The "El Niño" FAQ:
Frequently Asked Questions About
El Niño-Southern Oscillation (ENSO)
(Revised April 2003)
David B. Enfield1
1 NOAA/AOML, 4301 Rickenbacker Cswy, Miami, FL 33149
Preface - Since the mammoth El Niño event began around June, 1997, we have been amazed at the media coverage El Niño has garnered. It has been our observation, however, that much of the play in the media has ranged from superficial to misleading – and people still seem to be greatly confused. In what follows, we list some of the questions we have received from the ordinary "man in the street", along with our responses. This material does not necessarily reflect the official stance of the National Oceanic and Atmospheric Administration (NOAA).
Reader input in the form of questions or comments is welcome You may contact us at the above snail-mail address or send us e-mail at: David.Enfield@noaa.gov.
• What is "El Niño" and how does it start?
El Niño is an unusual warming of the tropical Pacific Ocean that occurs irregularly at about 3-6 year intervals in response to large scale weakenings of the trade winds that normally blow westward from South America toward Asia. Normally, the trade winds produce cool surface water in the eastern Pacific, through evaporation and the upwelling of colder water from below the surface. Simultaneously, they "corral" warm surface waters over in the far western Pacific. As the trade winds weaken, so does the containment of the warm water in the west and the maintenance of the coolness in the east. As a result, relatively warm water becomes ubiquitous all across the Pacific from New Guinea to South America. Although the immediate cause (wind weakening) is known and scientists have made much progress in understanding the phenomenon, the exact nature of the processes that govern its repetitive cycle are still not certain.
Most definitely. El Niño may be thought of as one of Earth's standard mechanisms for getting heat from the tropics (where more comes in from the sun than goes out) to the polar regions (where more heat returns to space than comes in). Ordinary winter storms also do this. Without these poleward transports of heat, the planet would be an unbearable hothouse in the tropics or too cold for habitation toward the poles. In the years between ENSO events, excess heat accumulates in the tropics and then gets "exported" during El Niño. It's somewhat like the accumulation of winter snow on a steep mountain slope. The snow cannot accumulate indefinitely, defying gravity; inevitably it must give way to avalanches. What happens during an El Niño "avalanche" is that a lot of excess heat gets transported poleward, most frequently through winter storms. That is why places like California and Chile have rougher winters in conjunction with El Niño.
El Niño is also responsible for a good deal of diversity in plant and animal life because the periodic stress it puts on biological systems is a stimulus to the evolutionary process. Moreover, the affected biota on both land and in the sea are much more resilient as a result of the need to survive these periodic upheavals in the environment.
• What is the history behind the term "El Niño"?
Our first knowledge of this came from Peruvian geographers, who at the end of the 19th century were interested in the unusual climate aberrations that occurred along the Peru coast in the odd year. They took note of what a knowledgeable ship captain said about the fishermen in northern Peru, who typically saw a switch from cold to tropical ocean conditions around Christmas of every year and attributed this to a southward setting, warm "El Niño current". The term was an obvious reference to the Christ child. We don't actually know how mythical this story might be, but that is the tale that got passed on. The geographers noted that in some years the onset of warm conditions was stronger than usual and was accompanied by unusual oceanic and climatic phenomena.
Starting with the arrival of foreign-based scientific expeditions off Peru in the early 20th century, the concept gradually spread through the world's scientific community that "El Niño" referred to the unusual events. The annual occurrence was forgotten, although one geographer (Eguiguren) lamented this inaccuracy. It was separately noted by Sir Gilbert Walker in the 1930's that notable climate anomalies occur around the world every few years. These were associated with what he called the Southern Oscillation (SO), a large fluctuation in atmospheric pressure. In the 1950's, Berlage observed that the SO-related climate anomalies generally coincided with El Niño occurrences. It wasn't until about 1960 that scientists came to realize that the warming off Peru is only part of an ocean-wide perturbation that extends westward along the equator out to the dateline. About the same time, the noted meteorologist Jacob Bjerknes proposed that El Niño was just the oceanic expression of a large-scale interaction between the ocean and the atmosphere and that the climate anomalies could be understood as atmospheric "teleconnections" emanating from the warm-water regions along the equator in the mid-Pacific. The catchy term "El Niño" is frequently abused in the popular vernacular through the tendency of people to confuse what is essentially an oceanic happening with the climate anomalies that are associated with it.
