations University (Tokyo), the National Center for Atmospheric Research (Boulder, Colorado), the United Nations Environment Programme (Nairobi), and the US National Science Foundation (Washington, DC). The entire workshop was carried on the Internet in audio by the Exploratorium in San Francisco, California ("www.exploratorium.edu/la_Niña/"). The purpose for convening such a meeting was to identify what is known, not known and what societies need to know about cold events in order to forecast their onset, growth, and decay several months in advance and to prepare for their societal impacts. (The terms La Niña and cold event will be used interchangeably in this report). This executive summary of the Summit is based on a draft of the full report of the workshop. The full report also contains brief discussion papers prepared by several of the participants from a variety of disciplines and countries. Given the growing interest in the output of the La Niña Summit as expressed by individual researchers, commercial organizations, resource managers, the general public and the media as well government agencies, this executive summary has been issued a few weeks in advance of the full report. The following pages attempt to capture the key aspects of deliberations during each of the workshop sessions: A review of the ENSO process of 1997-98; Problems associated with definitions of La Niña; Consideration of what constitutes 'normal' sea surface temperatures in the tropical Pacific; Teleconnections associated with La Niña; Climate change and the ENSO process; The issue of symmetry between El Niño and La Niña; The attribution of societal and environmental impacts to specific La Niña events; A review of media panel discussions; Perspectives on how well the 1997-98 El Niño was forecast; Forecasting the onset of a La Niña in 1998-99; The differences in forecasting El Niño and La Niña; and Monitoring La Niña. Each section provides highlights of a few (but not all) of the key points made during the individual sessions. Although discussion on each of these topics was centered in a specific workshop session, some of the topics were raised again directly or indirectly in other sessions. Thus, these are cross-cutting, not mutually exclusive, topics. For example, the concern about the need for a more precise definition of La Niña was expressed in most sessions. Symmetric impacts as well as teleconnections of El Niño and La Niña were also mentioned in different contexts throughout the workshop. The attribution issue was also revisited several times. El Niño, La Niña and the Climate Swings of 1997-98 Michael McPhaden (National Oceanic and Atmospheric Administration/Pacific Marine Environmental Labs) provided an overview of developments of the 1997-98 ENSO cycle in the tropical Pacific. He noted that the successes in observing the evolution of the 1997-98 El Niño, and in forecasting its impacts, were in striking contrast to the situation only 15 years ago when the last major El Niño in 1982-83 was not even detected until it was nearly at its peak. These recent successes were in large part the result of (1) the development of an observing system of satellites and ocean measurements, anchored by a buoy network spanning the equatorial Pacific (Figure 1) providing data in real time (within hours of collection); and (2) the development of computer forecast models for predictions. He reported that the onset of El Niño was heralded by warm sea surface temperature (SST) anomalies (i.e., deviations from normal) erupting in the tropical eastern Pacific during April-June 1997. By July 1997, SST anomalies in this region were the warmest observed in the past hundred years. By the end of the year, SST anomalies exceeded 5 degrees C, and were higher than the anomalies observed at any time during the 1982-83 event, which up to that time had been the strongest of the century. As the warm SST anomalies began to wane in early 1998, prediction models were largely consistent among themselves in forecasting an end to El Niño, and the development of a La Niña sometime during the second half of 1998. The surprise to El Niño researchers, though, was how quickly the tropical Pacific switched from warm to cold conditions. McPhaden commented that a temperature drop of 8 degrees C at one buoy location in the central Pacific over a relatively brief 30-day span had not been observed before, making the termination of the El Niño as stunning as was its relatively rapid onset. He suggested that the seeds for El Niño's demise, and for the birth of La Niña, were to be found below the ocean's surface. In late 1997, the buoy array detected a cold mass of water — situated at a depth of 100-150 m west of the Date Line — and documented its expansion eastward along the equator. McPhaden concluded by noting that during the past two years we have witnessed a spectacular display of climatic variability. The first part of that display, El Niño, is over, and the second, La Niña, is under way. A newly completed observing system, designed specifically for ENSO detection and forecasting, provided essential information at a level of detail never before possible. As a result, our climate crystal ball was sufficiently clear in 1997 to motivate national and local governments to undertake disaster preparedness, mitigation efforts, and other worldwide responses to developing El Niño conditions on an unprecedented scale. Key Points:
