El Niño-Southern Oscillation (ENSO)

The El Niño-Southern Oscillation (ENSO) is a series of linked atmospheric and oceanic events in which a reduction or reversal of the normal large-scale pressure systems acting in the equatorial regions of the Pacific Ocean results in dramatic changes in precipitation, currents, and wind patterns. Repercussions occur over wide areas from the interior of North America to India. Although not regularly periodic, El Niño phenomena generally occur every two to seven years.

Global Wind Patterns

El Niño is the term used for the oceanic aspects of one phase of a climatological phenomenon more generally referred to as an El Niño-Southern Oscillation (ENSO) event. El Niño is a Spanish term meaning “the Christ child” used by Ecuadoran and Peruvian fishermen to refer to unusually warm ocean surface currents that arrived around Christmastime at irregular intervals. "Southern Oscillation" refers to the accompanying effects in the atmosphere. ENSO events typically recur every two to seven years and involve dramatic changes in sea surface temperatures (SST), precipitation, and wind patterns, accompanied by nutrient upwellings. Such environmental fluctuations influence, and in some cases may control, the climate and ecology of large areas of Earth where the effects of ENSO events are most pronounced.

The large-scale wind patterns of Earth result from the different amounts of solar energy received at the equator and at the poles. Generally, a low-pressure region of ascending air can be found at the equator. This air rises, cools, and releases water vapor as rain, creating equatorial rainforests over land. High in the atmosphere, the dry air spreads out, eventually returning to Earth near the latitudes of the tropics, between 23 degrees and 30 degrees north and south. Most of Earth’s large deserts are located in this latitude band. To finish its circuit, this air travels across the surface of Earth toward the equator. Because of Earth’s rotation, however, its path is deflected, a phenomenon called the “Coriolis effect.” In the Northern Hemisphere, the observed deflection is to the right, and in the Southern Hemisphere, it is to the left. Such a deflection causes the air to travel obliquely toward the west in both hemispheres, resulting in the system of winds that are known as the trade winds. (Most of the United States lies north of this area and is under the influence of a similar set of winds blowing in the opposite direction, known as the prevailing westerlies.)

In one pattern called the Walker circulation, warm, wet air rises over Indonesia, spreads out, and then falls as cool, dry air on the eastern Pacific Ocean and the coast of Peru. The existence of this phenomenon became apparent to British scientist Sir Gilbert Walker as he collected data from Tahiti and Darwin, Australia, during the early part of the twentieth century. He found that the atmospheric pressures in these two locations varied together but out of phase with each other. That is, if the pressure rose in Darwin, it fell in Tahiti, and vice versa. Sometimes conditions were normal, with high-pressure zones over Tahiti and lows over Darwin, while at other times they were reversed. Walker called this phenomenon the Southern Oscillation. He showed that many other meteorological phenomena from around the world seemed to be tied to it, though his particular goal was to predict when the monsoons would fall in India.

As the trade winds move across the Pacific Ocean, they drag along great quantities of water. This water is at the sea surface, where it is warmed by incoming solar radiation and builds up in the western Pacific in the vicinity of Indonesia. The elevation of the sea there can be 40 centimeters (16 inches) higher than in the eastern equatorial Pacific. The temperature of the sea changes rather abruptly between the water that has been warmed by the sun and the water beneath it. The depth at which this change occurs is called the thermocline. Because the density of water varies with temperature, this abrupt temperature change corresponds to a density contrast that effectively acts as a barrier, and mixing of water across the thermocline does not readily occur. The buildup of warm surface water in the area east of Indonesia causes the thermocline to settle to as much as 200 meters (656 feet) deep. Off the coast of Peru, the thermocline is typically about 50 meters (164 feet) deep. The difference of 40 centimeters (16 inches) of water at the surface is balanced by a difference of 150 meters (492 feet) on the thermocline because the density contrast across the thermocline, between the cold water and warm water, is much less than the density contrast between the water and air at the surface.

Occurrence of El Niño and La Niña

El Niño and La Niña are opposing phases of the ENSO cycle. Every few years, for reasons that are not well understood and are the result of a complex atmospheric system, the strength of the trade winds decreases. Often, this is accompanied by an eastward spreading of the surface pool of warm water. These two phenomena are linked: Each can and probably does, to some extent, cause the other. As the trade winds collapse, a major readjustment of the thermocline occurs. As the small surface slope dwindles, the much larger slope on the thermocline also disappears. As the thermocline rises, it generates Kelvin waves, which are large-scale gravity waves. These waves race across the Pacific Ocean, deepening the thermocline to the east as they go.

The deeper thermocline and warmer surface water reduce the effectiveness of upwelling in supplying nutrients to the surface water. This causes dramatic decreases in the primary productivity of the oceans off the coast of Peru and often spectacular die-offs in many species of fish and the seabirds and other animals that rely on them. During the 1972 El Niño, die-offs resulted in the collapse of the anchovy fishing industry, although questionable fishery management policies exacerbated its decline.

