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Not So Frequently Asked Questions about El Niño Courtesy of Billy Kessler Oceanographer, Pacific Marine Environmental Laboratory/NOAA (adapted by Mark Hoover) Click on any question to go directly to its answer, or just scroll down and browse.
The name El Niño (referring to the Christ child) was originally given by Peruvian fishermen to a warm current that appeared every year around Christmas. What we now call El Niño seemed to them like a stronger version of the same event, and the usage of the term evolved over time until it only referred to the irregular strong events. It wasn't until the 1960s that people started realizing this was not just a local Peruvian occurrence, but was associated with changes over the entire tropical Pacific and beyond. In effect, El Niño was too big to be seen as the mega-event it is; it just seemed like a lot of unconnected unusual weather events around the world. The name El Niño as scientists now use it refers to the warm phase of a large warm/cold oscillation in the water and atmosphere of the Pacific region. The complete phenomenon is known as the El Niño/Southern Oscillation, abbreviated ENSO. The warm El Niño phase typically lasts for approximately eight to 10 months. The entire ENSO cycle usually lasts about three to seven years, and often includes a cold phase (known as La Niña) that may be similarly strong, as well as some years that are neither abnormally hot nor cold. However, the cycle is not a regular oscillation like the change of seasons; it can be highly variable in strength and timing. At present we do not fully understand what causes these changes in the ENSO cycle. The Southern Oscillation was named by Sir Gilbert Walker in 1923, who noted that "when pressure is high in the Pacific Ocean it tends to be low in the Indian Ocean from Africa to Australia." This was the first recognition that changes across the tropical Pacific and beyond were not isolated phenomena but were connected as part of a larger oscillation. Walker was Director of Observatories in India and was mostly concerned with variations in the Indian monsoon and the huge consequences that too much or too little monsoon rain could have in India. The first modern scientific description of the mechanics of El Niño/Southern Oscillation was made by Professor Jacob Bjerknes of the University of California, Los Angeles in 1969; our knowledge of the Earth's largest and most powerful weather engine is still incomplete. Why is El Niño frequently described as a disruption of the usual situation in the tropical Pacific? To understand El Niño, you have to think about the normal tradewind system in the tropical Pacific. The sun heats the equatorial regions more strongly than the rest of the globe, so a lot of heated air tends to rise from the surface there, to be replaced by inflow from the subtropics. Have you ever tried to walk straight across a merry-go-round while it was revolving? Then you know something seemed to push you sideways as you walked, and your actual path was curved diagonal line. That sensation of sideways motion is due to something called the Coriolis Effect, and it applies to both a merry-go-round and the spinning Earth. On Earth, inflowing winds from higher latitudes try to rush straight to the equator, but the Coriolis force turns these inflows to the right in the northern hemisphere and to the left in the southern, resulting in the great tradewind belts that blow towards the equator and westward over the width of the tropical Pacific. In the ocean, these winds tend to push the surface water towards the west, so sea level at Indonesia is usually about 50 centimeters higher than at the South American coast, which gives an idea of the power and steadiness of the tradewinds. As surface water is pushed west, cooler subsurface water is drawn up to the surface in the eastern equatorial Pacific, making the entire region about ten degrees Fahrenheit cooler than in the west. This cool water from the deep layers of the ocean is full of nutrients, and supplies food to the plankton, which form the base of the eastern Pacific food chain. That's why the eastern Pacific supports vast communities of fish and other sealife, as well as birds who eat fish. During normal conditions, as seawater journeys west, the sun steadily heats the layer of surface water over a huge area west of the International Dateline, known as the West Pacific Warm Pool. This air is also very humid because the warm water it's been riding over has been steadily evaporating. When this air rises in the west, heavy precipation falls over Indonesia and Southeast Asia as the moisture condenses back out as rain. As water condenses from vapor to liquid, it throws off heat. It is this heat—which can be vast—which energizes the atmosphere, creating storms. The warm pool is one of the major driving forces of world climate. Its heat strengthens the rising motion in the west and thereby reinforces the westward winds of the tradewind system. The rising, moisture-laden air pumps heat and water vapor into the upper atmosphere, where it can be carried great distances. The