El Niño & La Niña

BACKGROUND

The original definition of El Niño goes back to the eighteenth or nineteenth century when Peruvian sailors coined the term to describe a warm southward current that appeared annually near Christmas off the Peruvian coast. Hence the name El Niño, Spanish for "the Child," referring to the Christ Child. Throughout the year, a northward cool current prevails because of southeast trade winds, causing upwelling of cool, nutrient-rich water. However, during late December the upwelling relaxes, causing warmer and nutrient-poor water to appear, which signals the end of the local fishing season.

Over the years, the warm, southward current occasionally seemed more intense than usual and was associated with periods of extreme wetness along the normally very dry Peruvian coast. These events were called "years of abundance." In the early twentieth century, researchers found a strong inverse correlation, called the Southern Oscillation, between surface pressure over the Pacific and Indian Oceans, hence the saying, "When pressure is high in the Pacific and Indian Oceans." Researchers tried, but failed, to correlate the Southern Oscillation with Indian monsoon failures. In 1958–59, a strong "year of abundance" occurred, in which a large area of warm water in the Pacific Ocean extended from the South American coast westward to the International Date Line. Coinciding with the extensive warm water were wetness along the Peruvian coast, low surface pressure in the eastern Pacific, and high pressure in the western tropical Pacific. Consequently, scientists in the early 1960s concluded that these events were associated and occurred interannually. Since then, the term "El Niño" (or warm episode) has described not a local warm current, but warming of the tropical Pacific surface waters occurring every two to seven years and associated with changes in the atmospheric circulation in the tropical Pacific and worldwide.

MECHANISMS

Figure 1 depicts the typical atmospheric and oceanic circulations that exist in the tropical Pacific. The prevailing easterlies (NE and SE trades) converge over Indonesia in conjunction with the Asian monsoon, producing widespread convection. Additionally, warm water "piles up" in the western Pacific, due to the easterly winds. Further east, the SE trades and equatorial easterlies in the eastern and central Pacific produce upwelling of cool water along the equator and coast of South America.

As the El Niño event begins, the easterlies relax, reducing the amount of upwelling and allowing the western warm water to move eastward. As time goes on, the warm pool in the western Pacific grows and expands eastward toward the central Pacific (figure 2). Detailed monitoring of recorded El Niño episodes has revealed that once the warmest water reaches the International Date Line, anomalous convection usually appears in that region, accompanied by a weakening of the equatorial easterlies. This pattern typically occurs during the boreal winter (June–August) and may be preceded or followed by a warming that causes the Inter-Tropical Convergence Zone (ITCZ) to move farther south than normal, which contributes to enhanced rainfall across Ecuador and northern Peru, producing the "years of abundance."

In determining the atmospheric status of the tropical Pacific, climatologists devised the Southern Oscillation Index (SOI, Figure 3). It is the standardized sea level pressure difference between Darwin, Australia, and Tahiti, French Polynesia, in the central Pacific (Tahiti minus Darwin). Thus, when the surface pressure is high at Darwin and low at Tahiti, the SOI is negative (El Niño); conversely, when surface pressure is low at Darwin and high at Tahiti the SOI is positive. When the SOI is strongly positive, cooler than normal equatorial water appears throughout the central and eastern equatorial Pacific. This is called a cold episode or sometimes La Niña, "little girl." Climatologists prefer to use the acronym ENSO (El Niño/Southern Oscillation) to describe the warm (El Niño) and cold (La Niña) episodes that occur periodically across the tropical Pacific.

IMPACTS

When El Niño or La Niña develops, several consistent weather anomalies typically occur around the world. Figures

4 and 5 depict potential rainfall and temperature impacts from El Niño while figures 6 and 7 show potential rainfall and temperature impacts from La Niña. Most climate anomalies associated with El Niño are reversed during La Niña. In general, a majority of the impacts occur in climates that have significant oceanic influences and border the tropical Pacific. Thus, the regions of the world that show the highest correlation to warm or cold events are Indonesia, Australia, and the tropical Pacific islands. Weather anomalies (drought and excessive moisture) associated with El Niño and La Niña can have significant impact on agricultural production (i.e., poor crops due to failure of the Indian monsoon). However, several factors make the impacts on crop production less dramatic and sometimes nonexistent. These factors include the timing, duration, and intensity of ENSO events at various stages of crop development.

What is happening in the atmosphere during El Niño?

Several aspects of the atmosphere's behavior are remarkable and entirely unique to the ENSO phenomenon. Some normally arid tropical habitats are transformed into virtual gardens during El Niño. Abundant and reliable rains in other tropical areas become sparse and intermittent during El Niño. Extreme climates have also been experienced in the higher latitudes during ENSO, though these are by no means unique to ENSO. One marvels that the atmosphere, especially thousands of miles away from the equatorial Pacific, "knows" about the modest warming of those waters during El Niño! Yet, all regions of the globe are not equally affected, nor is ENSO's impact uniform throughout the year. How do we understand these atmosphere manifestations of ENSO?

