Compiled by Kelly Redmond from various sources.
Updated June 16, 1998, to add emphasis to La Niña effects.
El Niño is a warming of the Pacific Ocean between South America and the Date Line, centered directly on the Equator, and typically extending several degrees of latitude to either side of the equator. Coastal waters near Peru also warm. The warming is expressed as a departure from long-term average ocean temperatures, which are generally cool in the region, due to upwelling. El Niño is thus associated with a slackening, or even cessation, of the cold upwelling conditions which typically prevail in that area.
During a typical El Niño, the ocean warms 1 or 2 °C above its climatological average. A strong El Niño can warm by 3-4 °C over large areas, and even 5 °C in smaller regions.
Typically, El Niño is first noticed along the South American coast around Christmas (hence the origin from Peruvian fishermen of its Spanish name – "the child"). Farther west, in the open ocean, El Niño typically begins to appear about a month later (near the Galapagos) to about 4 months later (near the Date Line) than near the coast.
La Niña is essentially the opposite of El Niño. La Niña exists when cooler than usual ocean temperatures occur on the equator between South America and the Date Line. The name La Niña ("the girl child") was coined to deliberately represent the opposite of El Niño ("the boy child"). The terms El Viejo and anti-El Niño are also sometimes used. La Niña occurs almost as often as El Niño, but has been lesser known. La Niña and El Niño are but two faces of the same larger phenomenon.
Stronger than usual trade winds accompany La Niña. These winds, from the east, push the ocean water away from the equator in each hemisphere. (This is caused by the rotation of the earth.) Cold water from below rises to replace the warm surface water which has moved away from the equator.
The cool water acts as an impediment to the formation of clouds and tropical thunderstorms in the overlying air. This suppression of rain-producing clouds leads to dry conditions on the equator in the Pacific Ocean from the Date Line east to South America.
ENSO stands for "El Niño / Southern Oscillation". The acronym arose in the climate research community, and reflects an attention bias toward the warm phase of the entire cycle.
El Niño is just one phase of an irregular fluctuation between warmer than usual and colder than usual ocean temperatures in the region mentioned above. The cold phase has recently come to be known as "La Niña". The El Niño / La Niña "cycle" does not occur with strict periodicity. Historically, an El Niño usually recurs every 3-7 years, as does its (cold) La Niña counterpart.
The overlying atmosphere is tightly coupled to ocean temperatures and circulation patterns. An atmospheric pressure signal is seen throughout the tropics that is strongly linked to El Niño and La Niña. When barometric pressure is higher than usual in the western Pacific near Indonesia, pressure is lower than usual in the subtropical Pacific near Easter Island and Tahiti. This global-scale pressure signal, identified 70 years ago, is known as the "Southern Oscillation". Surface barometric pressure at Darwin, Australia, and the island of Tahiti are strongly anti-correlated: when one is higher than usual, the other is lower than usual. The difference, Tahiti minus Darwin, suitably normalized, is referred to as the Southern Oscillation Index (SOI), and is frequently used as a convenient, simple and reasonably accurate tool to monitor the status of El Niño / La Niña.
Because more attention has been devoted to El Niño, and noting the association between the Southern Oscillation in the atmosphere and El Niño (and La Niña) in the ocean, the research community began to refer to the combination as ENSO (El Niño / Southern Oscillation). This moniker is somewhat asymmetric: El Niño pertains to just one of the two phases of the Southern Oscillation.
It would be perhaps more accurate to refer to El Niño as the warm phase of the Southern Oscillation, and to La Niña as the cold phase of the Southern Oscillation. However, the term "ENSO" is firmly engrained.
Winter versus Summer: The most unambiguous signal is seen in the winter half-year, typically from October through March or April. A weaker signal may be seen in some parts of the West in summer or early autumn. Most of this discussion concentrates on winter.
Tropical Storms: Eastern Pacific autumn tropical storms, west of Mexico, appear to be less frequent in El Niño years, a tendency which is well-established in the Atlantic. However, those tropical storms that do occur have a greater than usual tendency to recurve into Mexico or the southwest U.S. Higher than usual water temperatures off the Mexican coast in El Niño years can help maintain their strength, or cause them to be stronger than they would otherwise be. Hurricanes need water temperatures of about 27 °C (81 °F) or more to sustain themselves.
Winter Circulation: During El Niño years, the storm track more frequently splits into two preferred branches. The Aleutian Low, in the Gulf of Alaska, is deeper than usual, and one branch of the jet stream departing from its vicinity heads toward the Queen Charlotte Islands and the southern coast of the main part of Alaska, bringing mildly increased storminess to those areas. A second branch of the jet stream is seen across the southern tier of the U.S. and northern Mexico, and with higher speeds than usual. Storms approaching the Pacific Northwest, and southwest Canada, are often split and weakened as they approach the shore, as their energy is shunted toward the north and/or the south.
Winter Precipitation: With El Niño, the period October through March tends to be wetter than usual in a swath extending from southern California eastward across Arizona, southern Nevada and Utah, New Mexico, and into Texas. There are more rainy days, and there is more rain per rainy day. El Niño winters can be two to three times wetter than La Nina winters in this region.