huge source of heat helps set the path of the jetstreams (stormtracks) that control and direct temperate-zone weather, much as a boulder in a riverbed determines the pattern of water flow, including wavy motions and ripples that extend well downstream of the rock. In effect, a huge mass of rising warm moist air acts like a big boulder stuck up into the atmosphere, deflecting the rivers of air flowing in the midlatitude jet streams. The effects of this ripple outward to affect much of the world's weather. During El Niño events, the normal pattern relaxes. The tradewinds weaken, particularly west of the Dateline, and the piled-up water in the west sloshes back east, carrying the warm pool with it. The region of rising air moves east with the warm pool, and so does the pumping of heat and moisture into the upper atmosphere. Deflections in the normal wind patterns distort the usual paths of the jetstreams, pushing them from their accustomed places, which eventually causes the changes in the weather that the rest of the world experiences. With weakened tradewinds, the deep cool waters coming up in the east slow their ascent. The food supply for plankton dwindles, and the effect travels up the food chain. When eastern sea surface temperature becomes warm, the east-to-west temperature contrast is small, and so the tradewinds weaken even further, leading to a complete collapse with essentially flat conditions across the entire equatorial Pacific. The most severe effects are found close to the equator. The usual pattern of deserts in Peru and Galapagos, and heavy rainfall over Indonesia and the west Pacific, reverses. Forest fires can occur in Indonesia (as has been happening in recent months, exacerbated by deliberate burning) and Australia, while Peru suffers flooding, with accompanying epidemics of cholera and other sewage-borne diseases. The food chain in the rich upwelling region is disrupted, so fish die off, which means hardship for the birds, mammals and people that survive on that stock. The warmer water near Central America spawns more and stronger hurricanes, which can go as far west as Hawaii. The entire sequence of the event lasts about one year, and events are usually separated by 2-7 years, in an irregular and not-well understood pattern. Should we take El Niño forecasts with a grain of salt? Remember that El Niño is not the only thing that influences weather. There are many other fluctuations and systems—some of which we are just discovering—and the weather we experience results from the tumultuous interplay of all these systems. Most of the interactions are poorly understood, particularly the longer-term ones, all the way up to Ice Ages, which may operate over periods of hundreds of thousands of years. As we get longer and longer records we become aware of more and more complexity, more cycles. Successive El Niños occur during different general conditions, at different times of year, and therefore have different total effects. Therefore, you can't simply speak of the isolated effects of El Niño on weather in, say, San Diego. There is only the ever-changing combination of influences. That is the main reason why we cannot produce reliable long-term forecasts. Scientists study El Niño partly for its own effects, but also partly as an example of how this kind of climate oscillation interacts with the rest of the climate system. (In other words, El Niño is a great weather and climate laboratory.) We know there are many such oscillations, and we would like to be able to fit the whole picture together. We hope that what we learn about the climate system from studying El Niño will help us understand other, less obvious, variability. Why don't you see much in the news about the causes of El Niño? The reason that you don't see much publicity about the causes of El Niño is that we don't understand the origins of the event. We do, however, have a pretty good understanding of how it evolves once it has begun, and that gives a useful ability to make forecasts six to nine months ahead for some regions. That is the information you see because that is the present state of reasonably secure knowledge. Of course, there are a variety of theories, and many scientists are working on various aspects of the genesis, which would presumably extend the predictive skill out another few months or even years. At several points over the last 20 years, we thought we had decent theories of what causes El Niño. Unfortunately (or perhaps fortunately for those who like scientific challenges), nature has shown that those theories were incomplete at best. For example, during the mid-1980s, a group at Columbia University developed a fairly simple theory and wrote a computer program to make predictions based on it. This was successful in predicting the 1986-87 and 1991-92 events almost a year in advance, and they were breaking out the champagne. Then along came the event of 1993, followed by another in 1994-95, and most prominently the present event, none of which developed according to the ideas in their theory. The champagne went back in the fridge. The main reason this is so difficult is that El Niños involve