El Nino 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 Nino is thus associated with a slackening, or even cessation, of the cold upwelling conditions which typically prevail in that area.
During a typical El Nino, the ocean warms a degree or two (C) above its climatological average. A strong El Nino can warm by 3-4 degrees C over large areas, and even 5 degrees C in smaller regions.
Typically, El Nino 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 Nino
typically begins to appear about a month later (near the Galapagos) to
about 4 months later (near the Date Line) than near the coast.
What is La Niña?
La Nina is essentially the opposite of El Nino. La Nina exists when cooler than usual ocean temperatures occur on the equator between South America and the Date Line. The name La Nina ("the girl child") was coined to deliberately represent the opposite of El Nino ("the boy child"). The terms El Viejo and anti-El Nino are also sometimes used. La Nina occurs almost as often as El Nino, but has been lesser known. La Nina and El Nino are but two faces of the same larger phenomenon.
Stronger than usual trade winds accompany La Nina. 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.
What is ENSO?
"ENSO" stands for "El Nino / Southern Oscillation". The acronym arose in the climate research community, and reflects an attention bias toward the warm phase of the entire cycle.
El Nino 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 Nina". The El Nino/La Nina "cycle" does not occur with strict periodicity. Historically, an El Nino usually recurs every 3-7 years, as does its (cold) La Nina 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 Nino and La Nina. 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 Nino/La Nina.
Because more attention has been devoted to El Nino, and noting the association between the Southern Oscillation in the atmosphere and El Nino (and La Nina) in the ocean, the research community began to refer to the combination as ENSO (El Nino/Southern Oscillation). This moniker is somewhat asymmetric: El Nino pertains to just one of the two phases of the Southern Oscillation.
It would be perhaps more accurate to refer to El Nino as the warm
phase of the Southern Oscillation, and to La Nina as the cold phase of
the Southern Oscillation. The term "ENSO" is, however, firmly
How does El Nino affect climate in the West?
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.
Eastern Pacific autumn tropical storms, west of Mexico, appear to
be less frequent in El Nino 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 Nino 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.
During El Nino 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.
With El Nino, 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 Nino winters can be two to three times wetter than La Nina winters in this region.
In the Pacific Northwest, El Nino 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 Nino 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 Nino.
In Hawaii, El Nino tends to bring dry winters. Drought is more likely during El Nino years, during the October-March period. This association is well known in the Hawaiian Islands.
In general, in all these regions, La Nina climate effects are
approximately, but not exactly, opposite to El Nino climate effects.
Winter temperatures with El Nino 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.
With El Nino 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 Nino 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.
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 Nino 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 Nino are typically delayed, in
some cases for several months. Thus, the effects of El Nino on
streamflow may not be manifest until late spring or summer. Usually,
El Nino results in less streamflow to the Pacific Northwest and
greater streamflow in the Southwest.
In southern California, Arizona, southern Nevada, New Mexico and
southern Utah, almost of the major flood episodes on mainstem rivers
have occurred during El Nino winters. None have occurred during La
Nina winters. The likelihood of flooding is considerably increased,
but flooding is not a guaranteed outcome.
How does La Nina affect climate in the West?
To a first approximation, it appears that the consequences of La Nina are nearly the opposite of El Nino in much of the U.S., including the West. In the previous discussion of El Nino effects, simply substituting the opposite words yields an approximately correct description.
Exceptions to La Nina / El Nino Opposition
1. The La Nina climate signal in the West seems more reliable than the El Nino signal. This is especially true in the Southwest. El Nino generally brings wet weather there in winter, but there are a number of exceptions. La Nina brings dry winters to the Southwest, and there are no exceptions, during the past 65 years. That is, La Nina brings much more consistent consequences in the Southwest.
In the Pacific Northwest, this appears to be not as true. La Nina generally brings cold, snowy, wet, active winters to the northern Cascades and the northern Rockies. There are a few exceptions to this picture among La Nina years. There appear, however, to be more such exceptions in El Nino years, to the dry, mild winter pattern these regions typically experience with El Nino.
2. For both El Nino and La Nina, the north/south dividing line between the opposing effects in the Southwest and the Pacific Northwest extends as a zone from about San Francisco to Cheyenne, Wyoming. The effects of both phases of ENSO are equally nebulous in this region. The effects of larger El Ninos extend farther north along the West Coast, and the effects of La Nina can extend south along the West Coast from the Pacific Northwest. Thus, northern California can be the recipient of moisture from the northern end of El Nino's effects, and the southern end of La Nina's effects.
3. In the central Sierra there are no large-scale winter floods associated with El Nino. All but one of the biggest floods have occurred in La Nina winters. However, not all La Nina winters have large floods, and many have small or average winter flood peaks. Thus, La Nina opens the door to, but does not guarantee, large scale rain-on-snow conditions associated with the biggest Sierra floods--more so than El Nino. The deep tap to abundant tropical moisture (the so-called "pineapple connection") associated with major Sierra floods has a higher likelihood of occurrence in La Nina years than in El Nino years, but in both cases is not common. These very large floods can be generated in just a few days, and the weather pattern during that time may poorly represent the overall character of the winter. Both 1996-97 and 1985-86 illustrate this point very well: without the short period of intense rains, these two years with the largest floods would likely have entered the record books as drought winters.
4. The overall atmospheric flow patterns differ substantially between La Nina and El Nino winters. El Nino winters tend to feature strong and persistent flow from the Pacific into North America, blocking the movement of cold Canadian air toward the south. La Nina winters have much more north-south movement of air masses, and alternations of temperature, particularly in the northern half of the West.
5. Although El Nino has received considerably more attention than
La Nina, evidence suggests that the types of weather associated with La
Nina winters have more deleterious effects to the national economy than
do those of El Nino.
Are there combined effects that are more likely?
In some areas, combined events can interact to accentuate the
effects of El Nino. 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.
Are big El Ninos different from average El Ninos?
This is one of the strongest El Ninos on record. There is some
evidence that the effects of large El Ninos (which constitute a small
sample) may be different from those of the typical El Nino. In
particular, heavier precipitation may occur farther north in Nevada
and California, and especially along the coast. For example, the
record El Nino 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.
How much confidence can we place in the predictions?
All forecasts are fallible(!). In the Southwest, not all El Ninos
bring wet winters, and in the Northwest, not all El Ninos 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 Nino the likelihood of a dry winter
is increased to 65-75 percent. For the 1997-1998 El Nino, 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
Is there a tie between El Nino and global warming?
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 Nino
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.
Are there long term trends associated with El Nino?
Yes. A number of climate indicators were noted to have changed in 1976, especially around the Pacific Basin. Prior to 1976, El Nino and La Nina occurred with about equal frequency, each at intervals of about 3-7 years. Since 1976, there have been 9 El Ninos (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 Nina in that interval (1988-89) and a rather weak La Nina (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.