A meteorological view of the ENSO phenomenon offers some answers. However it does not explain ENSO itself; for that, one needs to account for the origin of the oceanic conditions, and the coupled interaction of the ocean and the atmosphere is central to that problem.

So how does the atmosphere "know" about El Niño? It is useful to imagine a chain of atmospheric processes, with each link in this chain carrying information from the local vicinity of El Niño sea surface temperature (SST) anomalies throughout the global climate system. The first link is the tropical response of rain-producing cumulonimbus; critical because deep convection is the principal agent for exchanging heat from Earth's surface and thereby communicating El Niño's presence to the free atmosphere. Wet tropical climates tend to coincide with warm pool SST area in the western

Pacific, and the continental monsoons. During El Niño, rainfall increases over a distance of several thousand miles along the equator from the central to the eastern Pacific in response to the warming of the underlying SSTs. Reduced rainfall occurs on the periphery of this wet zone, and even the continental monsoons are not spared ENSO's influence. The opposite effect tends to be experienced during La Niña, although the west-east scale of rainfall anomalies over the equatorial Pacific is somewhat reduced compared to warm events.

The second link in the chain is the horizontal communication of El Niño's presence, and this involves the sensitivity of the atmosphere's circulation to shifts in organized cumulonimbus convection. Excited atmospheric wave motions adjust the climatological flow to the new tropical energy sources; not unlike the waves generated by a pebble dropped into a pond, although for ENSO the spatial scales of forcing are much larger and the atmosphere is readily forced. The major convection anomalies themselves are confined to within a few degrees of the equator during winter. However, associated with them is a circulation of mass and energy in the atmosphere that extends several thousand miles pole-ward into the subtropics. A deflecting force, due to the Earth's rotation, acts upon this outflow along its poleward course, thereby initiating wave-like patterns in the perturbed flow. In addition, the climatological circulation in higher latitudes acts to channel the course of this poleward flowing energy. This flow is directed from west to east, has concentrated westerlies along the jet streams east of Asia and North America, and is characterized by stationary waves of alternating low and high pressure.

Through interactions with this background flow, the resulting atmospheric response to El Niño also consists of a wave train pattern having alternating low and high pressure. In the Northern Hemisphere, the wind circulates parallel to these contours, with low pressure to the left of the motion's direction. The wave paths follow "great circle" routes that arc toward the pole, into the higher reaches of the Pacific-North American region, and then curve toward the equator into the subtropical western Atlantic. A similar wave pattern in the atmosphere exists in the Southern Hemisphere with an attendant influence on the climate of South America.

The anomalous wave patterns are referred to as the atmospheric "teleconnections," linking equatorial and high latitudes during ENSO. These are based on a historical composite of how the atmosphere has behaved during the ten strongest El Niño and La Niña events of the 1950–96 period. Several features to note are the stronger Pacific jet during El

Niño and an eastward shift of the stationary wave pattern over the Pacific-North American region during El Niño. These upper tropospheric changes alter the course of storms (cyclone and anticyclones) that control the daily weather fluctuations in the higher latitudes. Changes in statistical properties of storms (for example, their frequency, strength, or origin and track) account for the bulk of ENSO's signal in precipitation and surface temperature in the higher latitudes. Such "storm track" feedback constitutes a third essential link along the chain that is initiated by the equatorial Pacific SST anomalies.

Each El Niño event has a unique signature in its SST life-cycle, and the question of the atmosphere's sensitivity to such inter-El Niño variations is a matter of intense research focusing on the implications for seasonal climate predictions. The atmospheric events are being actively monitored by the scientific community, and the expected climate response continues to be assessed.

Global consequences of El Niño

The twists and turns in the ongoing dialogue between ocean and atmosphere in the Pacific can have a ripple effect on climatic conditions in far flung regions of the globe. This worldwide message is conveyed by shifts in tropical rainfall, which affect wind patterns over much of the globe. Imagine a rushing stream flowing over and around a series of large boulders. The boulders create a train of waves that extend downstream, with crests and troughs that show up in fixed positions. If one of the boulders were to shift, the shape of the wave would also change and the crests and troughs might occur in different places.

Dense tropical rain clouds distort the air flow aloft (5–10 mi above sea level) much as rocks distort the flow of a stream, or islands distort the winds that blow over them, but on horizontal scale of thousands of miles. The waves in the air flow, in turn, determine the positions of the monsoons, and the storm tracks and belts of strong winds (commonly referred to as jet streams) that separate warm and cold regions at the Earth's surface. In El Niño years, when the rain area that is usually centered over Indonesia and the far western Pacific moves eastward into the central Pacific, the waves in the flow aloft are affected, causing unseasonable weather over many regions of the globe.