the full complexity of ocean-atmosphere interaction on a global scale. That's about as complex as it gets. We have developed a reasonably good understanding of how the atmosphere works (at least in theory), once the sea surface temperature (SST) that drives the atmospheric circulation is known. (We are somewhat further behind when it comes to the ocean, which is much harder to observe). Atmospheric models work well enough to make short-term weather forecasts, because, in the short term, you can pretend the ocean is unchanging, due to the slow speed of ocean changes. But, when you consider longer-term phenomena like El Niño, it is not enough to specify the SST; you also have to think about how the ocean will evolve with the winds, and then how the altered ocean will modify the winds, and so on, in many tricky and sensitive feedback loops. We are just beginning to be able to see how these disturbances work, and then only in very idealized cases. Remember that for a long time meteorologists only talked to meteorologists, and oceanographers only to oceanographers. Now we are really at the initial stages of being able to think about these coupled problems. Nevertheless, it seems to me that we are able to do a lot of good for society even at our present stage of ignorance, since even without knowing what drives El Niño, we can recognize it, and then know (largely from statistics of past events) what the effects will be on regions far removed from the tropical Pacific. Does El Niño originate solely in the tropics, or do the midlatitudes play an important role? Big effects (scientists call them large-amplitude signals, just to be nettlesome) occur well outside the tropics during El Niño events. The question is, are these big effects precursors of El Niño, or are they simply a sideshow to the equatorial Pacific "main event"? Both points of view have their advocates. On one hand, no one doubts that the entire ocean-atmosphere system is interconnected. Each El Niño event occurs against the background of existing conditions, including the positions of the mid-latitude jetstreams. In addition, new theories of the slow changes of the tropical ocean circulation point to a key role for water masses that come from the subtropics or higher latitudes. These water masses are part of a global-scale overturning circulation, in which waters sink at high latitudes (due to evaporation or winter cooling making them more dense), and travel through the depths of the sea to the equator, where they well up to the surface. Then the waters flows back poleward on the surface, warming under the sun to complete the cycle as they replenish the subtropical sinking zones. The whole round trip can take decades. Researchers have shown that the sinking water masses can have different temperature and salinity properties, which contribute to the longer-term dynamics of the system, and it's thought that overlaps of these slow changes may be the reason why some periods have many El Niños (like the 1990s) while others don't, or why some El Niños are stronger than others. Whew. On the other hand, simplified computer models of the equatorial ocean-atmosphere system commonly develop multi-year climate cycles similar to El Niño. This indicates that at least the basic phenomena are a natural rhythm of a wide ocean spanning the equator, and theory bears this out. The prevailing sentiment among climate researchers is that the mid-latitude influences referred to above can change individual El Niños, but that basically, some form of oscillation would occur regardless of influences. In short, El Niño is an organic phenomenon. However, at present we don't know how El Niños begin. Therefore the tropical/extra-tropical debate cannot be said to be resolved. This topic is the subject of much current research. What are the differences between statistical and dynamical forecast models of El Niño? There are two main types of forecast. Statistical forecasts are based on historical records. Dynamical forecasts are based on computer models of the coupled ocean/atmosphere system. Each has its strengths and weaknesses, and the results from these can be quite different. That should tell you something about the state of forecasting science. Statistical forecasts tie observed weather conditions with records of conditions in other El Niños. Typically, sea surface temperature (SST) in key regions of the equatorial Pacific is used to define "El Niño periods." Alternatively an index known as the "Southern Oscillation Index" (SOI) is used, based on the surface barometric pressure difference between Tahiti and Darwin, Australia, on opposite sides of the Pacific. The advantage of the SOI is that records at those two locations go back almost a century, while we have only a few decades of SST observations in the mid-ocean. Finally, these records are indexed to, for example, rainfall in California, allowing a statistical forecast of the likelihood of reoccurrence of heavy rains in that region during an El Niño. These are the most common types of forecast, the kind that you see in newspapers and TV weather reports. In some places, such as the U.S. Gulf