The El Nino Phenomenon
The El Nino phenomenon occurs periodically, affecting the weather patterns in different regions of the world. It is caused by a change in the oceans’ surface temperature. This is accompanied by changes in atmospheric circulation and wind patterns throughout the tropical Pacific.
Impact on Rainfall Patterns
The resulting cascade of effects can have a significant impact on rainfall patterns across the tropics, including parts of South America and the United States. It also can alter the flow of the Pacific jet stream, the high-altitude winds that sweep from west to east.
Key Factors to Keep in Mind
There are a few key factors to keep in mind when studying the El Nino cycle:
Episode Frequency and Variability
It is important to understand that an episode of El Nino does not happen every year, and that no two episodes are the same. The intensity of an episode is based on many factors, such as how much the sea temperature in the equatorial Pacific has warmed or cooled, which affects wind patterns and other climate variables.
Historical Examples
For example, in the 1982-83 El Nino episode, the equatorial Pacific had a higher-than-normal ocean temperature and warmer-than-normal air temperatures around South America. The effects of the El Nino were felt around the globe, causing droughts in Australia and South America, flooding in central Chile, and record precipitation in parts of Africa and Asia. Historically, most El Nino cycles have been fairly short, typically lasting less than a year. However, there have been a few long-term periods of El Nino, such as the 1982-83 cycle and the 1997-98 cycle.
Forecasting El Nino
An El Nino event begins when the sea surface temperature (SST) in the eastern Pacific Ocean is above average for five consecutive months. The NOAA, the National Weather Service, and other agencies measure this by using their network of scientific buoys that collect data on sea and air temperatures, currents, winds, and humidity. These data are then analyzed to gauge the severity of an El Nino event. In order to accurately forecast an El Nino, scientists use a variety of methods, including dendrochronology, which is the study of tree rings. The thickness of the rings in trees can indicate whether a tree is experiencing more rain or drier conditions than usual. Thin rings indicate drier seasons, while fatter rings indicate rainier seasons.
Causes and Weather Effects
Although the exact cause of an El Nino is unknown, research suggests that it may be linked to changes in ocean chemistry and the weather system in the eastern tropical Pacific. During an El Nino, the trade winds that normally blow across the Pacific tend to weaken. This results in warm water pushed back toward the west coast of South America.
Regional Weather Effects
During an El Nino, the weather in the Southern United States is generally wetter than normal. This is primarily because the Pacific jet stream moves south of its neutral position. This can make it easier for the Gulf of Mexico to have more rain than normal, and it can cause more flooding in parts of the Southeast. Similarly, in the northern part of the United States and Canada, El Nino periods are often dryer than normal. This can also make it more difficult to deal with floods.
