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El Niño & La Niña


Both weather and climate tend to be quite variable, with short and long time scale variations. Long time scale variations are generally associated with changes in atmospheric circulations, which lead to changes in weather, temperature and rainfall patterns around the world. One of these naturally occurring circulations is the El Niño/Southern Oscillation (ENSO) cycle. This occurs in the tropical Pacific Ocean near the equator when there is a reversal of the surface air pressure at opposite ends of the Pacific Ocean.

Normal Conditions
Normally the trade winds in this region blow towards the west from a region of higher pressure in the eastern Pacific to a region of lower pressure in the western Pacific. These winds enhance upwelling (the rising of cold water from the deep ocean towards the surface) in the eastern Pacific off the coast of South America. Therefore, sea surface temperatures, as seen on the figure below, are cool off the coast of South America and significantly warmer in the western Pacific.





El Niño
Every few years, this pressure pattern breaks down, resulting in higher pressure in the western Pacific and lower pressure in the eastern Pacific. This causes the winds to slow or even blow towards the east instead of towards the west. This wind reversal brings the warmer water from the western Pacific towards South America. This warming of the ocean is El Niño.




This large area of warmer ocean temperatures can have an effect on the global wind patterns, which in turn affects the temperature and rainfall patterns around the world. The following graphics show the effects that a strong El Niño pattern can have globally.


El Niño effect during December through February

El Niño effect during June through August




La Niña
Eventually the El Niño event will begin to subside and the pressure and wind patterns will return to normal. Sometimes, the trade winds, which are blowing towards the west, can be very strong. This strong flow causes the cold surface water in the eastern Pacific to move out over the central Pacific. This cooling of the central and eastern Pacific Ocean is La Niña.




This pattern can also have an effect on the global wind patterns and thus the temperature and rainfall patterns throughout the world. Generally, the impacts of La Niña tend to be opposite those of El Niño. The following graphics show the effects a strong La Niña pattern can have globally.


La Niña effect during December through February

La Niña effect during June through August

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