Coast, the correlations are quite robust and the statistical forecast is fairly reliable. In others the correlations are weak, and their predictive value is low. The strength of statistical forecasts is that they are based on events that actually did occur, but they can fail; because of the complexity of the climate system, El Niño doesn't repeat itself. It is not very accurate to isolate the specific effects of El Niño by taking the average of previous events. (The average daytime high temperature for Chicago in July may be 88 degrees, but what does that tell you about how warm it will be at 4:00 PM on the next Fourth of July? Not much.) Anyway, different conditions every year blur the statistics and reduce confidence in such a forecast. Another problem with statistical forecasts is that we do not have good, long-term records of many of the important quantities of interest. Once you go back further than the mid-1950s, the ocean records are sparse and ambiguous, making it hard to determine which are strong El Niño years and which are weak (or even whether or not there really was an El Niño at all). Using only the good data, you see only a handful of events, and the statistics become quite unreliable. (If you weighed three apples in a bushel, you'd probably still not know the weight of the average apple very well.) Many of the differences among statistical forecasts reported in the media are due to the choice of different averaging periods, or in other words, thin records. Of course, they don't usually tell you that. Dynamical forecasts are based on computer models—equations, really—which are especially useful because specific processes can be analyzed and dissected in idealized and simplified terms. In other words, a good computer model focuses on what's important, and ignores what's not. For a while during the 1980s it appeared that much of the El Niño cycle was tied to "planetary waves" bouncing around the Pacific, and these could be decently simulated in a simple model. However, this theory failed to predict the series of El Niño events during the 1990s, and it appears that we must simulate the full complexity of ocean-atmosphere interaction. Remember Einstein's saying? "We must strive to make the world as simple as possible, but not more simple." Modeling El Niño and other climate fluctuations is a task of huge difficulty. The hardest part is not knowing what to include, but what to leave out. Nevertheless, many feel that as computers become faster and as our understanding of the physical processes of weather becomes better, we will rely more and more on the dynamical forecasts. They have the tremendous advantage of working forward from actual present conditions, and so avoid the problem of statistically averaging a number of events that differ in important details. In addition, for low-frequency events like El Niño, it might take centuries to observe enough occurrences to really improve statistical confidence. How can I understand the conflicting El Niño forecasts I find on the Web for this coming winter in California? There's a lot of confusion about what El Niño will do this winter. The media have given it a lot of play—more than is justified by the uncertainty of the forecasts. Please remember that weather forecasting is not an exact science, particularly when we are forecasting months ahead. However, some things are certain:
One of the best forecasts is what we call "persistence." That is, when a pattern is established it tends to remain. If the winter begins rainy, then probably it will continue as such. Lots of rain in November is a good indication that this El Niño has set up the jetstream to direct moisture to California (as opposed to further east), and it would then be more likely to continue. There was above-normal rain in California this November, but not a deluge, so that may tell you something about later in the winter. One of the hardest things for the lay public to get good information on is what's behind various forecasts. A lot of what you hear is based on statistics of past events. But remember, there haven't been very many El Niños since we started realizing it is a global phenomenon, and not just a bunch of unconnected weirdness. There have been 10 since 1950 and only six since 1970. That's not a very good basis for statistics, particularly when we observe that different events evolve in different ways. It's like measuring five kids in a classroom. Would that give you a good estimate of the average height of all the kids? Maybe you'd happen to get the five shortest. So statistical forecasts (noting that El Niño "usually" brings rain to California) are not on a good foundation. When you surf the Web sites looking for information, you can easily find statistics telling you different things. It doesn't mean they're wrong, or trying to mislead, it just means that we have a very small sample of a highly variable phenomenon. A final thing to remember is that El Niño is not the entire story. Many other oscillations are going on at the same time, so whatever the effects of El Niño, we see them all jumbled up with many other signals. Since we really have only a relatively