Problems Associated with Definitions of La Niña James O'Brien (Center for Ocean/Atmosphere Prediction Studies, Florida State University) led discussion on this topic. Cold events in the central and eastern equatorial Pacific have been referred to in many ways by various scientists and media personnel. For the most part, La Niña (or cold event) has been defined in terms of El Niño, that is, with El Niño as its reference point. For example, one can find in the media and in professional journals such statements as the following: El Niño's counterpart, the inverse of El Niño, El Niño's sister, anti-El Niño, the opposite of El Niño, the flip side of El Niño, El Niño's lesser known twin, and non-El Niño. In a strict sense, La Niña is an extreme cooling of the central and eastern equatorial Pacific over a period of several months. It is not just any drop in temperature below (i.e., cooler than) the long-term average. For some parts of the globe, La Niña is perceived as an extreme case of normal in that nothing unusual is expected to occur with regard to climate anomalies. Problems in the evaluation and use of La Niña-related information arise because of the lack of consensus on a single, universally accepted description of what constitutes a La Niña event. The lack of a widely accepted definition creates problems for users of ENSO information who seek to identify correlations between La Niña events and, for example, crop yields, fish landings, or rainfall anomalies in a particular location. Also, the way in which one defines La Niña in quantitative terms determines how many events have occurred. The more strict the definition, the fewer the events and the smaller the sample of La Niña cases to assess for its physical characteristics or for its environmental and societal impacts. Thus, scientists point out that there have been about twice as many El Niño events as La Niña events since the early 1970s. Key Points:
What is 'Normal' with Respect to Sea Surface Temperatures in the Tropical Pacific? Joseph Tribbia (Climate and Global Dynamics/NCAR) opened the session, noting that views differ about what constitutes normal sea surface temperatures in the tropical Pacific. Some observers suggest that there are only two states, warm and cold. Others suggest that there are two different states, El Niño (warm) and non-El Niño (i.e., everything else). Still others suggest that there are three states of ocean temperatures in the central Pacific: extreme warm, extreme cold and normal. Each of these states can be quantitatively defined and each can be associated with the occurrence of weather and climate anomalies (i.e., extreme meteorological events) around the globe. The public, fostered by media stories, video clips, and interviews with scientists, tends to believe that the El Niño phenomenon is "the culprit" in bringing about poor climatic conditions and death and destruction in many countries, rich and poor alike. Only recently, following the 1997-98 El Niño event, now considered to have been one of the two largest in a century, have the media shown any serious interest, though short-lived, in La Niña, in part because of its expected adverse impacts. Adverse impacts include an increase in the number of tropical storms and hurricanes in the tropical Atlantic and in the Gulf of Mexico. It has been known for some time that extreme meteorological events (e.g., droughts, floods, tropical storms) can be associated with a normal range of SSTs in the tropical Pacific. However, the relatively short time series for global sea surface temperatures makes it difficult to determine what really constitute normal. Key Points:
Teleconnections Associated with La Niña George Kiladis (Aeronomy Lab, NOAA) opened the workshop session on La Niña teleconnections by defining the term "teleconnections" as the influence of SST variations in the tropical Pacific on regional and local climate regimes around the world. He then provided a brief overview of the probable physical causes and effects of teleconnections during El Niño and La Niña (Figure 2). Physical scientists are researching how events in the tropical Pacific transmit a signal through the atmosphere to distant places on the globe. Since tropical convection represents the primary "heat engine" for the global atmospheric circulation, changes in the location of this convection alter the global circulation, which in turn is manifested as climate anomalies. For example, as convection shifts westward from the tropical Pacific into the Indian Ocean during La Niña, the jet stream over the Pacific is weakened, thereby affecting the downstream wave activity (high and low pressure weather systems) moving into North America. Several maps were then presented to the participants depicting the global temperature and precipitation composites for both La Niña and El Niño events (Figure 3). It was suggested that there was some degree of linearity between El Niño and La Niña impacts over many regions, meaning that cold events in several locations produced the opposite climate anomalies to those occurring during warm events. However, the reliability of the teleconnection signals becomes less as one moves farther away from the tropical Pacific, the "center of action" of ENSO. Thus, even though a given teleconnection might still be defined as statistically significant, and most certainly related to the entire population of warm or cold events, the probability of that signal occurring during any one event may not necessarily be very high because of the