Weather-wise, El Niño events can cause severe weather events, including such extremes as flooding in the U.S. Southeast and California, ice storms in the Northeast, and tornado outbreaks in Florida. During a La Niña year, the climate in various places around the world can change; for example, during a La Niña event, the average winter temperatures are warmer than normal in the Southeast United States and cooler than normal in the Northwest—just the opposite as what happens during El Niño years, in which temperatures in the winter are warmer than normal in the North Central states and cooler than normal in the Southeast and the Southwest.

The environment also suffers during a severe El Niño. Large numbers of fish and marine plants may die; the decomposing dead material depletes the water’s oxygen supply, leading to the bacterial production of huge amounts of smelly hydrogen sulfide. A greatly reduced fish (especially anchovy) harvest affects the world’s fishmeal supply, leading to higher prices for poultry and other animals that normally are fed fishmeal. Anchovies and sardines are also major food sources for marine mammals such as sea lions and seals; when the food source is in short supply, not only do many sea lions and seals starve, but also a large proportion of the infant animals die.