few years of decent observations, picking these signals apart is partly a matter of guesswork. How do models used to predict El Niño work? How accurate are they? Computer models of the climate system let us examine the results of ideas that are too complicated for the human mind to crunch through. For an (overly simple) example, we know that the sun heats ocean surface water during the day and cools off at night. Say the amount of heating is determined just by the length of the day. That means the water would heat up in the summer, since the days are long and the nights short, and cool off more during the winter. We could write these ideas down in equations, specify the values of heating due to various amounts of sunlight, use a computer to solve the equations, and get a plot of what the predicted water temperature would be at any time in the future. We might have one calculation for each square kilometer of ocean, or if we're really being precise, one for every square hectare (1000 square meters, about 2.5 acres). The ocean is huge, so that is going to be millions and millions of equations. A computer can do millions of such calculations in a second; I can manage about one a minute. Computers have changed the way we study weather for just this reason. In reality, of course, we have much more complicated ideas of how the climate system works. For example, there are clouds, and the clouds not only block the sun during the day, cooling off the water, but also tend to insulate it at night, preventing cooling. Do these balance out? Depends on how the amounts of cooling and insulating are specified. Numbers that describe these relationships have to be estimated from observations, then programmed into the model. In addition, clouds are not independent of the water temperature; for example very warm water tends to produce a lot of evaporation, leading to tropical rainstorms. But if there is wind, the clouds may be blown somewhere other than where they were formed. So the pattern of where clouds occur can quickly become extremely complicated. Since we have already specified in our model that the clouds affect the water temperature, that, in turn, means the pattern of water temperature gets more complicated, which feeds back on the cloud pattern, and so on. Things continue to get more complicated. When air rises over warm water, other air must flow in from the sides to make up the deficit. Therefore if water temperatures are not uniform, there will be wind. When there is wind, this causes ocean currents, which moves water of various temperatures around. If the winds are such as to move the surface water away from some region, colder water from below may be pulled up. Cold water weighs more than warm water and the difference in density also causes currents to flow. So you can see that computer models of the climate system may start with some fairly simple ideas, but quickly become extremely complicated in practice. We use the world's biggest computers in this field, and still they're not fast enough. As we get more observations, we learn more about the system, and modelers are constantly struggling to represent these processes more accurately. One of the main difficulties is that while we know pretty well how the system will change over a short time (like a day or so), once we ask for longer predictions we come up against the problem that we don't know the initial state perfectly. Therefore there will be some error in the forecast since it won't be starting from exactly where the real system starts from. For a one-day forecast, the error probably won't be too great; perhaps some clouds in the wrong place. If we run the model further into the future, soon those wrong clouds will produce erroneous water temperatures, which will produce an even worse cloud pattern, and pretty soon the whole solution is garbage. Because we can never know the exact state of every bit of air and water, there is an inherent limit to the predictability of a system as complicated as the ocean-atmosphere system. In general, I don't take forecasts seriously more than a few months in advance. For example, right now most of the models are suggesting that El Niño will wind down by early summer 1998, and next winter will be a strong La Niña (the opposite phase, in which it is abnormally cold in the tropical Pacific and many of the effects of El Niño are reversed). Frankly, I don't have much confidence in a forecast that far ahead. However, the scientists who make these forecasts do it publicly as a means of "ante-ing up" to the forecast competition. Believe me, this is a real competition. Everyone wants to be the first to develop a successful El Niño model. And you can't not publish a forecast and then claim later that you had it right. So you see a lot of long-range forecasts, but that doesn't mean that anyone, including the authors, necessarily has much confidence in them. Unfortunately, with the media frenzy about El Niño this past year, many of these experimental forecasts were trumpeted around the newspapers and TV shows as if they were truth. Why can't I find any information about links between El Niño and