large amount of climate "noise" (random fluctuations in the atmosphere). This is especially true in the extratropics, where large internal climate variability is dominant, as opposed to the tropics, where climate variability is determined by SSTs to a much greater degree. In addition, weak and moderate La Niña events might not be strong enough to generate climate anomalies in distant locations. Kiladis then noted what he considered to be among the robust (reliable) La Niña teleconnections. These included a tendency for wetter-than-normal conditions and a risk for flooding in southern Africa and the monsoon regions of India, Indonesia, and northern Australia, and drier-than-normal conditions, sometimes leading to drought, over eastern Africa, the western equatorial Indian Ocean, southern South America, and the southern Plains and southeastern portions of the United States. In general, tropical surface temperatures tend to be below normal, with robust signals appearing even as far away from the tropical Pacific as Africa. The most pronounced extratropical temperature signals during La Niña are seen over North America in winter, where there is a tendency for colder-than-normal conditions over Alaska, western Canada, and the central Plains of midwestern Canada and the northern United States, and warmer-than-normal tendencies over the southeastern United States. Key Points:
The Impacts of La Niña on Specific Countries and Sectors Several participants were asked to present their thoughts in brief discussion papers that focused on country- and sector-specific information about La Niña, specifically, and the ENSO cycle in general. Many countries have, until now, shown little, if any, interest in cold events. In fact some of these countries have only recently become concerned about El Niño research and about research about the impacts of El Niño on key sectors of their societies. The countries represented in the discussion papers in the full report are the following: Cuba (Lino Naranjo Diaz, La Niña Impacts in Cuba), Climate Change and the ENSO Process Kevin Trenberth (CGD/NCAR) opened the discussion session on climate change by noting that interest in the relationship between the ENSO process and global warming of the atmosphere has been increasing in recent years. Some studies have suggested that (1) the unusual behavior of the tropical Pacific's sea surface temperatures in the first half of the 1990s (considered by some observers to have been one continuous El Niño from 1991 to 1995) and (2) the apparent climate regime shift after the mid-1970s are the result of influences of human-induced global warming of the atmosphere. Others have challenged that view, arguing that there is not enough evidence as yet to identify with certainty the linkages between these two processes, one natural and one possibly human-induced. Because ENSO is involved with the movement of heat, it is conceptually easy to see how increased heating from the buildup of greenhouse gases can "interfere." The instrumental observational record is not really long enough to sample all of nature's variety, especially with regard to variations on decadal time scales. One approach to this then is to use proxy data, such as those provided from tree rings, and the annual layers of coral or tropical glaciers. Some reconstructions using these methods show El Niño variations quite nicely. However, their decadal variability is compromised by the impacts of non-climatic effects. Another problem with proxy data is that the actual climate is changing, and so the results do not simply give the variability that ocurs in an unchanging climate. Another approach is to use models. At this point, no global climate model is able to simulate El Niño events as realistically as desired to build confidence in the results. Several models, but not all, suggest increased warming in the eastern Pacific, so the mean climate becomes a bit more El Niño-like. Some models suggest that strong El Niño/La Niña swings may occur, but none simulate El Niño with sufficient fidelity to have confidence in their results. The natural variability of the atmosphere must be identified at both the interannual and interdecadal time scales and a more extensive time series of SST changes in the equatorial Pacific must be developed before one can begin to assert with confidence that linkages between these two processes have an acceptable degree of reliability. Key Points:
Symmetry Between El Niño and La Niña Events Martin Hoerling (NOAA-University of Colorado, CIRES Climate Diagnostics Center) noted that the tropical Pacific Ocean undergoes an oscillation, albeit irregular, between warm (El Niño) conditions an cold (La Niña) conditions. This variability occurs relative to a climatological mean, or normal ocean state. The overall distribution resembles a bell curve, or Gaussian, shape, and the El Niño and La Niña events reside in the less-frequented tails of that curve. But is that distribution a perfect bell shape, or is it somehow skewed? For example, can the sea surface temperatures become anomalously colder by as many degrees as they can become anomalously warm? Furthermore, does the atmosphere, by which we refer to the rainfall, pressure and wind systems, respond symmetrically with respect to El Niño and La Niña? In other words, is the atmospheric response to La Niña merely the mirror image of its response to El Niño? Furthermore, does the atmosphere react twice as strongly, if the ocean