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what is el nino? El Niño  Southern Oscillation refers to the cycle of warm and cold temperatures, as measured by sea surface temperature (SST) of the tropical central and eastern Pacific Ocean.  El Niño  is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The  El Nino  phenomenon, which occurs every three to five years, is  caused  by warmer water in the eastern Pacific Ocean. This water is warmer because as trade winds lessen or reverse their direction, winds from the west push warm surface water to the east in the direction of the continent of South America. Global climate La Niña impacts tend to be opposite those of El Niño impacts. In the tropics, ocean  temperature  variations in La Niña also tend to be opposite those of El Niño. During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.