Author: Donna Conneely
The surface temperature of the Pacific Ocean near the equator can have far-reaching influence over weather patterns all over the planet — even in areas thousands of miles away. When equatorial Pacific Ocean surfaces go through periods of higher than normal temperatures, an El Niño year is the result. When the temperatures are lower than normal, a La Niña year occurs. While there is no strict timetable for these fluctuations, they do seem to recur at intervals of 3-7 years.
The El Niño phenomenon was first observed centuries ago by fishermen who worked off the coast of South America. The fishermen noticed that every 3-7 years the fish they relied on for a living disappeared. Because this usually happened in December, they called this period “El Niño”, after the Christ Child.
Under normal conditions, the dependable trade winds move from east to west across the Pacific, dragging warmer water to the western Pacific. This movement of the warmer water sucks up colder water from the depths of the eastern Pacific — like a giant water wheel.
When the warmer waters, dragged by the trade winds, move to the western Pacific, they cause a sea surface temperature increase of about 8 degrees. This in turn causes more warm air to rise, along with more evaporation — which means more rainfall. On the other hand, the eastern Pacific experiences lower rainfall as the colder water that has risen from the deep decreases evaporation.
During an El Niño year, the trade winds do not blow as they normally do, and scientists have not yet figured out why this happens. When the trade winds fail, the push of warmer water to the western Pacific does not occur. The dragging up of colder water from the deep is likewise greatly reduced. So we see a reversal of the normal patterns of rainfall, with much greater precipitation in South America, Central America, and parts of North America. Conversely, droughts occur in areas that are normally very rainy at that time of the year, notably in Australia and Indonesia.
Not all El Niño years are alike, and conditions caused by the phenomenon cannot be accurately predicted. However, it is thought that many of the world’s most devastating floods have been caused by El Niño conditions. At the same time, El Niño years have also caused some of the most destructive droughts in history.
Scientists cannot predict an El Niño year far in advance, but weather agencies can observe the signs that indicate an El Niño year is forming, and issue a warning. The National Weather Service and the Climate Prediction Center are two agencies that work to provide citizens, scientists, and researchers with timely information about the formation of an El Niño event. Armed with this information, business and homeowners can step up flood protection measures and prepare for possible floods or major storms.
Recently the National Weather Service announced that there is a 50% chance that El Niño conditions are developing for this year, meaning the likelihood of flooding, severe weather events, and increased rain is heightened for areas affected by El Niño patterns.