El Nino, formally known as the El Nino Southern Oscillation, is a major five-year climate pattern over the Pacific Ocean, which affects land regions all around that ocean, including at considerable distances from the coast. It is characterized by unseasonally warm weather, whereas the opposite part of the cycle, referred to as "La Nina," is characterized by below-average temperatures.
To understand how El Nino affects the weather, it is important to understand the difference between climate and weather. Climate refers to long-term trends and tendencies in temperature, precipitation (rainfall and snowfall), and humidity. In contrast, weather refers to short-term changes in these same factors. Forecasting the weather involves predicting very specifically what will happen in the next several days, whereas understanding climate involves predicting very generally what may happen in the next several years. Because of the extreme complexity of the atmosphere, in some ways it is much easier to predict the general long-term trends of climate than the detailed, short-term reality of the weather. El Nino is a climate pattern which, on average, skews the daily weather towards higher temperatures.
El Nino is caused by the surface temperature of the tropical Pacific Ocean. Approximately every five years, the surface temperature of the ocean warms or cools by more than half a degree Celsius (1 degree Fahrenheit). When that happens, it tends to last anywhere from several months to over a year, unleashing substantial but temporary climate changes. The actual cause of this is unknown, but when the waters warm, this leads to the atmospheric changes we known as El Nino.
While we do not understand the causes, we can chart the effects. Surface pressure starts to rise over the southern Pacific and the Indian Ocean, affecting Australia and Indonesia. Farther north, and east, air pressure falls. Warm water from the western Pacific is drawn eastward, largely pulling rainfall with it, so that the western Pacific sees a drop in precipitation while the eastern Pacific (including North and South America) sees an increase in precipitation. At sea, this water tends to be poor in nutrients and decimates southeastern fish populations.
As the ocean currents shift, more than just rainfall changes with it. Technically, meteorologists separate just the ocean current change (called El Nino) from the atmospheric consequences, which they refer to as the Southern Oscillation. The warm pool of moving water raises temperatures and causes both more precipitation (usually rain) and more storms, particularly thunderstorms. In the Americas, this leads to unusually wet, warm winters, including flooding in South America. In western North America, the winters are not usually wetter (and can be drier), but they are normally wetter. Farther east, towards the Atlantic coast, the long-distance effects of El Nino can be opposite, causing wetter and cooler winters.
El Nino must be understood not just on its own, but as a part of a cycle: it is (from an American perspective) the warm phase, whereas the opposite part of the cycle is a cool phase known as La Nina. La Nina sees the eastern Pacific waters cool, resulting in increased rainfall or snowfall in the western U.S. and Canada but drier conditions, even drought, in western South America (Peru and Chile).
- Sources and More Information -
Environment Canada. "El Nino."
Franklin Institute. "El Nino: Hot Air over Hot Water."