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 degrees F in the "warm pool" located in the central and western Pacific.

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 with an on average every 3-4 years.

There have only been two strong La Niña events. Although scientists have looked for a correlation between La Niña and tornadoes, there just isn't enough data to make any conclusions.

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 science community today. Research will help us separate the natural climate variability from any trends due to man's activities. We cannot figure out the "fingerprint" of global warming if we cannot sort out what the natural variability does.

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.

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.

I've discussed the supposed effects of El Niño on the weather, in general, on my Pet Peeves page (item # B.12). Although El Niño (and its counterpart, La Niña) certainly can influence the general circulation pattern around the world, and that pattern (in turn) alters the probabilities of certain events, it's absurd to say that a particular tornado was "caused" or "spawned" or "driven" by El Niño.

This question does not have a simple or straightforward answer, since this is not a settled issue. Fundamentally we are not exactly sure why the Pacific should have an El Niño/La Niña cycle and the Atlantic not. We observe that this is the case and can think of reasons why this makes sense, but if we had no observations to tell us the answer in advance I don't think we would deduce it from theory.

La Niña is defined as cooler than normal sea-surface temperatures in the central and eastern tropical Pacific ocean that impact global weather patterns. La Niña conditions recur every few years and can persist for as long as two years.

It would have no effect. The time and space scales are completely different. El Niño represents the large-scale, but slow, redistribution of heat across the tropical Pacific. A tsunami is a short-lived pulse in which the entire thickness of the ocean shifts position a small distance.

No! It is wrong to think of this as an oscillation, simply swinging back and forth. There can be several El Niños in a row, as we had in the early 1990s. Many scientists are coming to the view that there may not be such a thing as La Niña, or at least that it is not just the opposite of El Niño. Perhaps there is just the normal situation that is disturbed every few years by an El Niño.

Teleconnection modes can be visualized as the preferred ways the atmosphere oscillates (in a manner roughly akin to a drum head) on time scales of weeks and longer. El Niño disturbs the atmosphere by moving the west Pacific warm pool (region of sea surface temperature greater than 28C, usually found from Indonesia to the dateline) eastward. The warm pool is one of the main sources of heat driving the atmosphere, comparable to a continent during summer.

No. During El Niño winters, for example, Florida receives significantly more rain than normal, which reduces the risk of wildfires in the spring and early summer. Reservoirs fill and skiers and sport fisherman are happy in many places. The formation of desert lakes in northern Peru form a temporary habitat for vegetation, freshwater fish and farmers.

There are two kinds of El Niño phenomena in the Atlantic. One is a spin-off from the Pacific El Niño due to transmission through atmospheric fluctuations. This atmospheric signal from the Pacific El Niño is the same phenomenon that causes the Atlantic to experience fewer hurricanes during El Niño years.

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.

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.

El Niño and La Niña occur on average every 3 to 5 years. However, in the historical record the interval between events has varied from 2 to 7 years. According to the National Centers for Environmental Prediction, this 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.

The reason that you don't see much publicity about the causes of El Niño is that we don't understand the origins of the event. We do, however, have a pretty good understanding of how it evolves once it has begun, and that gives a useful ability to make forecasts 6-9 months ahead for some regions. That is the information you see because that is the present state of reasonably-secure knowledge.