As the thermocline deepens and warmer surface waters move east, the area in the Pacific equatorial region where the ascent of warm, moisture-laden air produces intense precipitation shifts to the east. This area, called the Intertropical Convergence Zone (ITCZ), is generally found over the warmest water. During strong ENSO events, the Walker circulation may be completely reversed, with warm, moist air rising over coastal Peru, where cool, dry air normally falls. When this happens, regions that have not received a drop of rainfall for years are suddenly inundated with precipitation.

The paths and severity of many storms tracking across the continental United States are affected by the location of the ICTZ. During ENSO events, the jet stream is often moved or disrupted by the heavy storm activity above the zone. Sometimes this results in more precipitation across certain sections of the United States and sometimes less, but typically the result is abnormal weather.

Eventually, El Niño weakens, and the trade winds return to their usual strength. Cold water wells up from the depths to replace the warm, extra water as it dissipates, bringing nutrients to reestablish the high productivity of the waters. As warm surface water builds up in Indonesia, the thermocline there deepens again, and the ICTZ moves toward its earlier location. If, as sometimes happens, the return of the trade winds cools the surface waters beyond their normal temperatures, a phase called La Niña begins.

The ocean/atmosphere system oscillates between the two states of La Niña and El Niño. What drives it from one state to the other remains a matter of considerable speculation. Although the complexities of the ocean/atmosphere system would seem more than adequate to produce almost any sequence of events, some researchers have suggested the involvement of yet another complex system, Earth’s tectonic engine. ENSO events may also be correlated with eruptions of large equatorial volcanoes, which inject dust into the air and dramatically impact the atmospheric components of the cycle.

There is a great deal of interest, both scientific and general, in learning more about ENSO events. Computational capabilities and theoretical developments have progressed rapidly, and although scientists do not yet understand this complex interaction of air and water, they have begun to deploy water temperature sensing buoys across the Pacific to study it.

Study of El Niño

El Niño-Southern Oscillation events are large-scale phenomena. They last longer than one year, involve massive amounts of air and water, and affect a substantial percentage of Earth’s surface. During the course of an ENSO event, the changes that have already occurred in the system influence the changes that will occur as it progresses. Winds affect sea surface temperatures, and sea surface temperatures affect the winds. Such feedback interactions make it difficult to develop models with predictive capability. In the past, computer models used for atmospheric studies often oversimplified the ocean, and those used for oceanic studies oversimplified the atmosphere. ENSO events involve a tight coupling of ocean and atmosphere, and any successful model will need to treat both with sophistication and detail.

Large computer models are used to study ENSO events. As they evolve, deficiencies in the available data set are identified, and additional data are acquired. Much of the necessary data are of the type normally gathered during almost any routine oceanographic study: temperatures, salinities, and surface wind velocities and directions. Researchers would like to have such data from before, during, and after an ENSO event. Because these events recur so frequently, almost any data must be collected within a few years of an event. The difficulty lies in figuring out which data are important. Although there is agreement on what typically happens during an ENSO event, the specifics vary widely from event to event. Just when scientists think they understand it well enough to venture forth with a predictive model, nature comes along and shows them that their model is inadequate.

Still, modelswhether sophisticated computer models or simple physical modelscan further the understanding of ENSO events. One simple physical model uses a clear Pyrex baking pan as the ocean basin; water as the deep, cool, ocean water; vegetable oil as the warmer, less dense, surface water; and a hair dryer (set to blow unheated air) as the trade winds. If the flow of air produced by the hair dryer is directed across the oil, the oil piles up at the far side of the baking pan. The surface of the oil will form a slight slope, with the higher elevation at the far side of the baking pan. A more pronounced, readily visible slope will form where the oil meets the water. Friction from the air produced by the hair dryer pushes surface oil toward the far side of the pan, but as it builds up, it increases the pressure on the water beneath it. This causes the water beneath the oil to move back toward the near side of the baking pan. Eventually a wedge of oil sits over the water, its upper slope maintained by the hair dryer “wind,” and its lower slope positioned where the pressures at depth are the same. (The fraction of a centimeter of additional oil instead of air at the top surface is compensated by a considerably greater deflection of the oil-water boundary because the density contrast between oil and water is much less than that between oil and air.) This represents the normal state in the equatorial Pacific: a shallow thermocline off the coast of Peru; winds moving off the South American continent at Peru, preventing rainfall; a deep thermocline in Indonesia; and air moving from east to west across the ocean, becoming warm and moist, rising over Indonesia, and subsequently drenching it with rain.

In the model, the equivalent of an El Niño can be produced by turning off the hair dryer. The forces that maintained the slope on the upper surface of the oil are gone, so the oil sloshes down and its surface becomes horizontal. A wave forms where the oil meets the water, a boundary that corresponds to the thermocline. Such a wave is called an internal wave because it occurs between two layers of water with contrasting densities but in general is not visible at the surface. The wave races across the baking pan just as the waves race across the Pacific during an El Niño.