Starting in about 1975, oceanographers and meteorologists began to combine their efforts to expand and refine the Bjerknes hypothesis by systematically studying the El Niño and the Southern Oscillation together in what we now call "El Niño–Southern Oscillation", or ENSO. The advent of powerful computers and modern measurement systems has caused a rapid acceleration in our understanding of ENSO, especially since the large event of 1982/83.
• How often do ENSO events occur, and are they always so strong?
On average they occur every 3-6 years but only irregularly and not as predictably as the astronomically controlled tides. As measured by the degree of warming, every other event tends to stronger or weaker, with the strong ones occurring only at 8 to 15 year intervals. The intervening weak and moderate events do not typically bring such disastrous consequences. The events of 1982/83 and 1997/98 were unusually strong, equaled historically only by events in the late 1800s. Really big events like 1982/83 and 1997/98 occur only a few times in a century.
• I've heard about something called "La Niña". What's that?
"La Niña" refers to certain years between El Niño events in which most of the El Niño characteristics are reversed, including the tropical Pacific ocean temperatures, which are colder than average. Because it is contrary to El Niño (cold instead of warm) and its climatic effects are frequently opposite as well (droughts instead of floods, and vice versa), it is given the contrasting feminine name in opposition to the masculine "El Niño". The way we characterize ENSO anomalies is to subtract long-term averages from our data. During the intervening years between El Niño events the equatorial Pacific Ocean is somewhat cooler than that average, since together with the warm El Niño events they must sum to the average. So you might say that La Niña is an inevitable consequence of how we calculate the anomalies. However, in some years the equator is unusually cold, though never as cold as a strong El Niño is warm. This prompts some scientists to think that La Niña is a physical phenomenon in its own right, or at the very least, the opposite phase of an "ENSO cycle". Scientists have never predicted La Niña and it is difficult to agree on what constitutes La Niña. La Niña events with significant impacts are fairly unusual.
• Is El Niño something new and unusual in the Earth's climate?
No. El Niño has been occurring at least since people started putting thermometers in the ocean around the middle of the nineteenth century. Moreover, archived documents left by the Spanish colonists in Peru confirm that El Niño impacts such as occur now (flooding, marine life disturbances, etc.) have been felt in Peru ever since the first conquistador (Francisco Pizarro) set foot there in the early 16th century. And, as far as we can tell from paleo-climatic indicators such as geological evidence & tree rings, El Niño has been occurring for at least thousands of years, probably much as it has during this century. It will probably continue to occur as long as our climate system works the way it has since the most recent ice sheets of the late pleistocene receded (i.e., needing to get rid of excess tropical heat as explained in the question Does El Niño play a special role in Nature?).
• So, did El Niño also occur during the ice ages?
We don't know yet. The global climate was very different then and the need for a heat-exporting mechanism may not have existed. As you might guess, however, scientists are anxious to know this because it will help us to understand how sensitive the features of our present climate system (such as ENSO) are to significant changes in the climatic background state. Clearly we are altering our present climate, so we need to understand what kinds of changes in weather systems might be expected as a result.
• So, will the global warming due to greenhouse gases (exhaust from our cars and factories) cause ENSO events to become more frequent or more severe?
We don't know that for certain. Some studies suggest this may be true while others cast doubt on the idea. Part of the problem is that natural changes in the frequency and intensity of ENSO events have occurred in the last five centuries for which records exist, and it is hard for us to distinguish those from recent characteristics that might otherwise be attributed to greenhouse warming. This is also a subject of great interest in research. Unfortunately, while ENSO intervals are well matched to the political time scale that governs our research funding (3-4 years), global warming is not.
• Do weather patterns change during El Niño?
Absolutely, and rather drastically in the case of the stronger events, such as 1997/98. If all that occurred during an El Niño were a warming of the equatorial ocean where nobody lives, El Niño would not occupy the public awareness as it now does.
• El Niño gets talked about in terms of both climate and weather. What is the distinction between these two things and how does it help us to understand the effects of El Niño?
"Climate is what we EXPECT,
weather is what we GET."