The impacts of El Niño upon climate in temperate latitudes show up most clearly during wintertime. For example,

most El Niño winters are mild over western Canada and parts of the northern United States, and wet over the southern United States from Texas to Florida. El Niño affects temperate climates in other seasons as well. But even during wintertime, El Niño is only one of a number of factors that influence temperate climates. El Niño years, therefore, are not always marked by "typical" El Niño conditions the way they are in parts of the tropics.

BENEFITS OF EL NIÑO PREDICTION

Scientists are now taking our understanding of El Niño a step further by incorporating the descriptions of these events into numerical prediction models—computer programs designed to represent, in terms of equations, processes that occur in nature. Such models are fed information mostly in the form of sets of numbers, describing the present state of the atmosphere-ocean system (for example, observations of wind speeds, ocean currents, sea level, and the depth of the thermocline along the equator). Updated sets of numbers that the models produce indicate how the atmosphere-ocean system might evolve over the next few seasons or years. The results thus far, though by no means perfect, give a better indication of the climatic conditions that will prevail during the next one or two seasons than simply assuming that rainfall and temperature will be "normal."

Peru provides a prime example of how even short term El Niño forecasts can be valuable. There, as in most developing countries in the tropics, the economy (and food production in particular) is highly sensitive to climate fluctuations. Warm (El Niño) years tend to be unfavorable for fishing and some of them have been marked by damaging floods along the coastal plain and in the western Andean foothills in the northern part of the country. Cold years are welcomed by fishermen, but not by farmers, because these years have frequently been marked by drought and crop failures. Such cold years often come on the heels of strong El Niño events.

Since 1983, forecasts of the upcoming rainy season have been issued each November based on observations of winds and water temperatures in the tropical Pacific region and the output of numerical prediction models. The forecasts are presented in terms of four possibilities: (1) near normal conditions, (2) a weak El Niño with a slightly wetter than normal growing season, (3) a full blown El Niño with flooding, and (4) cooler than normal waters offshore, with higher than normal chance of drought.

Once the forecast is issued, farmer representatives and government officials meet to decide on the appropriate combination of crops to sow in order to maximize the overall yield. Rice and cotton, two of the primary crops grown in northern Peru, are highly sensitive to the quantities and timing of rainfall. Rice thrives on wet conditions during the growing season followed by drier conditions during the ripening phase. Cotton, with its deeper root system, can tolerate drier weather. Hence, a forecast of El Niño weather might induce farmers to sow more rice and less cotton than in a year without El Niño.

Countries that have taken similar initiatives include Australia, Brazil, Ethiopia, and India. Although tropical countries have the most to gain from successful prediction of El Niño, for many countries outside the tropics, such as Japan and the United States, more accurate prediction of El Niño will also benefit strategic planning in areas such as agriculture, and the management of water resources and reserves of grain and fuel oil.

Encouraged by the progress throughout the 1990s, scientists and governments in many countries are working together to design and build a global system for (1) observing the tropical oceans, (2) predicting El Niño and other irregular climate rhythms, and (3) making routine climate predictions readily available to those who need them for planning purposes, much as weather forecasts are made available to the public today. The ability to anticipate how climate will change from one year to the next will lead to better management of agriculture, water supplies, fisheries, and other resources. By incorporating climate predictions into management decision, humankind is becoming better adapted to the irregular rhythms of climate.

THE PACIFIC DECADAL OSCILLATION

The Pacific Decadal Oscillation (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time. Fisheries scientist Steven Hare coined the term "Pacific Decadal Oscillation" in 1996 while researching connections between Alaska salmon production cycles and Pacific climate (his dissertation topic with advisor Robert Francis).

Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, twentieth century PDO "events" persisted for 20–30 years, while typical

ENSO events persisted for 6–18 months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics—the opposite is true for ENSO. Several independent studies find evidence for just two full PDO cycles in the past century: "cool" PDO regimes prevailed in 1890–1924 and again in 1947–1976, while "warm" PDO regimes dominated in 1925–1946 and from 1977 through (at least) the mid-1990s. Shoshiro Minobe has shown that twentieth century PDO fluctuations were most energetic in two general periodicities, one of 15–25 years, and the other of 50–70 years.

Major changes in northeast Pacific marine ecosystems have been correlated with phase changes in the PDO; warm eras have seen enhanced coastal ocean biological productivity in Alaska and inhibited productivity off the west coast of the contiguous United States, while cold PDO eras have seen the opposite north-south pattern of marine ecosystem productivity.

Causes for the PDO are not currently known. Likewise, the potential predictability for this climate oscillation is not known. Some climate simulation models produce PDO-like oscillations, although often for different reasons. The mechanisms giving rise to PDO will determine whether skillful decades-long PDO climate predictions are possible. For example, if PDO arises from air-sea interactions that require 10-year ocean adjustment times, then aspects of the phenomenon will (in theory) be predictable at lead times of up to 10 years. Even in the absence of a theoretical understanding, PDO climate information improves season-to-season and year-to-year climate forecasts for North America because of its strong tendency for multi-season and multi-year persistence. From a societal impacts perspective, recognition of PDO is important because it shows that "normal" climate conditions can vary over time periods comparable to the length of a human's lifetime.

