What is El Niño?
One of the most important sources of year-to-year climate variation in the Southwest is the El Niño phenomenon of the tropical Pacific Ocean. El Niño is a natural but largely unpredictable condition that results from complex interplay among clouds and storms, regional winds, oceanic temperatures, and ocean currents along the equatorial Pacific.
Under "normal" conditions, the tropical trade winds blow from east to west,
Figure 1. Schematic diagram of normal and El Niño conditions in the Pacific Ocean. From NOAA El Niño website.

ponding up warm water in the western Pacific. In the eastern Pacific, the trade winds pull up cold, deep, nutrient-rich waters along the equator from the Ecuadorian coast to the central Pacific. The warmth of the western Pacific results in a particularly vigorous hydrologic cycle there with towering cumulus clouds and tropical storms that "radiate" atmospheric waves and disturbances across vast regions of the globe. Heat and moisture lofted into the upper atmosphere by the clouds and storms are distributed by high-altitude winds across vast regions of the globe.

During an El Niño, this situation is disrupted and the trade winds weaken, thus reducing the upwelling of cool waters in the eastern Pacific and allowing the pool of warm water in the west to drift eastward toward South America. As the central and eastern Pacific warms, atmospheric pressure gradients along the equator weaken, and the trade winds diminish even more.

These changes in sea-level pressure of the atmosphere are characteristic of the strongest El Niño and were identified as the "Southern Oscillation" of the global atmosphere by Sir Gilbert Walker in the early decades of this century. A chicken-and-egg relation exists between the changes in ocean temperatures and changes in winds (and atmospheric pressure gradients); the two sets of changes reinforce and drive each other but neither is clearly or universally "the" initiator of El Niño. Ocean temperatures and surface winds interact to form the complex process, El Niño-Southern Oscillation (ENSO). The interactions can be set off by subtle changes in one or the other, by buffeting from other parts of the tropics, or from regions beyond the tropics. Such a complex interplay and its uncertain (and variable) origins are the primary limitations on our ability to predict El Niño.

As the waters of the central and eastern Pacific warm, the powerful tropical Pacific storms begin to form farther east than usual (Fig. 1). As the distribution of storms spreads east along the equator, their influence on global weather systems also changes. Most notably, for our purposes, the jet stream over the North Pacific Ocean is invigorated and pulled farther south than normal, where it collects moisture and storms and carries them to the southwestern United States and northern Mexico.

During an El Niño, the trade winds are too weak to cause upwelling of nutrient-rich waters off the coasts of Ecuador and Peru. Generations of South American fisherman thus have recognized these conditions by the disappearance of their standard catch, commonly during December and January, every three to seven years. Because of the near coincidence in timing between these conditions and Christmas, the fishing communities have called the phenomenon "El Niño", for the Christ child. The geologic record suggests that El Niño conditions have been a part of earth's climate for at least several thousand years.

An El Niño event usually lasts for several seasons, and, along with its other effects, represents an interruption of the "normal" seasonal cycle of the tropical climate. After a few seasons, and usually during spring time (in the Northern Hemisphere), the seasonal cycle reasserts itself and the tropical ocean cools back to the normal east-to-west sea-surface temperature gradients. Sometimes the warm El Niño events give way to unusually cold sea-surface temperatures and unusually strong trade winds, a condition now called La Niña. On other occasions, La Niñas may begin on their own, without an immediately preceding El Niño. The effects of the El Niño and La Niña on global climate are, in part, mirror images of each other. For example, drought is a common occurrence in the southwestern United States during La Niña, in contrast to the wet years associated with El Niño.

The Next El Niño
The warming associated with the most recent El Niños, however, has been unusually persistent. The most recent El Niño waxed and waned for over four years in the early to mid 1990s. Some scientists raise the possibility that heating of the atmosphere by global warming could increase the frequency and duration of El Niño events, creating longer periods of flooding and drought (Trenberth and Hoar, 1996). Such a possibility may undergo initial scrutiny in the near future, because recent observations of wind, water temperature, and fish in the Pacific Ocean suggest the onset of the next El Niño.

El Niño as a Window to Future Climate?
Many influences in the global climate system, other than El Niño, determine the climate, hydrology, and landscape of the Southwest. Nevertheless, the transient El Niño is certainly the best studied example of a global climate change, having coherent influences (varying rather smoothly from place to place) that arise from recognized physical origins. El Niño is also of a scale, both spatially and in terms of the magnitude of temperature deviations, that is comparable to much of the greenhouse change projected in climate-model experiments. The ways in which water and land resources are managed and maintained in the presence of El Niño-like variability, which has time scales of only several seasons to several years, provide snapshots of the kinds of responses that might be necessary in a climate-changed world. Qualitatively, the primary difference is that an El Niño is "always" interrupted, whereas any new state or mode of global climate would result in dominant conditions and climatic variability unlike those of today.

The social and geological impacts of El Niño: A global reach
The effects of El Niño are widespread, and any El Niño event may touch the lives of more than a billion people around the globe. The impacts can be devastating, as illustrated by some of the effects of the unusually strong El Niño of 1982-83: Drought (sometimes with associated wildfires) in many nations of the western and southwestern Pacific Rim, southern Africa, southern India and Sri Lanka, Spain, Portugal, northern Africa, and parts of South and Central America; severe cyclones that damaged island communities in the Pacific; flooding over wide areas of South America, in western Europe, as well as in the Gulf Coastal states and some Caribbean islands; and severe storms in the western and northeastern United States (Glantz, 1996).
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Impacts of El Niño and Benefits of El Niño Prediction

When weather from El Niño visits the Southwest United States
It is not surprising that destructive weather can affect the Southwest during El Niño years, because much of western North America lies in the path of storms that can be carried by the atmospheric circulation patterns spawned by El Niño. Below we examine some effects of El Niño events on streamflow, on lake-level change of Great Salt Lake, and on landslide potential. In general, El Niño events increase precipitation and landslide activity in southern California and the Southwest. A recent example is the 1992-93 winter in the Los Angeles area and northern Arizona. On the other hand, El Niño may bring drought to the northern Rocky Mountains, part of which lie within the Upper Colorado River Basin.
El Niños vary widely from event to event, with different sequences and distributions of sea-surface warming and wind-pattern changes. These variations result in large differences in the weather of the Southwest depending on the particular pattern, strength, and timing of El Niño influences on storm tracks. Weather typically differs markedly from north to south during an El Niño event (wet in south, dry in north) but also usually varies greatly within one region from event to event. Areas near the boundaries between regions that receive more rain during El Niño and regions that receive less are especially prone to event-to-event differences. As an example, San Francisco lies near the transition between wet and dry El Niño zones, and thus the El Niño effects can be quite variable. Las Niñas also have differing influences from episode to episode. As a result, the San Francisco area may receive heavy rains (or drought) during either El Niño or La Niña.