[Robert A. Heinlein]
To understand the twin concepts of climate and weather and why they should be affected by El Niño, think of the atmosphere as a huge pot of fluid (our atmosphere is a mixture of gaseous fluids, after all) on a stove with heating elements (hot spots of warm tropical ocean temperatures) located under certain portions of the pot. This creates a characteristic circulation (climate) in the pot, with warm fluid rising over the hot spots and moving away toward the cooler regions where it sinks (the Earth's higher latitudes). The circulation has embedded turbulence, or eddies (weather), that are inherently less predictable than the average circulation itself, but nevertheless conform to a statistically expected behavior (again, climate). As thermal anomalies develop in ocean temperatures during El Niño, displacing cold water in certain regions, the distribution and intensity of the hot spots under the pot are changed; naturally, so too does the circulation pattern in the pot change, along with the statistical expectations for the turbulence (i.e., the weather). Although we cannot predict the precise nature of the altered weather far in advance, we do know more or less how different it will be on average.
• Are weather patterns variable from one El Niño to the next, & why?
They do indeed vary to some extent. Here's the accurate, if somewhat technical explanation from NOAA/Climate Prediction Center:
"In response to the more uniform pattern of heating in the tropics during an El Niño the wintertime jet streams in each hemisphere tend to be more uniform from east to west and extend farther east than normal. However, the timing, location and magnitude of the ocean warming varies from one El Niño to the next, which results in variations in the patterns of tropical rainfall and deep tropical heating. These conditions contribute to variations in the precise location, strength and structure of the mid-latitude jet streams over both the North and South Pacific from one El Niño to the next, and thus to the variability in weather patterns and storm tracks over North and South America.
"A second major reason for the variability in weather patterns from one El Niño to the next is simply that El Niño is not the only factor influencing the weather and climate. In particular, the atmosphere exhibits considerable variability on time scales ranging from days to seasons to years, and this variability often reflects nothing more than the normal chaotic behavior of the atmosphere. This description is particularly applicable to areas such as eastern North America, the North Atlantic, Europe, etc., which are heavily influenced by features such as the North Atlantic jet stream. "
There will continue to be surprises associated with future El Niño events. Scientists have really only focused on ENSO as a large scale phenomenon since the mid 1970s. We have not witnessed all the forms these events can take nor have we recognized all the ways they can affect societies and ecosystems.
• Is the occurrence of a strong "El Niño" always synonymous with disaster and Armageddon?
No. Again, a word of caution from NOAA/CPC is appropriate here:
"When interpreting the climate information linked to El Niño it is important to note that while abnormal temperature and rainfall patterns can and sometimes do result in severe climate conditions, they do not imply calamitous conditions in many instances. For example, the above-normal precipitation expected in the Southwest and southern plains states implies a reduced chance for wintertime drought such as occurred during November 1995 - May 1996. In Florida the above-normal rainfall expected this winter indicates reduced chances of wildfires. In California, the potential impacts from El Niño can be severe. However, the above-normal rainfall across the state during the 1992-93 El Niño resulted in an end to severe long-term drought conditions that had persisted since 1986/87, and to a much-needed replenishment of water reserves."
• What are the known effects of El Niño on marine life?
El Nino affects marine life mainly through the drastic changes that occur in the Pacific ocean, especially along the equator and the Pacific coasts of North and South America. The two principal factors are (1) the intense warming in regions of normally cool, upwelled water, and (2) the reduction in the supply of high, subsurface nutrients that normally upwell in the same regions. During El Niño changes occur in the distribution and abundance of many species. During the milder El Nino events, the cold-loving Peruvian anchovy becomes scarce off Peru and more prevalent in the cooler Chilean waters to the south. In some instances the anchovy has been replaced by population increases of pelagic species that do better in warmer water, such as sardine and Spanish mackerel. Not only does the anchovy not like the warmer water, but the associated decrease in nutrients has a negative impact on the abundance of its principal food source: the microscopic algae (phytoplankton) that are normally so plentiful along the productive Peru coast. During strong events many other species are also affected and changes in species distributions can be seen as far away as the Gulf of Alaska.
• So, what happens to marine fauna during the stronger events?