MADDEN-JULIAN OSCILLATION

Weather is not as predictable in the tropics as in mid– latitudes. This is because in mid–latitudes the weather variables (clouds, precipitation, wind, temperature, and pressure) are largely governed by the upper–tropospheric Rossby waves, which interact with surface weather in a process called baroclinic instability. In the tropics there is no such dominant instability or wave motion, and therefore the

weather is less predictable for the 1–10 day period. Until recently it was believed that tropical weather variations on time scales less than a year were essentially random.

In 1971 Roland Madden and Paul Julian stumbled upon a 40–50 day oscillation when analyzing zonal wind anomalies in the tropical Pacific. They used 10 years of pressure records at Canton (at 2.8°S in the Pacific) and upper level winds at Singapore.

The oscillation of surface and upper–level winds was remarkably clear in Singapore. Until the early 1980s little attention was paid to this oscillation, which became known as the Madden and Julian Oscillation (MJO), and some scientists questioned its global significance. Since the 1982-83 El Niño event, low-frequency variations in the tropics, both on intra-annual (less than a year) and inter-annual (more than a year) timescales, have received much more attention, and the number of MJO–related publications grew rapidly.

The MJO, also referred to as the 30–60 day or 40–50 day oscillation, turns out to be the main intra–annual fluctuation that explains weather variations in the tropics. The MJO affects the entire tropical troposphere, but is most evident in the Indian and western Pacific Oceans. The MJO involves variations in wind, sea surface temperature (SST), cloudiness, and rainfall. Because most tropical rainfall is convective, and convective cloud tops are very cold (emitting little longwave radiation), the MJO is most obvious in the variation of outgoing longwave radiation (OLR), as measured by an infrared sensor on a satellite.

The OLR signal in the Western Hemisphere is weaker, and the recurrence interval for the eastward propagating OLR anomalies in the Eastern Hemisphere is about 30–60 days. How exactly the anomaly propagates from the dateline to Africa (i.e. through the Western Hemisphere) is not well understood. It appears that near the dateline a weak Kelvin wave propagates eastward and poleward at a speed exceeding 10 m/s.

NORTH ATLANTIC OSCILLATION

There are several prominent, recurring modes of lowfrequency variability over the extra tropical North Atlantic and Europe. Perhaps the most well known of these patterns is the North Atlantic Oscillation. The NAO exhibits little variation in its climatological mean structure from month to month and consists of a north-south dipole of anomalies of opposite sign, with one center located over Greenland and the other spanning the central latitudes of the North Atlantic at 35–40°N. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States, and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track, and in large-scale modulations of the normal patterns of zonal and meridianal heat and moisture transport, which in turn result in changes in temperature and precipitation patterns often extending from eastern North America to western and central Europe.

Strong positive phases of the NAO are often associated with above-normal temperatures in the eastern United States and across northern Europe and below-normal temperatures in Greenland and across southern Europe and the Middle East. The positive NAO phase is also associated with above-normal precipitation over northern Europe and Scandinavia and below-normal precipitation over southern and central Europe. Opposite patterns of temperature and precipitation anomalies are typically observed during strong negative phases of the NAO. During prolonged periods dominated by one phase of the NAO, abnormal height and temperature patterns are also often seen extending well into central Russia and north-central Siberia.

NORTH ATLANTIC–ASIAN MONSOONS INFLUENCE EACH OTHER'S PATTERNS

Like ballroom dancers on a crowded floor, climatic phenomena like El Niño, the Asian monsoons, and the North Atlantic influence each other's patterns. Sediments from the floor of the Arabian Sea near Oman were studied by researchers looking for evidence of the strength of monsoons in the region over the past 10,000 years. There is a suggestion that the link between the North Atlantic climate and the Asian monsoon is a persistent aspect of global climate. The link was demonstrated previously by various researchers, but the new research examines a much longer time period (the past 10,000 years). The new study reveals substantial natural variation in climate and the monsoon in a time prior to any significant human influence. The new information may lead to improved predictions of the monsoon in the coming decades.

The significance of these results lies in demonstrating a pattern of persistent variability in monsoons throughout the Holocene (from 10,000 years ago to the present) that may be linked with episodic warming and cooling of the North Atlantic. The results highlight the need to improve our understanding of abrupt and difficult-to-predict weakening in monsoon strength, which could accompany major climate shifts in the North Atlantic in the future.