global warming? The reason you won't find much information connecting El Niño and global warming is that we (meaning the mainstream scientific community) don't really have too much useful to say about it at this point. While we know that El Niño occurs on the background of the large-scale climate, and assume that as the background changes that some aspects of El Niño might also change, we are nowhere near the ability to say what those changes might be. So rather than speculate about such a politically charged subject, we usually keep our mouths shut. The above cautionary note does NOT, however, mean that one should discount the possibility. Since we see that El Niños in different years vary greatly in their strength it appears the process may be quite sensitive to changes in the background state. Much of the uncertainty in the question of whether greenhouse warming is affecting the ENSO cycle revolves around the problem of how one would measure the statistical significance of changes in recent El Niños. Some say that the string of warm El Niño events during the 1990s are evidence that a general warming trend is starting to change the weather; others say that these variations are within normal limits. The fact is we have only a few events to talk about, which means there is no statistical rigor to any argument for or against this idea. It is simply shooting the breeze. We won't have good statistics about El Niño for another hundred years or so (perhaps even longer if it is truly chaotic), so I don't bother with such arguments at all. To me the interesting stuff is the dynamics and thermodynamics anyway, and on that front we stand a chance of making progress in my lifetime. Is there a scale for the intensity of El Niño, like the Richter scale or the typhoon classification? The most widely used scale is known as the Southern Oscillation Index (SOI), which based on the surface (atmospheric) pressure difference between Tahiti and Darwin, Australia, on opposite sides of the Pacific. It was noted as far back as the 1920s that these two stations were anticorrelated, so that when Tahiti pressure is high, Darwin pressure is low. This reflects the very large scale of the phenomena, since one would not usually expect such a close relation between such faraway places. When Tahiti pressure is high, it indicates that winds are blowing towards the west (normal tradewinds), and when it is low, that winds are blowing to the east (El Niño). A major advantage of the SOI is that time series at these two locations extend back to the 1880s, so we can see the distribution of El Niño events back much further than we can see in records of ocean temperatures. The SOI is given in normalized units of standard deviation, a way of judging the distribution ("bell-shaped curve") of all the recorded intensities. This can be used as an intensity scale. For example, SOI values for the 1982-83 El Niño were about 3.5 standard deviations, so by this measure that event was roughly twice as strong as the 1991-92 El Niño, which measured only about 1.75 in SOI units. By this standard, the present El Niño is about as strong as 1991-92. However, the sea surface temperature anomaly is larger than in 1982-83, and some say that is a more important measure. This shows that there is no single number that summarizes the intensity of events. Why do diagrams of El Niño show a pointy wedge of warm water pointed west from South America? You see the westward-pointing wedge in plots of anomalies, not in plots of the actual temperatures. Anomalies mean the normal temperatures in each location have been subtracted from the observed values at the time of the plot. (When you put your hand on your forehead, thinking you might have a fever, you are checking for an anomaly.) An anomaly plot therefore shows whether the water is warmer or cooler than its normal state, and the normal state is different in different places. For example, if on some (very unusual!) day the temperature was 20°C (68°F) everywhere in the ocean, that would be normal along the coast of Baja California, but anomalously warm in Seattle, and anomalously cold in the Philippines. An anomaly map for this hypothetical flat 20°C day would show positive values in mid latitudes and negative values in the tropics. In the tropical Pacific, normal sea surface temperatures (SSTs) are much colder in the east than in the central/western Pacific (say 23°C (73.4°F) in the east vs. 29°C (84.2°F) in the west. Further, the cold water in the east is concentrated in a band along the equator. That may sound strange, since we usually think of the equator as warm, but upwelling of deeper, and hence colder, water occurs on the equator in the east, resulting in the cool water found there. During El Niño, water of about 28°C is found to stretch across the Pacific along the equator from Indonesia to Peru. This is a near normal temperature in the central and western Pacific, but it gets progressively more anomalous along the equator to the east, because it's usually colder there. That's why there appears to be a wedge. What have been the major new developments in instruments for measuring the temperature