In general, species that like warmer water become more prevalent in the cool-water regions off the coasts of North and South America. Both coasts frequently see increases in certain nearshore benthic (bottom) fauna, such as shrimp and scallops, which reproduce and survive better in the warm water (they are not great migrators). Migrating species of offshore pelagic (mid-water) fish, such as dolphinfish (also known as dorado, or mahi-mahi) typically invade the normally cool coastal waters in greater numbers; other tropical species, including popular sportfish like yellowtail, may be found far poleward of their normal distribution (much to the liking of deep sea fishermen in California). Cold-water fish such as salmon may be found closer to the poles, migrating from Oregon-Washington to the Gulf of Alaska. The marine ecosystems of both continents, from the microscopic phyto- and zooplankton to the largest predator fishes, may be altered for up to a year during a strong event such as 1997/98.
• What happens to species that can't migrate, such as corals?
Corals have a hard time during the more intense events. In 1982/83 (a very strong event) many of the coral species of the Galapagos Islands were killed in large numbers. Fifteen years later in 1997/98 many of the previous survivors perished as well. Many coral species depend on a symbiotic relationship they have with algae-like species (called zooxanthellae) that live in the gastrodermal tissues of the corals and increase the availability of food. When the water warms too much, the zooxanthellae disappear; the white color of the coral skeleton ceases to be obscured by the darker zooxanthellae and the corals are said to "bleach". If the waters remain warm for too long, the prolonged bleaching stresses the coral metabolism to the breaking point and they die. The good news is that corals are capable of re-establishing themselves in decimated areas, possibly after several years, by means of immigrating larvae from distant surviving populations.
Oddities and potpourri about El Niño:
• I heard somewhere that El Niño slows down the rotation of the earth. True?
El Niño cycles cause a correlated but small fluctuation in the "length of day" (it gets longer), so yes, this is true. The reason it happens is that the entire Earth system (solid earth, air and water) must conserve its total angular momentum (related to the speed of rotation around the earth's axis), like a spinning top, or a twirling ice skater. During El Niño, the average eastward speed of the winds around the globe increases, which is related to the reason California and Chile get more and stronger winter storms (see Does El Niño play a special role in Nature?). Since the angular momentum of the air increases, the rotational speed of the solid earth must decrease, in compensation. Of course, the resulting increase in the length of the day is very small, so don't worry about arriving early for your next appointment...
• February 25, 1998: "When will this freaking rain stop???"
This complaint came from a Californian as the severe El Niño winter extended into spring. Starting at the beginning of February, a seemingly endless series of storms, one after another, assaulted the California coast with flooding, erosion and misery. Some were stronger than others but all were significant in comparison with those of a normal winter. Instead of tracking across the Pacific south of the Aleutians and into the Gulf of Alaska, they crossed Hawaii straight into California, picking up moisture and energy from the heat-laden tropical Pacific (El Niño). From there they continued across the desert southwest, dropping rain and snow from Arizona to Texas, where they picked up new moisture from the nearby Caribbean and Gulf of Mexico, finally continuing eastward to drench the southeastern states, frequently spawning tornadoes as they passed. During the 1982/83 event, a series of 13 storms followed a similar pattern starting at the beginning of January and did not let up until early April.
• During an El Niño event, when do the weather problems hit California, and how long do they last?
Off and on throughout the winter, if El Niño is strong then. Then it snows a lot in the mountains and it rains a lot elsewhere in California. Some of that is actually good, such as for skiers. In both 1982/83 and 1997/98, the worst effects were felt after the New Year. Possibly the worst thing that can happen is in the spring if El Niño is still going strong as occurred in 1983. Then you can get a rapid wetting and melting of the large snowpack in the western mountains, producing severe flooding in some areas. This can be very bad. It's something the folks who manage water supplies (dams and such) have to watch out for.
• In the summer of 1997 a Pacific hurricane threatened California and there were predictions that increased hurricane frequency and intensity could spell disaster, that monsoons would hit northern California, etcetera.
For the most part, such talk was unwarranted. First of all, monsoons are not hurricanes, they are periods of significant seasonal change in tropical weather and wind direction. All major continents are affected by monsoons, and they may be stronger or weaker in association with El Niño. However, a hurricane in California is only remotely possible, even during an El Niño. The possibility that California could get hit by a major hurricane is very small, especially during the period that El Niño is expected to produce its strongest impacts (California's winter).