NOAA and university researchers used fossils of the plankton Globigerina bulloides to estimate wind intensity. During a monsoon, the seasonal reversal of winds brings moisture from the ocean onto land. The winds also blow surface waters off shore, causing an upwelling of colder, nutrient-rich water where the microscopic marine animals can thrive. By counting the amount of G. bulloides present in different layers of the sediment and using radiocarbon dating, the scientists were able to approximate monsoon strength from 10,500 years ago up to the present. The resulting record showed a natural variation in the monsoon from one century to the next. This provides new evidence that the strength of Asian monsoon varies substantially on century to millennial time scales, and the need to understand this if we're going to ensure human and ecological sustainability in Tibet, China, India, and the rest of Southeast Asia.

While researchers aren't sure of the exact causes of the link between the North Atlantic and the Asian monsoon, earlier research showed the amount of snow on the Tibetan plateau may play a critical role. As the land warms in the spring, the air rises above the land causing a pressure gradient that drives the monsoon. More snow on the plateau in spring or early summer uses up all the sun's heating because it has to be melted and evaporated before the land can warm. So the more snow you have in winter, the weaker the monsoon the following summers. There is speculation that when the North Atlantic is cold, areas downwind like the Tibetan plateau stay cold longer, allowing more snow to persist and setting up a weakened monsoon. The monsoon–snow cover link may lead to a stronger or more variable monsoon in the coming century as the Northern Hemisphere continues to warm faster than the tropics.

Other studies show that changes in the amount of sunlight correlate to variations in both the North Atlantic climate and the Asian monsoon. The researchers aren't certain if the sun affects each system directly or if solar radiation influences the North Atlantic circulation, which in turn affects the monsoon. In an earlier study, evidence from sediments in the same region showed an increase in monsoon strength in the past 400 years.

YEAR 2000

A strong La Niña at the beginning of 2000 weakened during July and August, but was still evident at year's end. As a result, cooler than normal temperatures throughout the eastern equatorial Pacific held down temperatures in the tropics. However, temperatures in the non-tropical Northern Hemisphere continued to average near record levels. Temperatures north of 20°N were the second warmest on record during the December 1999–November 2000 period. In addition, annual anomalies in excess of 2°F were widespread across Canada, Scandinavia, much of Eastern Europe, and the Balkans.

FREQUENTLY ASKED QUESTIONS ABOUT EL NIÑO AND LA NIÑA

What is the difference between La Niña and El Niño?

El Niño and La Niña are extreme phases of a naturally occurring climate cycle referred to as El Niño/Southern Oscillation. Both terms refer to large-scale changes in sea-surface temperature across the eastern tropical Pacific. Usually, sea-surface readings off South America's west coast range from the 60s to 70s°F, while they exceed 80°F in the "warm pool" located in the central and western Pacific. This warm pool expands to cover the tropics during El Niño, but during La Niña, the easterly trade winds strengthen and cold upwelling along the equator and the west coast of South America intensifies. Sea-surface temperatures along the equator can fall as low as 7°F below normal during La Niña. Both La Niña and El Niño impact global weather patterns.

How often does La Niña occur and how long does it last?

El Niño and La Niña occur on average every three to five years. However, in the historical record the interval between events has varied from two to seven years. According to the National Centers for Environmental Prediction, the twentieth century's previous La Niñas began in 1903, 1906, 1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988, and 1995. These events typically continued into the following spring. Since 1975, La Niñas have been only half as frequent as El Niño.

La Niña conditions typically last approximately nine to 12 months, though some episodes may persist for as long as two years.

Does a La Niña typically follow an El Niño?

No, a La Niña episode may, but does not always, follow an El Niño.

Why do El Niño and La Niña only occur in the Pacific?

This question does not have a simple or straightforward answer, since this is not a settled issue. Fundamentally, no one is exactly sure why the Pacific should have an El Niño/La Niña cycle and the Atlantic not.

A principal difference between the Atlantic and Pacific is the width of the equatorial region. The Pacific is more than twice as wide at the equator. This is important to its capacity to sustain El Niño/La Niña because of the peculiar dynamics of equatorial waves. Equatorial waves are not the familiar surf or swell seen on the surface, but very large-scale motions that carry changes in currents and temperature over thousands of miles. The period of these waves is measured in months, and they take typically three months to more than a year to cross the Pacific. Surprisingly, these waves do not spread out equally in all directions like waves made by dropping a rock in a lake, but preferentially propagate eastward or westward. When winds blow over a large area of the ocean consistently for a month or more, equatorial waves are usually generated, and these then modify conditions over a very large region, including places far removed from where they were generated. For example, winds over the far western Pacific make waves that carry the signal to the coast of South America, even though the winds in the South American region may not change at all. The subsurface changes due to the arriving waves can then cause sea surface temperature changes, entirely due to winds occurring many thousands of miles to the west.

With the huge distances across the Pacific, one side of the ocean can be reacting to conditions due to one set of waves, while the other can be doing something completely different. As the waves propagate back and forth, a cycle can be set up that oscillates (El Niño/La Niña). The much smaller Atlantic, on the other hand, is not large enough to sustain much of an oscillation, since the waves cross it so quickly, often in only a month or so. This does not allow a cross-ocean contrast to be created, nor an oscillation to be set up. Some indications suggest that some kind of weak oscillation may in fact occur in the Atlantic, but it never reaches the amplitude of that in the Pacific.

