When was the last time El Nino hit? What are the differences between El Nino and La Nina?

El Nino and La Nina are two phases of the El Nino Southern Oscillation that involves changes in equatorial water temperature in the Pacific Ocean. El Nino is the warm phase of ENSO, and brings wetter conditions across the southern tier of the United States and parts of South America, and drought in the western Pacific. La Nina is the cold phase of ENSO.

Global climate impacts of La Nina tend to be opposite those of El Nino, NOAA explains, with the impacts of El Niño and La Niña at higher latitudes including North America most clearly seen in wintertime.

"In the continental United States, during El Niño years, temperatures in the winter are warmer than normal in the North Central States, and cooler than normal in the Southeast and the Southwest. During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest," NOAA explains.

The name El Nino means "the little boy" or "Christ child" in Spanish, and was given to the warming sea waters by South American fishermen who noticed the phenomenon and its impact on their fishing. El Nino events result in a reduction of nutrients at the surface of the central and eastern Pacific Ocean, and fewer fish. La Nina, or "the little girl," was named as the opposite event, a cooling of surface waters, more upwelling of nutrients, and better fishing.

Past El Nino events have brought higher than normal temperatures November through January to the Pacific Northwest, the Midwest, and New England, and colder than normal temperatures to the Sierras, southwestern Wyoming, southern New Mexico and parts of Texas in 1941, 1942, 1958, 1964, 1966, 1973, 1983, 1987, 1988, 1992, and 1995.

El Nino brought drier than normal conditions those same months to the Rockies of Idaho, Montana, and Wyoming, and wetter than normal across the southern tier of the United States, up into the Midwest, and parts of southern New England.

Past La Nina events have brought colder than normal temperatures November through January to the U.S.-Canadian border region in 1943, 1951, 1956, 1965, 1974, 1976 and 1989, and warmer than normal temperatures to the Southwest from Nevada, across central Wyoming to Texas and Louisiana. La Nina brought wetter conditions to the Pacific Northwest and northern Rocky Mountains, and drier conditions across the southern tier of the United States those same years.