temperature anomalies are doubled? Symmetry of course would greatly simplify the problem of predicting the expected effects of tropical Pacific SST anomalies on climate, but the problem is somewhat more complicated than this simple linear symmetric view of the world. Hoerling presented an overview of the issue of symmetry between El Niño and La Niña, using as a tool the results from several sophisticated numerical models of the atmosphere and their sensitivity to ENSO extremes. His purpose was to study the atmosphere's sensitivity in the extreme tails of the ENSO bell-shaped distribution, since the extreme events undoubtedly exert the largest impact, and for which any departures from symmetry would be critical to understand. Hoerling provided some examples from the observational data alone that, if one were to separate out the events according to their varying intensities, one would find that the weaker events tend to behave in a linear fashion. It is for the larger ENSO events that one tends to see asymmetry or nonlinear behavior in atmospheric and oceanic anomalies. His main point was that in several parts of the globe, including North America and even the tropical Pacific, the wintertime (i.e., December, January and February) climate anomalies associated with the extreme La Niñas were weaker than their El Niño counterparts. Also, the spatial patterns were not mirror images of each other, but instead the centers of maximum anomalies over the Pacific-North American region were shifted several thousand kilometers with respect to one another. These impressions, drawn from empirical analysis, were substantiated by results based on general circulation model simulations of the atmospheric response to warm and cold tropical Pacific Ocean temperature anomalies. Taken together, the observational and model results cast doubts over the simple notion of symmetry. He also noted that the climate system was much more deterministic with respect to tropical Pacific rainfall: a single ENSO event produces a reliably detectable signal across the tropical Pacific. Over North America, however, one needs to witness several El Niño or La Niña events in order to get a statistical sampling of the climate signal related to the SST forcing, and to be able to distinguish that from the the range of climate states that occur naturally in the absence of extreme ENSO conditions. In response to questions raised about the influence of temperature changes in other oceans, it was noted that SST changes in the Indian and Atlantic oceans and convection processes in the maritime region of Indonesia and Malaysia have been less well studied than those in the tropical Pacific. Key points:
Symmetry Between the Societal Impacts of El Niño and La Niña Michael Glantz (NCAR) opened the discussion on the symmetry of impacts by first drawing attention to the sharp increase in access to the Internet, which has helped to proliferate the number of websites devoted to various scientific and societal aspects of the ENSO cycle. At first, websites focused mainly on El Niño, but interest in La Niña has increased in recent times. Almost every website refers to the El Niño-La Niña process as one of symmetry in impacts, meaning that it believes that the opposite impacts of about equal strength will accompany either El Niño or La Niña. But a closer look at the local and regional societal and environmental impacts should prompt a call to move away from such a simple view of symmetry in impacts of the ENSO cycle. It is necessary and more useful to talk about societal impacts in terms of tendencies and probabilities of occurrence. Because people and governments have been blaming El Niño, rightly or wrongly, for many climate-related problems faced by their citizens and economies, a "Buyer Beware" label should accompany the information on (including forecasts of) ENSO extremes provided in some of these sites. Factors that cast a shadow on the claim of symmetry include, but are not limited to, the following: scientists do not agree on a precise list of La Niña or El Niño years; composite maps are misleading in that they average out the impacts of weak events with the strong ones; a reliance on a previous El Niño event as an analogue to identify or forecast impacts is also misleading because the set of impacts from one event to another will likely differ, depending on the influences of regional and local climate processes around the globe; and the number of observed events is too small as yet for reliable scientific projections. However, there are some locations around the globe where the societal impacts of El Niño and La Niña appear in general to exhibit some "usable" degree of symmetry: for example, Uruguay, Argentina, Brazil, Australia, Indonesia, Papua New Guinea, Peru, Ecuador, Chile, the US Gulf states, Central America, western Canada, and southern Africa. At present, there are several impacts maps that depict in a general way the impacts of El Niño and La Niña on precipitation and temperature in a number of countries around the globe. Most of these maps, although seemingly different, are graphic variations of those produced in the late 1980s. However, because these maps were produced in the late 1980s, it is time to update as well as to assess them critically. Although they tend to show symmetry of physical and, therefore, societal impacts, the sample size of La Niña events used to prepare the maps is small. Also, these maps are composites of the impacts of a