• Why is the idea of a California hurricane so implausible?
The ocean temperatures off California – including during a strong El Niño – are so cold that they would grind down a hurricane to nothing more that a strong, rainy windstorm. Historical statistics indicate that a major California hurricane is no more likely in an El Niño year than in a normal year. Such an event has never occurred since hurricane records began, so we can only guess that such a thing might happen once in a millennium or so.
• OK, but amid all this talk of California hurricanes, we also heard TV forecasters saying that El Niño had knocked down the chance of hurricanes. This confused me!
You have to keep your oceans straight. In the Pacific, El Niño has little discernible effect on hurricanes close to the continent. Atlantic hurricanes are an entirely different matter. There, you see, El Niño almost always reduces the frequency of storm maturation from what it would otherwise be, just as the TV weathercasters say. This is because El Niño produces increased wind shear ("scissors effect") over the tropical North Atlantic in the region where storms born off northwest Africa try to mature into hurricanes. This shear, or wind difference between the high and low levels of the atmosphere, tears many of the developing storms apart before they can become serious threats.
• Where else in the Americas are severe effects felt?
Best known, of course, are the coastal regions of Ecuador and northern Peru, where so much rain falls that flooding, landslides, erosion and consequent property destruction reach disastrous proportions. The moisture causes explosive infestations of insects and associated problems with public hygiene that favor the development of water-born and mosquito-vectored diseases (cholera, malaria, dengue, etc.). In the deserts of Peru and Chile, entire "pampas" bloom with flowers where nothing but a barren expanse is normally found. In the far north of Peru, shallow lakes appear on the desert, with ecologies of plants and fishes that temporarily sustain migrant human populations. Farther south, both Chile and Argentina, and even southern Brazil, experience severe storms during the austral winter (June-August). Northeastern Brazil goes through severe drought in March-May, with consequences for its agriculture. Dry conditions over much of Central America and northern South America also affect agriculture and additionally cause deficits of hydroelectric energy and drinking water.
• What about other regions of the world?
El Niño impacts are typically quite severe over southeast Asia, including Malaysia, Indonesia, New Guinea, Borneo and northern Australia. Throughout much of that region drought is common, agriculture is affected and tropical forest fires become a problem. Although western Pacific typhoons (overall) are no more frequent than normal, they tend to affect areas farther to the east, as far as Tahiti, that are otherwise less affected. To the west there is typically a failure of the summer (wet) monsoon over the Indian subcontinent, whose populations depend critically on the monsoon rains. Finally, droughts are known to occur in southern Africa and northeastern Africa.
• Are the effects of El Niño all bad?
No. During El Niño winters, for example, Florida receives significantly more rain than normal, which reduces the risk of wildfires in the spring and early summer. Reservoirs fill and skiers and sport fisherman are happy in many places. The formation of desert lakes in northern Peru form a temporary habitat for vegetation, freshwater fish and farmers.
• Is there an El Niño in the Atlantic Ocean?
There are two kinds of El Niño phenomena in the Atlantic. One is a spin-off from the Pacific El Niño, due to transmission through atmospheric fluctuations. This atmospheric signal from the Pacific El Niño is the same phenomenon that causes the Atlantic to experience fewer hurricanes during El Niño years. The result of that "teleconnection" is that the tropical Atlantic usually experiences a smaller but anomalous warming several seasons after the maximum warming in the Pacific (which usually occurs in December).
The other "Atlantic El Niño" effect is a non-synchronous and aperiodic warming that occurs along the equator, entirely due to internal Atlantic dynamics. It is only "El Niño-like" in the sense that those dynamics are similar to the Pacific case, but it has no correlation with Pacific events. Moreover, the magnitude of the warmings is much smaller, as is the typical period between events. Hence, it is not of much consequence and should probably not be called "El Niño", as this would create unnecessary confusion.
• Is it true that El Niño years are good wine years in Europe?
Not convincingly so. There is some statistical evidence that European winters may be slightly more severe during El Niño. However, the consistency of this is fairly low and there are numerous historical exceptions to the pattern. The effects during the summer growing season are even harder to document.