Although this physical model demonstrates many of the basics of an ENSO event, no physical model is capable of capturing the entire natural event because its scale is so enormous. The rotation of Earth plays a major role, and the nature of currents and waves as they move directly along the equator is also quite significant. Computer models can incorporate these effects, however, and developing such models is an active area of research.

Additional data regarding the ocean are also required. The Pacific Ocean between Indonesia and Peru has yet to be studied in enough detail to enable scientists to refine their models as much as they would like. The processes that occur during an ENSO event involve large areas and take place over a considerable period of time. Monitoring the sea to detect gradual changes in sea surface temperatures, depth of the thermocline, and surface wind velocity and direction is a complex, expensive project. As more data are acquired, understanding of ENSO events will improve.

Significance

The El Niño Southern Oscillation cycle is as much a part of Earth’s weather and climate as the cycle of seasons. Just as people spend about one-fourth of the year in winter, people spend about one-fourth of their lives in an El Niño phase. Therefore, El Niño events are not truly anomalous. Unlike winter, however, the timing of the ENSO cycle is not regular. For example, from 1991 through 1994, three El Niños developed. Analysis of historical records shows that El Niños have occurred at nearly the same frequency since at least 1525 when record-keeping began in Peru. In addition to historical records, evidence for prehistoric ENSO events is found in tree rings, flood deposits, ice cores, and other geological and biological record keepers. These, too, suggest that the frequency and severity of ENSO events have been fairly constant for at least several thousand years.

The severity of El Niños can vary remarkably. Substantial effort has been made to identify predictors that would permit an early estimate of the occurrence and severity of ENSO events. Several promising predictors of the timing or intensity have emerged but have been abandoned as additional events transpired in which the predictor was present but the expected development did not occur.

Sir Gilbert Walker’s suggestion that local weather in widely separated parts of the planet might be closely related was ridiculed by some of his contemporaries. However, his belief has been borne out by the data and understanding acquired subsequent to his early work. ENSO-related changes can also be seen in the Indian and Atlantic Oceans. Some droughts in Africa and India and most droughts in Australia seem connected to ENSO events. Additionally, weak floods on the Nile River, records of which have been kept since 622 CE., seem directly related to El Niño events.

Droughts and floods can never be fully prevented. However, many of the ensuing hardships could be avoided or reduced if they could be predicted. People and governments in affected areas could take preventive measures: Drought-resistant or flood-resistant varieties of crops could be planted, dikes could be reinforced, deforestation could be halted, and water could be temporarily impounded. Before these measures are taken, however, considerable confidence in any prediction is required. Preparing for a flood may worsen the effects of a drought. Someday, however, it may be possible to predict ENSO events with the same degree of accuracy with which meteorologists predict the arrival of a significant winter storm.

ENSO is also frequently discussed in the context of climate change. Climate change skeptics often claim that the existence of natural, unpredictable shifts in warming and cooling cycles can explain the trend of increasing average temperatures referred to as global warming, therefore denying the theory that humans have contributed to such warming (anthropogenic climate change). However, most scientists reject this belief. For example, 2015 set a new record for global average high temperatures, beating out the previous record set in 2014. Scientists around the world, including with NASA and the US National Oceanic and Atmospheric Administration (NOAA), acknowledged that the El Niño experienced in 2015 (considered the strongest on record to that point) helped boost temperatures but determined that the year would have set the record even without the ENSO effect. Virtually all mainstream researchers agree that while ENSO can have an effect on local and even global temperatures, it is a relatively short-scale process, and climate change is responsible for the general trend of increasing temperatures. Scientists continue to study the connections between ENSO and global warming, with many suggesting that the latter causes more extreme versions of the former. This holds critical implications for ecosystems around the world, including humans.

Principal Terms

convection cell: the cyclic path taken when warmer, less dense material in one part of a fluid or gas rises, cools, contracts, and becomes denser, then descends again to its original level

El Niño: the warm extreme of the Southern Oscillation cycle, with unusually warm surface water in the equatorial Pacific Ocean and subdued trade winds

ENSO: acronym for El Niño Southern Oscillation, used to denote the combined atmospheric/ocean phenomenon

La Niña: the cold extreme of the Southern Oscillation cycle, with unusually cold surface water in the equatorial Pacific Ocean and enhanced trade winds

Southern Oscillation: the reversal of atmospheric pressures that occurs between opposite sides of the tropical Pacific Ocean

thermocline: the depth interval at which the temperature of ocean water changes abruptly, separating warm surface water from cold, deep water

trade winds: winds blowing from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere and converging at the Intertropical Convergence Zone, which wanders back and forth across the equator during the course of a normal year

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