set of events, but such composites mix the impacts of weak and strong events together. Thus, there appears to be symmetry but considerable detail is lost. However, if one were to separate the weak from the strong events, different maps would emerge, as suggested by Kiladis and Hoerling in other sessions of this workshop. While weaker events tend to behave in a similar and symmetric fashion, the stronger ones appear to manifest nonlinear behavior in the ocean or the atmosphere. Perhaps one can speak of symmetry between the societal and environmental impacts of El Niño and La Niña in much the same way that one refers to the greenhouse analogy when discussing the greenhouse effect with regard to the atmosphere. While the analogy of the atmosphere as a greenhouse has its beneficial uses (e.g., educating the public), it is inaccurate and misleading if carried too far. The same applies to the notion of the symmetry of ENSO impacts on environment and society. Key Points:
The Attribution of Societal and Environmental Impacts to Specific La Niña and El Niño Events Just about everything that happened between September 1997 and May 1998 that was either unwanted or unexpected was blamed on El Niño, i.e., the devastating ice storm in Quebec and the northeastern US, killer tornadoes in Florida and Alabama, heat wave and drought in Texas, coastal storms in California, and so on. However, just because there are weather and climate extremes that occur while an El Niño or La Niña event is in progress, there may be little reason to link them in terms of cause and effect. Gerald Meehl (Climate and Global Dynamics Division, NCAR) identified two major issues involved with attribution. The first is to what do we attribute the actual cause of La Niña. This involves mechanisms of ocean-atmosphere coupling, and are captured to various degrees in the coupled climate models used to forecast El Niño or La Niña. This type of attribution was left to other discussion sessions. Instead, the focus was on a second type of attribution. This concerns what type of climate anomalies or impacts we can attribute to La Niña. Here we separate the manifestations of occurrence of El Niño or La Niña from the climatic impacts of individual events. For the former, a number of studies have documented low-frequency decadal time-scale variability in the tropical Pacific that could affect the manifestation of El Niño and La Niña impacts. Thus, the record-setting El Niño of 1997-98 could be characterized as having a contribution from (1) coupled processes on the interannual time scale; (2) a warm phase in the decadal oscillation, and (3) a longer-term warming trend in the eastern Pacific. These effects, superimposed upon one another, could combine to produce the large El Niño we experienced in 1997-98. Similarly, these factors could affect the manifestations of individual La Niña events. Thus, we could think of attributing El Niño or La Niña occurrences to a combination of processes acting on different time scales. In the latter category of how we can attribute seasonal mean climate anomalies to individual El Niño or La Niña events, the concept was developed of a seasonal mean anomaly as a collection of meteorological events, or a shift of the probability distribution of certain conditions in a season in a certain region. It is very important that such attributions to La Niña events be reliable, because actions taken by societies will likely be taken based on the belief that La Niña events tend to spark certain kinds of adverse conditions, such as Midwestern drought conditions in the summer of 1988. If such an attribution proves to have been incorrect, societies will have wasted scarce resources to respond to unlikely adverse events. While there is a strong, almost irresistible, temptation to attribute every individual meteorological event to La Niña or El Niño, most of these attributions can not as yet be supported by scientific proof, with some exceptions (Figure 4). It was suggested that the best way to view attribution was to think of the forcing of sea surface temperature changes from the tropical Pacific as having the effect of shifting the statistical probabilities of a set of meteorological events over the course of a season toward wetter or drier, warmer or colder conditions. Key Points:
Media Panel Review of Reporting on the 1997-98 El Niño The media had a major role in fostering awareness among policy makers and the public of an upcoming El Niño in 1997-98 and its potential for devastating impacts around the globe several months in advance of its onset. As it happened, that event did turn out to be one of the two strongest in the 20th century. Media coverage of El Niño was so intense and often sensationalized that the media were accused of "hyping" El Niño in a negative way by generating "gloom and doom" scenarios, many of which were either premature or eventually proved to be incorrect. One workshop participant suggested that for all practical purposes "El Niño might as well have been the Spanish word for hype". Even some members of the media have begun to assess their performance during the recent El Niño, contending however, that the constant headline coverage of El Niño was dictated to them by the interests of the public in the topic. The tendency of the press coverage of El Niño, for example, to focus on adversity has been reflected in various newspaper headlines (Figure 5). Media representatives pointed out that a serious language problem