• I have heard about predictions. What exactly is being predicted?
Two kinds of prediction are being made: (1) prediction of "equatorial warm events" in the Pacific and (2) predictions of the impacts of those events. The term "warm event" refers to a large scale warming in the tropical Pacific and is frequently a euphemistic way of saying El Niño event or "El Niño-like" without actually saying "El Niño". Sometimes these events are too weak to be called "El Niño" in any universally agreed- upon way, and to call it such in advance (given our poor ability to predict its magnitude) would be to irresponsibly encourage unwarranted speculation about its impacts, which will be minimal for the marginal cases. These predictions are done routinely once or several times a year by a number of scientific groups around the world, utilizing various kinds of numerical and/or statistical models of how the oceans and atmosphere interact with each other. Some of the predictions may be found through internet links to the World Wide Web (WWW). NOAA/CPC provides the U.S. "official" predictions. The International Research Institute for Climate Prediction (IRI) disseminates predictions with a global scope.
The predictions of impacts are done more empirically and more informally, and usually only in connection with a warm event that is already known to exist and is strong enough to be called El Niño without controversy. A prediction of impacts is typically based on what we know historically about climate responses to (or correlations with) El Niño occurrences. It is a probabilistic statement that certain climatic conditions can be expected more or less frequently than normal due to the existence of El Niño conditions. Sometimes these predictions are nothing more than the response of a scientist to a reporter or an interviewer. The U. S. National Oceanic and Atmospheric Administration (NOAA) routinely publishes formal climate outlooks and advisories for the coming season (available via the WWW and elsewhere). These are based on applied research by NOAA and others. When a strong El Niño is in progress, these outlooks typically warn about unusual conditions expected from El Niño.
• How far in advance can we predict the existence and the intensity of an El Niño event?
Accurately? In terms of timing and magnitude, hardly at all. In terms of saying that an "equatorial warm event" (only vaguely defined in magnitude) will start within a particular six-month time frame, we can typically foresee that up to a year in advance. Some prediction models succeed in doing this on one event and then go bust on the next. There's a good example of that in 1997. Accordingly, we have come to rely more on a "consensus forecast" of many models rather than on any one model. But the long-lead accuracy still leaves a lot to be desired. Most models correctly anticipated a "warm event" for the 1997/98 winter as early as one year earlier. However, none of the predictions anticipated that strong anomalies would already be in place by June, 1997, while their predicted magnitudes were small compared with what actually occurred.
• Some have suggested that the dire predictions we heard during the 1997 summer (once the El Niño was underway) were premature. Is that true?
There was a lot of misinformation and unfounded speculation, especially early in the event when we had no accurate way of knowing how strong it would be. Although the event indeed proved to be very strong, there was little basis at the time for making those dire projections. But even within the parameters of those projections the interpretations in the media were inaccurate, misleading and unnecessarily alarming. The misinformation included things like the NY Times saying (in July) that sea temperatures (not their anomalies) were as large or larger than ever recorded. In fact, the measured temperatures on the equator were well below previous El Niño highs; it was the difference between the measured temperatures and what is normal for that time of year that was extraordinary. The distinction, in terms of climate impacts, is crucial. The prediction (in June 1997) that Californians would see a winter with several times their normal rainfall did prove accurate, but no attempt was made to prevent that projection from unnecessarily alarming people in places where no danger was likely. When Reuters carried these news releases to places like Ecuador and Panama (where strong El Niños really ARE disastrous), it caused a general panic. Banks closed credits for crop planting and insurance companies refused to offer crop insurance, even though under the direst scenario the severe agricultural impacts could not occur until much later. Clearly, we have a long way to go with how we handle ENSO-related information in the public eye.
• If the predictions of a watery grave are not reliable, can we make any predictions at all?
Certainly. We can say that "a warm event will occur" with enough advance notice to be useful and that will probably prove to be correct in most instances. The projections of climate impacts, given that an event will probably occur, are more tenuous. Such projections are probabilistic, aimed at the center of our expectations, not at the fringes of what is remotely possible, and they are still not terribly accurate, though much better than anything we could do 20 years ago. If regional ENSO climate outlooks are correct 3-4 times out of every five, then people who use them are ahead of the game over the long term.