In the Pacific Northwest, El Niño tends to bring drier winters. The area affected in this manner includes Washington, Oregon, and the more mountainous portions of Idaho, western Montana and northwest Wyoming. This area of influence extends well up into Canada, and coincides very well with the Columbia River Basin on both sides of the U.S. / Canada border.
In between these regions, including central and northern California, northern Nevada, southern Oregon, northern Utah, southern Wyoming, and much of Colorado, the effects of El Niño are ambiguous. No strong association in either direction (toward wet or dry) can be discerned.
Farther north, from the Queen Charlotte Islands around toward Kodiak Island, the relationship again switches sign, and southern Alaska tends to have wetter winters with El Niño.
In Hawaii, El Niño tends to bring dry winters. Drought is more likely during El Niño years, during the October-March period. This association is well known in the Hawaiian Islands.
In general, in all these regions, La Niña climate effects are approximately, but not exactly, opposite to El Niño climate effects.
Winter Temperature: Winter temperatures with El Niño conditions tend to be warmer than usual from Washington and northern Oregon across the northern tier to Montana, and also along the West Coast. Conversely, cooler than normal temperatures are seen in the far southeastern portion of the West, especially in southeastern New Mexico.
Snowpack: With El Niño conditions, precipitation and temperature effects combine to accentuate the effect on snowfall. In the Southwest, there is a slight tendency toward cooler winters, and a strong tendency toward wet winters, which makes higher elevation snowpack deeper. In the Pacific Northwest, El Niño winters are warmer and drier than usual, so that at a given elevation 1) less precipitation occurs, and 2) the freezing level is higher, so the type of precipitation is more likely to be rain, and 3) the accumulation season is shorter. All three conspire to produce a smaller snowpack accumulation by the end of winter in the Pacific Northwest.
Streamflow Effects: Most streamflow in the West is produced by melting snow in the spring (in general, about 75 percent). At lower elevations rain can be an important component of streamflow. El Niño effects on streamflow are magnified versions of the effects on the climate elements. Because of hydrologic lags (snow does not usually begin to melt until spring), the effects of El Niño are typically delayed, in some cases for several months. Thus, the effects of El Niño on streamflow may not be manifest until late spring or summer. Usually, El Niño results in less streamflow to the Pacific Northwest and greater streamflow in the Southwest.
Likelihood of Floods: In southern California, Arizona, southern Nevada, New Mexico, and southern Utah, almost of the major flood episodes on mainstem rivers have occurred during El Niño winters. None have occurred during La Niña winters. The likelihood of flooding is considerably increased, but flooding is not a guaranteed outcome.
To a first approximation, it appears that the consequences of La Niña are nearly the opposite of El Niño in much of the U.S., including the West. In the previous discussion of El Niño effects, simply substituting the opposite words yields an approximately correct description.
Exceptions to La Niña / El Niño Opposition:
In some areas, combined events can interact to accentuate the effects of El Niño. For example, in the southern West, during the winter months vigorous storms are more likely, precipitation amounts are heavier, the frequency of precipitation is higher, and events are more likely to persist. Thus, ground saturation is likely to be greater, and landslides are more frequent. With more frequent and vigorous storms, coupled with wet soils, trees are more likely to topple. In the northern West, by contrast, such circumstances are proportionately less likely.
There is some evidence that the effects of large El Niños (which constitute a small sample) may be different from those of the typical El Niño. In particular, heavier precipitation may occur farther north in Nevada and California, and especially along the coast. For example, the record El Niño of 1982-83 brought heavy precipitation as far north as Oregon and Washington, the only major exception in the last 70 years to the typical dry winter response expected in the Pacific Northwest.
All forecasts are fallible(!). In the Southwest, not all El Niños bring wet winters, and in the Northwest, not all El Niños bring dry winters. The most appropriate way to use these forecasts is to "hedge one's bets" in the indicated direction. In the Southwest, typically the likelihood of a wet winter is increased from 50 percent (a coin toss) to about 65-75 percent likely. In the Pacific Northwest and northern Rockies, for a typical El Niño the likelihood of a dry winter is increased to 65-75 percent. For the 1997-1998 El Niño, these likelihoods are even higher than usual in the most affected areas, as much as 80 percent for a dry or wet winter, in the two respective areas.
This is a matter of considerable speculation in the climate research community. It is plausible that a warmer earth would produce more and stronger El Ninos. There is some evidence that the earth has warmed over the past two decades, and there is no doubt that El Niño has been much more frequent in that time. If the evidence of a warming earth is taken at face value (not universally accepted), there still remains a wide spectrum of opinions on whether we are seeing a manifestation of human modification of global climate, or whether the natural climate system would be exhibiting this behavior anyway.
Yes. A number of climate indicators were noted to have changed in 1976, especially around the Pacific Basin. Prior to 1976, El Niño and La Niña occurred with about equal frequency, each at intervals of about 3-7 years. Since 1976, there have been 9 El Niños (using a 6-month average of the Southern Oscillation Index of -0.50 as the criteria), or one every 2.2 years. There has been just one moderate La Niña in that interval (1988-89) and a rather weak La Niña (not even counted by some) in 1996-97. Longer perspectives, since 1860, indicate that the 1976-1997 period is quite unlike any other in the record. This is a source of considerable puzzlement at this time.