of the ocean? There have been two major developments. First, satellite coverage has gotten way better. Second, we've developed a simple, inexpensive design for buoys that can be deployed in mid-ocean and remain active for a year or more. The Pacific Marine Environmental Laboratory in Seattle maintains the TAO (Tropical Atmosphere Ocean) buoy array across the equatorial Pacific that measures sea surface temperatures, surface winds, air temperature and humidity, and subsurface temperature (sometimes currents, too) in the upper 500 meters of the ocean, at about 70 locations from Galapagos to Australia. Together, all the buoys constitute a HUGE instrument, and it lets us see things we never saw before. The buoys transmit data daily (in some cases, hourly) to orbiting satellites, which feed the data to the weather forecast network, and also to users on the Internet, for research and prediction. The big advantage of moored buoys is their high temporal resolution...that is, their ability to show us what is happening while it's happening. Also, knowledge of subsurface conditions is critical to understanding how the sea surface temperature is likely to change over the next few weeks and months. By combining satellite and buoy data, we get a better calibration of both. For the first time, we are now visualizing on graphic displays what's going on, rather than trying to make sense of reams of numbers on computer paper. Which is, if you think about it, a very natural way for humans to make sense of anything. Do volcanoes or sea floor venting cause El Niño? The idea that volcanoes cause El Niño events originally gained prominence because of the eruption of Mt. Chichon in Mexico in February 1982 (preceding the El Niño of 1982-83), and the eruption of Mt. Pinatubo in the Philippines in June 1991 (preceding the El Niño of 1991-92). However, when the incidence of El Niños is compared to the incidence of volcanic eruptions it becomes clear that the relationship is coincidental. There have been numerous large volcanic eruptions around the world and almost as many El Niños, and there's almost always an eruption at some time preceding any El Niño. Scientists are now convinced that this relation is coincidental. Certain experiments bear this out. For example, several computer models predicted the onset of the 1991-92 event as early as January 1991, based on observations of the ocean and atmosphere, well before Pinatubo. That indicates that the ocean-atmosphere system was already generating the El Niño, and Pinatubo occurred coincidentally. Computer models integrating the equations of fluid motion and the flow of heat routinely produce El Niño-like variability completely on their own. This shows that El Niño is a natural variability mode of the ocean-atmosphere system, rather like a thunderstorm is. Experiments to examine the modes of variability of the climate system suggest that it's prone to instabilities, ranging from storm systems lasting a few hours or days, to El Niño, with a cycle of several or more years, to longer-term fluctuations that we are just beginning to explore, measured in decades on up to thousands of years.. There is no need to invoke volcanoes. None of this is to say that volcanoes don't affect the climate. They most certainly do, and since El Niños occur against the background existing climate, there is little doubt that volcanic eruptions that eject large amounts of dust into the stratosphere can modify the El Niños, possibly in important ways. (There is a big distinction between "modify" and "cause" El Niño here.) As far as deep-ocean vents modifying the ocean temperatures, researchers now think that this source of heat does contribute to the long-term evolution of the ocean state. We can trace the chemical signatures of sea floor venting carried for quite a distance in the deep currents. Those traces are useful for estimating the deep flows, which are difficult and expensive to measure directly since they are so slow. However, we observe that the heating due to deep venting becomes diluted in the vast reaches of the abyssal ocean and therefore does not make quick changes in the ocean state. These affects are felt over decades or centuries, not on the relatively rapid time scale of El Niño. It is indeed tempting to look for simple causes of complex oscillations like the El Niño cycle. Unfortunately (or perhaps fortunately for those of us who like scientific challenges), it seems that the ocean-atmosphere system is well capable of generating these oscillations on its own, and the task now is to understand how this happens. Volcanoes and sea floor venting are part of the slowly changing background state to which phenomena like El Niño are added, and they increase the complexity of the task. Come back February 10 to find out more about how scientists are visually mapping El Niño. Anatomy of El Niño | Chasing El Niño | El Niño's Reach Dispatches | Resources | Mail | Site Map | El Niño Home Editor's Picks | Previous Sites | Join Us/E-mail | TV/Web Schedule About NOVA | Teachers | Site Map | Shop | Jobs | Search | To print PBS Online | NOVA Online | WGBH © | Updated November 2000 |