existed between them and the scientific community. Causes of this problem include, but are not limited to, the following: very few science writers have scientific backgrounds; there is a tendency among scientists and the media to overstate conditions (the media tend to highlight extremes and shifts, while the scientists tend to present their efforts in a more positive light); time constraint is an important factor as it is difficult for the media to maintain interest in long term scientifically based issues; the media have the added difficulty of presenting a range of views thereby having to take note of outlying as well as minority views on scientific issues, even when those views might lack much support. It was proposed that scientists and the media should work closely together, exploring new ways to educate each other more effectively on the ENSO cycle and its impacts on environment and society. There was confusion between forecasting the El Niño phenomenon and forecasting its societal impacts several months following its onset. Successful forecasts of the latter (e.g., societal impacts) do not necessarily stem from forecasts of the former (e.g., the physical event). It appears that the media's interest in La Niña is following on the coattails of one of the biggest El Niño events in a century. It became clear very quickly that much less is known about La Niña events than El Niño, as it had received much less attention (until now) from the scientific community than El Niño had. Note that these media headlines (Figure 6) on La Niña are considerably less sensational than those for El Niño stories. Key Points:
How Well Did We Forecast the 1997-98 El Niño? Forecasting El Niño (and La Niña) is still fraught with difficulties, with some researchers suggesting that forecasting the onset of La Niña was a more difficult task than forecasting the onset of El Niño. Anthony Barnston, (NOAA's National Center for Environmental Prediction, Climate Prediction Center) began this session noting that, based on an analysis of 15 statistical and dynamical models, most models did forecast some degree of warming of the tropical Pacific sea surface temperatures one to two seasons prior to the onset of the 1997-98 El Niño in the Northern Hemisphere spring of 1997. However, none had detected its strength until the event was already on its way to becoming strong in the early summer of 1997. The dynamical and statistical models performed with about the same level of success during this episode. He noted that the most comprehensive dynamical models performed better than the simple dynamical ones. Once the El Niño had developed in mid-1997, a larger set of the models was able to forecast its peak in late 1997 and dissipation and reversal to cold conditions in late spring and early summer 1998. Because ENSO extremes usually begin to develop in the Northern Hemisphere spring or early summer and persist through the following winter, forecasting the tendencies of the teleconnected impacts in extra-tropical North America for the winter (when impacts are most pronounced) at lead times of about five months is not difficult. Given the strength of the 1997-98 El Niño and the consequent skill of the 5-month lead forecasts of U.S. impacts in the winter of 1997-98, the success of these forecasts of impact tendencies was noticed to an unprecedented extent by the media and, hence, the general public and policy makers. Because of the need for forecasts to be expressed verbally and also to be precise enough for meaningful utility and verification, a simple numerically based verbal classification system for describing ENSO-related forecasts was proposed. Key Points:
Forecasting the Onset of a La Niña in 1998-99 Nicholas Graham (International Research Institute/Scripps Oceanographic Institute) opened discussion on forecasting La Niña by noting that, as of mid-July 1998, the observed trend in sea surface temperatures in the tropical Pacific in the region referred to as Niño3.4 was from a warm event toward a La Niña. Cold SSTs were beginning to appear in the central Pacific. It was suggested that the relatively rapid decline of El Niño conditions in such a short period of time was more similar to those for the 1972-73 event than for the 1982-83 event. Several models projected months in advance a shift from a warm event in mid-1998 to a cold event by the winter of 1998-99. Although the dynamical and statistical models performed about the same, it was expected that in the future the dynamical models (producing an ensemble of forecasts) would produce more reliable forecasts than the statistical ones. It was noted that several models have different but consistent biases and that such biases should be taken into account in their forecasts. It is important to know the model biases, so that their forecasts can be judged against them. The behavior of the tropical Pacific in the first half of the 1990s, when the SSTs in the eastern Pacific (in regions referred to as Niño1 and Niño2) changed from a warm event to normal and back to a warm event a few times, showed that not every El Niño has been followed by a La Niña. Furthermore, one should not expect that the precipitous drop in SSTs in May and June 1998 would necessarily lead to a precipitous onset of a strong La Niña event later in the year (Figure 7). Key Points:
