If the El Nino is particularly strong, its effect in the ocean can reach as far north as the California coast. The opposite phase of the El Nino is called La Nina. La Nina and El Nino are the two opposite phases of the same cycle. During La Nina, the tradewinds are especially strong, and the warm pool in the western Pacific and cold water at the equator in the central and eastern Pacific are particularly well-developed. Sea level is especially high in the western Pacific.
Rainfall is strong in the western Pacific, with little rainfall in the eastern Pacific.
La Nina affects from the tropical Pacific can reach far to the northeast and southeast through the atmosphere. The cycle thus affects South and Central America and when especially strong, can impact the United States. During an especially strong El Nino, such as occurred in 1997, record rainfall is found in California, tornados and storms in the southeast U.S. El Nino occurs irregularly, but generally every three to seven years, which is the timescale for east-west propagation of large, slow ocean waves across the equatorial region. Major progress has been made in predicting an El Nino about one year in advance because the sequence of events in an El Nino is often the same. Thus detection of early signs of El Nino, such as the appearance of warm water in the eastern tropical Pacific or a change in the strength of the trade winds, often allows prediction of changes in rainfall and air temperature later in the year throughout the Pacific region.