A second reason that the Pacific is more important in this regard is that the fundamental driver of the whole ocean-atmosphere circulation is heat. The large width across the Pacific allows the existence of a huge pool of warm water in the west. The smaller distances across the Atlantic mean that the Atlantic warm pool is much smaller. The Pacific warm pool is a gigantic source of heat that is one of the main controls of the atmosphere. When the warm pool shifts east (during El Niño) or shrinks west (during La Niña), the effects reverberate around the world, causing the weather disruptions associated with this cycle. In the Atlantic, there is simply not enough of a warm pool to make that much difference to worldwide weather. So even if there is an analogue to El Niño in the Atlantic, it does not have the power to cause weather disturbances that affect more than local conditions.

Why do El Niño and La Niña occur?

El Niño and La Niña result from interaction between the surface of the ocean and the atmosphere in the tropical Pacific. Changes in the ocean impact the atmosphere and climate patterns around the globe. In turn, changes in the atmosphere impact the ocean temperatures and currents. The system oscillates between warm (El Niño) to neutral (or cold La Niña) conditions on an average of every three to five years.

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 El Chichón 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 time series of El Niño is compared to the time series of volcanic eruptions, it becomes clear that the relationship is coincidental. There are numerous large volcanic eruptions around the world and almost as many El Niños. In that situation there is almost always an eruption at some time preceding any El Niño. Scientists are now convinced that this relationship 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 the state of the ocean-atmosphere system at that time well before Pinatubo erupted. This indicates that the ocean-atmosphere system was already generating the El Niño, and Pinatubo erupted coincidentally. Computer models integrating the equations of fluid motion and the flow of heat routinely produce El Niño-like variability completely on their own. Of course, computer models are not reality, but these experiments suggest that El Niño is a natural mode of variability of the ocean-atmosphere system, as much as, for example, a thunderstorm. While we do not have a complete picture of how the El Niño cycle operates, these models (and a developing theoretical understanding) suggest that the fluid envelope of the Earth is prone to developing various kinds of instabilities, ranging from storm systems lasting a few hours or days, to El Niño, to longer-term fluctuations that we are just beginning to explore. There is no reason to think that external processes such as volcanoes are a necessary element.

None of this is to say that volcanoes do not affect the climate. They most certainly do, and since El Niños occur against the background existing climate, there is little doubt volcanic eruptions that eject large amounts of dust into the stratosphere must modify the frequency, character, and strength of El Niño events, possibly in important ways. The distinction is between "slowly modifying the background" and "causing" El Niño.

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. The chemical signatures of undersea vents are of great interest as tracers of the slow deep circulation of the ocean, and therefore these signatures are studied carefully. (The deep circulation is so slow that its currents cannot be measured directly, so we look at tongues of chemical tracers to estimate the speed, direction, and transport of the flows.) Numerous scientific papers discuss these questions, studying a variety of chemical constituents. What is consistently found is that the traces spread extremely slowly through the water column and are vastly diluted. There is little doubt that over very long periods the effects of undersea venting on the ocean are large, both for their heat and for their contribution to the chemical makeup of the ocean. However, these effects occur on timescales of thousands of years, and certainly do not produce the kind of rapid signals that characterize El Niño. To trigger an El Niño event, one would look for a signal that produced surface variability on a month to month or year to year timescale, and undersea venting has never been observed to do that.

It is indeed tempting to look for nice clean causes for complex oscillations like the El Niño cycle. Unfortunately, it seems that the ocean-atmosphere system is 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 against which phenomena like El Niño occur, and add to the complexity of the task.

Why isn't there much publicity about the causes of El Niño and La Niña?

The reason that there is not much publicity about the causes of El Niño and La Niña is that we do not understand the origins of the events. We do, however, have a pretty good understanding of how they evolve once they begin, and that allows us to make forecasts six to nine months ahead for some regions. This is the information that is publicized because it is 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.

The fact is, at several points over the past two decades scientists thought working theories of what causes El Niño had been firmly established. Unfortunately, nature has shown that those theories were at best incomplete. For example, during the mid-1980s, a group at Columbia University developed a fairly simple theory and wrote a computer model to produce predictions based on it. This was successful in predicting the 1986–87 and 1991–92 events almost a year in advance. Then along came the event of 1993, then another in 1994–95, the most prominent El Niño of the late 1990s, neither of which developed according to the ideas in their theory.

