El Niño and La Niña alternate about on a 2-7 year cycle.

Usually winds blow from South America and Central America along the equator in the direction of the other side of the Pacific Ocean, towards Australia. Sometimes, this direction of flow can flip, and winds will blow towards South and Central America. The flipped weather pattern is called El Niño, and when the ordinary situation is more powerful(trade winds blowing towards Australia), then the weather pattern is La Niña. La Niña and El Niño set off a bit of chain reaction, a domino effect, that can cause changes in rain and affect crops from Australia all the way to the other side of the U.S.

La Niña (wind towards Australia) pushes warm water away from the South and Central American coast and causes upwelling of cold water. This usually means that the southern and south-central U.S. experiences drier conditions while the Pacific Northwest (Oregon, Washington) gets more rain. During El Niño, southern California and southern states in the U.S. are generally wetter, but the Pacific Northwest is drier. The water along the equator and off the South and Central American coasts is warmer, and less nutrient-rich, which makes fish less plentiful during that time.

This is a video from NOAA which explains El Niño and La Niña episodes:
El Niño and La Niña.

El Niño and La Niña episodes typically last nine to 12 months, but some prolonged events may last for years. While their frequency can be quite irregular, El Niño and La Niña events occur on average every two to seven years. Typically, El Niño occurs more frequently than La Niña.