• So, does our knowledge that an ENSO is occurring allow us to predict when and where things like winter mudslides will occur?
No, no more than we can with earthquakes. We can say their likelihood is greater, over a season, and if summer brush fires have occurred in a particular hilly location (making it more susceptible to mudslides), we can put out a danger alert and perhaps take measures to offset that probability of occurrence. Climate prediction is a game of likelihood, a way of rigging the coin toss so that our call comes up better than 50:50. But there are no guarantees.
Nevertheless, once the event is underway and we have the observations to tell us how strong it is, we can then make some intelligent guesses about how it will develop, and based on past experience we can make some fairly good projections about impacts for the coming season. To whom those projections will prove useful will depend on where they live and what time of year it is. By the time the next big event comes around (10-15 years from now?) our advisories should be much better.
• What are the principal changes that take place in the equatorial Pacific Ocean when El Niño occurs?
Before El Niño occurs the Pacific trade winds blow steadily from east to west across the Pacific. To maintain its balance the ocean must “lean into” the wind by being higher in the west than in the east (like a person leaning windward in a gale), thus creating a “pressure gradient force” toward the east in opposition to the westward wind stress. Under these normal conditions the height of the ocean surface in the western equatorial Pacific is greater than in the eastern Pacific by a few tens of centimeters. The near surface temperature in the west is about 10°C (20°F) greater than in the east and the thickness of the upper warm layer of the ocean is about 120 meters in the west as opposed to only 30-40 meters in the east.
At the onset of El Niño conditions the trade winds slacken across much of the basin. The eastward tilt of the sea surface is then unbalanced and a series of ocean responses occur over a half-year period that lead to a more nearly flattened sea surface with less east-west contrast in temperatures and upper layer depths. The most noticeable change to an observer is the remarkable increase in the equatorial sea surface temperatures over the eastern half of the basin, which is normally much cooler.
• Why do the east-west differences in sea level height initially exist and what happens to reduce them?
Because the upper layer of warm water is so much warmer and thicker in the west, the density of a column of water there is less than in the east, where the water is colder and denser. The volume in the west must therefore be greater and the column height is higher. Under strong trade winds, characteristic ocean circulations exist that maintain these density and volume differences. As the winds fail the ocean circulations change so as to distribute the upper layer water more evenly across the Pacific. The upper layer in the west becomes thinner and in the east it becomes warmer and thicker. With the volumes and densities being more equal the sea level difference is lessened or eliminated.
• Does this mean that the warm water initially found off New Guinea in the western Pacific winds up off the coasts of Peru and Ecuador?
No. The process of adjustment is wavelike — more like a slosh in a bathtub or a lava lamp. Parcels of water do not have to travel great distances in order to reduce the upper layer thickness in the western Pacific or to expand the layer in the east. Drifting buoys released by research vessels show that eastward setting currents on the equator do move parcels toward South America but only over a fraction of the basin width. If this seems strange, consider the familiar experience of a person swimming just beyond the surf at a beach. As a swell overtakes the swimmer, a surge of water carries the person only a short distance toward the beach, yet the deformation of the water column and sea surface (the wave crest) travels a comparatively immense distance.
• Then how does the water off South America become so warm if not by importing warm water all the way from the western Pacific warm pool?
The eastern Pacific warms because El Niño suspends the local process normally responsible for its extraordinary coolness. Because the upper layer in the eastern Pacific is normally so thin, the colder water of the deep ocean is much closer to the surface. The processes of mixing and upwelling due to wind action then bring this available colder water to the surface, producing an impressive 10°C (20°F) cooling compared to elsewhere in the tropics. When El Niño causes the upper layer (in the east) to thicken considerably, the underlying cool water is depressed to greater depths and becomes unavailable to the upwelling and mixing, which are comparatively shallow processes. The normal cooling fails to occur and we are left with a warm anomaly.
Web links to other El Niño information:
Others who have contributed to this FAQ: D. Michael Enfield; Michael H. Glantz; Christopher W. Landsea; Stanley Goldenberg; Peter Glynn; David Atlas; Lisa Davis; Graham Jenkins, Kurt Baldenhofer.