The defining characteristic that determines whether a period of time will be dominated by El Niño or La Niña is the temperature of the east-central Pacific Ocean. During El Niño years, the Oceanic Niño Index (ONI), a measure of the departure from normal sea surface temperature in the east-central Pacific Ocean, needs to increase more than 0.5 °C (0.9 °F) away from normal conditions for at least five successive overlapping three-month seasons. In contrast, during La Niña episodes, the ONI shows a cooling of more than 0.5 °C (0.9 °F) for the same amount of time. Whereas El Niño episodes are characterized by a drier western Pacific and southeastern Africa, a wetter eastern Pacific, unseasonably warm conditions in parts of Asia and North America, and reduced tropical cyclone activity in the North Atlantic–La Niña episodes, in general, produce the opposite effects.
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