The main reason this is so difficult is that the processes that cause El Niño and La Niña involve the full complexity of ocean-atmosphere interaction on a global scale. Now that the sea surface temperature (SST) driving the atmospheric circulation is known, a reasonably accurate understanding of how the atmosphere works (at least in theory) has been developed. With a basic understanding, atmospheric models can make short-term weather forecasts, because the ocean changes rather slowly. However, when one considers longer-term phenomena like El Niño and La Niña, it is not enough to specify the SST; one must consider how the ocean will evolve under 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 fundamentally coupled disturbances work, and generally 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.

Does El Niño have a purpose?

El Niño is part of the natural rhythm of the ocean-atmosphere system, as much as winter cold or summer thunderstorms or any other weather phenomenon.

In a complicated system like this, each feature fills a role in the grand scheme of things. The exact role cannot be pinpointed, but scientists do observe that these events drain the west Pacific of heat that is built up over several years by the trade winds. In any case, El Niño does not exist in isolation, and any changes in it would reverberate around the whole system in unpredictable ways. Further, as part of the natural environment of the Pacific basin that the animals, fish, birds, and plants have adapted to over the millennia (it is known that El Niños have occurred throughout history), it is not clear that stopping El Niño would even be desirable. Even if it was possible to make El Niño disappear, it is unclear what the outcome would be.

What is the relationship between El Niño/La Niña and global warming?

The jury is still out on this. Are we likely to see more El Niños because of global warming? Will they be more intense? These are the main research questions facing the scientific community today. Research will help us separate the natural climate variability from any trends due to human activities. We cannot figure out the "fingerprint" of global warming if we cannot sort out what the natural variability does. We also need to look at the link between decadal changes in natural variability and global warming. At this time we cannot preclude the possibility of links, but it is too early to say there is definitely a link.

Is El Niño or La Niña responsible for a specific hurricane/tropical storm/drought/fire/flood/winter storm?

It is inaccurate to label individual storms or events as La Niña or El Niño events. Rather, these climate extremes affect the position and intensity of the jet streams, which in turn affect the intensity and track of storms. During La Niña, the normal climate patterns are enhanced. For example, in areas that would normally experience a wet winter, conditions would likely be wetter than normal.

It is impossible to prove that El Niño or La Niña cause a particular event, just as it is impossible to say that winter caused a particular snowstorm—it is the likely suspect.

We cannot run experiments to see what a parallel Earth without El Niño or La Niña would do. But a group at NOAA's Climate Diagnostic Center in Boulder, Colorado, is trying something similar using numerical forecast models. First, they run the model with the actual conditions and produce weather forecasts, just like the regular ones. Then they make another run, in which everything is the same as the first one except that they change the Pacific SST to be like a "normal" year. The difference between these forecasts gives an indication of the effect of El Niño and La Niña conditions on the specific weather events being forecast.

What impacts do El Niño and La Niña have on tornado activity across the country?

Since a strong jet stream is an important ingredient for severe weather, the position of the jet stream determines the regions more likely to experience tornadoes. Contrasting El Niño and La Niña winters, the jet stream over the United States is considerably different. During El Niño the jet stream is oriented from west to east over the northern Gulf of Mexico and northern Florida. Thus this region is most susceptible to severe weather. During La Niña the jet stream extends from the central Rockies east-northeastward to the eastern Great Lakes. Thus severe weather is likely to be further north and west during La Niña than El Niño.

What are the impacts of La Niña?

Both El Niño and La Niña impact global and U.S. climate patterns. In many locations, especially in the tropics, La Niña (or cold episodes) produces the opposite climate variations from El Niño. For instance, parts of Australia and Indonesia are prone to drought during El Niño, but are typically wetter than normal during La Niña.

In the United States, La Niña often features drier than normal conditions in the Southwest in late summer through the subsequent winter. Drier than normal conditions also typically occur in the Central Plains in the fall and in the Southeast in the winter. In contrast, the Pacific Northwest is more likely to be wetter than normal in the late fall and early winter with the presence of a well-established La Niña. Additionally, on average, La Niña winters are warmer than normal in the Southeast and colder than normal in the Northwest.

How is La Niña influencing the Atlantic and Pacific hurricane seasons?

Dr. William Gray at the Colorado State University has pioneered research efforts leading to the discovery of La Niña impacts on Atlantic hurricane activity, and to the first—and presently only—operational long-range forecasts of Atlantic basin hurricane activity. According to this research, the chances for the continental United States and the Caribbean Islands to experience hurricane activity increase substantially during La Niña.

Does El Niño create dangerous conditions for marine life, and will it have a lasting effect on marine animals?

First, there is no question that El Niño has serious effects on life in many regions.

Second, it should be remembered that El Niño is part of the normal rhythm of Earth, and of the environment that marine life has evolved to face. A plant or creature will not last long in a place in which it can only handle ideal conditions. With natural variability, some winters are colder than others, some years drier, etc. El Niño is part of this normal climate, along with other influences that are less known. Living things may have varied success in these natural fluctuations. So El Niño may cause a temporary die-back of some forms of marine life in some regions, or reduce the survival rate of young, but it probably does not have a lasting effect. For example, El Niño devastates the population of seabirds off Peru by reducing the fish stock on which they live. But those birds will bounce back soon after El Niño is gone. If, however, El Niño became more frequent, then one might find the overall composition of marine life changing in that region. But the individual events themselves probably do not cause a permanent change.

