Every night as The Late show comes on, you hear David Letterman blame everything from the sweltering New York weather, to the free lance prostitutes that roam the area of Times Square, on some weather phenomena called El Nino - which is sometimes referred to as everything but the scientifically correct name. Night after night, the news reporters on television are speaking of this devastating force that is casing irregular weather patterns not only in the United States, but all over the world. I may speak for myself when I say that I had not actually ever heard of this before, but I feel pretty confidant that I am not the only ignorant person when it comes to this natural spectacle. What I would like to do is fully explain all aspects of the natural occurrence that is known as El Nino.
Why is it called El Nino? El Nino was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year. El Nino means The Little One in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas.
Why does El Nino occur? El Nino results from interaction between the surface layers of the ocean and the overlying atmosphere in tropical Pacific. It is the internal dynamics of the coupled ocean-atmosphere system that determine the onset and termination of El Nino events. The physical processes are complicated, but they involve unstable air-sea interaction and planetary scale oceanic waves. The system oscillates between warm to neutral conditions with a natural periodicity of roughly 3-4 years. External forcing from volcanic eruptions has no connection with El Nino, nor do sunspots.
How often does El Nino occur? El Ninos usually occur irregularly, approximately every two to seven years.
How do we detect El Nino's? In the tropical Pacific Ocean, El Nino's are detected by many methods, including satellites, moored buoys, drifting buoys, and sea level analysis. Many of these in-situ ocean observing systems were part of the Tropical Ocean Global Atmosphere (TOGA) program, and are now evolving into an operational El Nino/Southern Oscillation (ENSO) observing system. Large computer models of the global ocean and atmosphere, such as those at the National Centers for Environmental Prediction use data from the El Nino/Southern Oscillation observing system as input to predict El Nino. Other models are used for El Nino research, such as those at NOAA's Geophysical Fluid Dynamics Laboratory, at Center for Ocean-Land-Atmosphere Studies, and other research institutions.
Should we take El Nino forecasts with a grain of salt? El Nino is not the only thing that influences weather. There are many other fluctuations and systems, some of which are just being discovered, and the weather we experience results from the tumultuous interplay of all these systems. Most of the interactions are poorly understood, particularly the longer-term ones, all the way up to Ice Ages, which may operate over periods of hundreds of thousands of years. As we get longer and longer records we become aware of more and more complexity, more cycles. Successive El Ninos occur during different general conditions, at different times of year, and therefore have different total effects. Therefore, you can't simply speak of the isolated effects of El Nino on weather in, say, Houston. There is only the ever-changing combination of influences. That is the main reason why we cannot produce reliable long-term forecasts.
Is there a scale for the intensity of El Nino, like the Richter scale or the typhoon classification? The most widely used scale is known as the Southern Oscillation Index (SOI), which based on the surface (atmospheric) pressure difference between Tahiti and Darwin, Australia, on opposite sides of the Pacific. It was noted as far back as the 1920s that these two stations were anticorrelated, so that when Tahiti pressure is high, Darwin pressure is low. This reflects the very large scale of the phenomena, since one would not usually expect such a close relation between such faraway places. When Tahiti pressure is high, it indicates that winds are blowing towards the west (normal tradewinds), and when it is low, that winds are blowing to the east which causes El Nino. A major advantage of the SOI is that time series at these two locations extend back to the 1880s, so we can see the distribution of El Nino events back much further than we can see in records of ocean temperatures. The SOI is given in normalized units of standard deviation, a way of judging the distribution ("bell-shaped curve") of all the recorded intensities. This can be used as an intensity scale. For example, SOI values for the 1982-83 El Nino were about 3.5 standard deviations, so by this measure that event was roughly twice as strong as the 1991-92 El Nino, which measured only about 1.75 in SOI units. By this standard, the present El Nino is about as strong as 1991-92. However, the sea surface temperature anomaly is larger than in 1982-83, and some say that is a more important measure. This shows that there is no single number that summarizes the intensity of events.
In conclusion, it seems that maybe this El Nino is not as a big of a deal that it may first seem. Yes, it does cause many more storms, and possible disasters, but it only rolls around every 7 or so years. Hopefully, this has helped everyone to understand this phenomenon in a better fashion. I know that the research did at least inform me.