The Identification of Differences in Forecasting El Niño and La Niña Stephen Zebiak (Director of Modeling, International Research Institute) opened the session by noting that after 15 years or so of research, we have come to think of ENSO warm and cold events as a process, an oscillation, and not as a sequencing of random events. In the context of an oscillation, the same physics describes the whole state including both extremes. However, the succession of extreme warm and cold states have systematic differences. For example, it is believed that the cold states do not depart from an average condition to the same extent that warm states do. Also, some decades have been inactive and others have been dominated either by cold events (earlier in the century) or by warm events (in recent decades). The physical differences between the extreme warm and cold events relate to changes in the thermocline depth, the spatial extent of convective activity, and a shift in the zone of convection among others. However, even though some of the physics at work in the two extremes are different, the predictability of El Niño and La Niña is not much different. Four key factors were noted as limiting forecast skill: model flaws, flaws in the way that data are used, gaps in the observing system, and the inherent limits of predictability. It was not clear whether the terms used to describe the varying strengths of El Niño — weak, moderate, strong, very strong, extraordinary — could be used to describe the varying strengths of La Niña events. La Niña events have been fewer, so the number of cases for statistically reliable assessment is small. Questions about the definition of La Niña once again surfaced. Some argued, as noted earlier, that perhaps there are two states in the tropical Pacific, El Niño and non-El Niño (or normal). Such a distinction would have a major effect on looking for similarities and differences between forecasting La Niña and forecasting El Niño. Key Points:
Monitoring La Niña Historically, El Niño researchers have used regions in the tropical Pacific referred to as Niño3 and Niño3.4 to generate indices of SSTs that are convenient for describing the evolution of an El Niño event (Figure 8). This session focused on whether these historical indices were appropriate for describing and understanding the character of La Niña as well. Stated another way, is what we believe to be an appropriate monitoring system for El Niño equally appropriate for monitoring La Niña? Antonio Busalacchi (National Aeronautics and Space Administration's Goddard Space Flight Center) presented the argument that the TOGA TAO array was originally set up as an equatorial waveguide array to monitor the development of El Niño (warm) events. Construction of the system began in 1985 in large part as a response to the 1982-83 event. It was completed in by the mid 1990s. While noting that the development of the 1997-98 El Niño event was captured by the TAO array (8 deg N and S latitudes), it appears that the changes in ocean temperatures below the surface that signaled the trend toward a cold event were taking place outside the 8 N and S latitude band. In fact, the first sign that a change was in the offing was detected by the SeaWiFs satellite instrument and showed up in satellite color images of biological productivity a few degrees north of the Equator in the area of the Galapagos Islands. The return of upwelling in the near equatorial region signaled a shift away from a warm event a couple of months before other indicators captured physical changes in the ocean and atmosphere environments. Such occurrences underscored the need to expand the TOGA TAO array toward the west and the east (zonally) and poleward (meridionally). Because ENSO's teleconnections are affected by local and regional factors, it was urged that similar monitoring systems be established for the Indian and Atlantic oceans as well. A case was made for the need to monitor salinity changes in the tropical Pacific in order to provide yet another indicator of changes in the ENSO oscillation. Key Points:
Working Group Sessions The final session of the La Niña Summit involved dividing the participants into two groups. One group was asked to discuss what it is that the user/application/impacts community needs and wants from the ENSO research community. The second group was asked to focus on the needs and wants of the scientific research community. The user/application/impacts community discussions centered on the following suggestions:
Discussions in the research community group centered on the potential utility of a mutually agreed-upon multivariate index to include factors denoting magnitude as well as threshold criteria, and discrete categories. Suggestions were also raised concerning the possible value of "official" forecast products (this generated a heated discussion within the group as well as within the full workshop; some participants claimed that it would serve to indirectly censor information on El Niño or La Niña not considered to be part of the mainstream perspective). Concern was raised, and steps called for, related to the issue of identifying accountability and credibility of each of the growing number of ENSO and ENSO-related forecasts. The comments compiled by each of the working groups were presented to the full workshop for brief discussion in the time remaining. Thus, it is important to note that the results of these discussions were only expected to be suggestive and did not represent an exhaustive list of needs. To reference this paper: |
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