Since normal variations can have large swings, such as those that occur during El Niño, the boundaries of where particular forms of life can survive are somewhat smaller than they would be if the climate were more constant. For example, palm trees can survive in Seattle, Washington, during most years, since it has generally mild winters. But every ten years or so a killing frost occurs, so natural palms do not occur there. But many people grow them in gardens and find that they thrive with only occasional protection.

Is there a scale for the intensity of El Niño?

The most widely used scale is known as the Southern Oscillation Index (SOI), which is based on the surface (atmospheric) pressure difference between Darwin, Australia and Tahiti, French Polynesia. It was noted as far back as the 1920s that these two stations were anti-correlated, so that when Tahiti pressure is high, Darwin pressure is low. This reflects the very large scale of the phenomenon, since one would not usually expect such a close relation between such

faraway places. When Tahiti pressure is high, that indicates winds blowing towards the west (normal trade winds), and when it is low, winds blow to the east (El Niño).

Scientists use the Southern Oscillation Index for three main reasons, even though it is an indirect measure of El Niño and these locations are not ideally sited for this purpose. First, the time series at Darwin and Tahiti are more than 100 years long, and there is no other record that would allow us to categorize the El Niño cycle that far back. Second, the measurement of atmospheric pressure is simple (it is just the height of a column of mercury in a barometer) and not subject to calibration problems, as, for example, thermometers are. A column of mercury is just a measure of length, which is accurate and easily convertible between inches or centimeters, whereas thermometers are inherently less accurate since they rely on a carefully made glass tube that may be subject to expansion or irregularities; also the placement of the thermometer (in the sun or shade or breeze, near buildings, etc.) can have a large effect. For example, temperature measured in cities shows a long-term rise associated with the heat generated by urban activities and the increased absorption of solar heat by pavement compared with forest. This is one thing that makes it difficult to detect the signature of greenhouse warming. Therefore, pressure measurements are highly desirable for interpreting long records. Third, pressure tends to be similar over wide regions, whereas more directly important quantities (SST and winds) can have many local effects that make it hard to interpret single point measurements as representative of the large-scale situation.

The SOI is given in normalized units of standard deviation. It 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 El Niño of the late 1990s is about as strong as 1991–92. However, the sea surface temperature anomaly, November 1997, was about as large as in 1982–83, and some might say that is a more important measure. This shows that there is no single number that summarizes the intensity of events.

Why is predicting these types of events so important?

Better predictions of the potential for extreme climate episodes like floods and droughts could save the United States billions of dollars in damage costs. Predicting the onset of a warm or cold phase is critical in helping farmers and water, energy, and transportation managers plan for, avoid, or mitigate potential losses. Advances in improved climate predictions will also result in significantly enhanced economic opportunities, particularly for the national agriculture, fishing, forestry, and energy sectors, as well as social benefits.

How do scientists detect La Niña and El Niño and predict their evolution?

Scientists from NOAA and other agencies use a variety of tools and techniques to monitor and forecast changes in the Pacific Ocean and the impact of those changes on global weather patterns. In the tropical Pacific Ocean, El Niño is detected by many methods, including satellites, moored buoys, drifting buoys, sea level analysis, and expendable buoys. Many of these ocean observing systems were part of the Tropical Ocean Global Atmosphere (TOGA) program, and are now evolving into an operational El Niño/Southern Oscillation (ENSO) observing system. NOAA also operates a research ship, the KA'IMIMOANA, which is dedicated to servicing the Tropical Atmosphere Ocean (TAO) buoy network component of the observing system. Large computer models of the global ocean and atmosphere, such as those at the National Centers for Environmental Prediction, use data from the ENSO observing system as input to predict El Niño. Other models are used for El Niño research, such as those at NOAA's Geophysical Fluid Dynamics Laboratory, at the Center for Ocean-Land-Atmosphere Studies, and other research institutions.

How are sea surface temperatures monitored?

Sea surface temperatures in the tropical Pacific Ocean are monitored with data buoys and satellites. NOAA operates a network of 70 data buoys along the equatorial Pacific that provide important data about conditions at the ocean's surface. The data is complemented and calibrated with satellite data collected by NOAA's Polar Orbiting Environmental Satellites, NASA's TOPEX/POSEIDEN satellite, and others.

How are the data buoys used to monitor ocean temperatures?

Observations of conditions in the tropical Pacific are essential for the prediction of short term (a few months to one year) climate variations. To provide necessary data, NOAA operates a network of buoys that measure temperature, currents, and winds in the equatorial band. These buoys transmit data that are available to researchers and forecasters around the world in real time.