When an El Nino occurs, the ocean temperature becomes unusually warm in the Equatorial Pacific. According to the National Oceanic and Atmospheric Administration, El Nino is an oscillation of the ocean-atmosphere system in the tropical Pacific. When winds pushing ocean water west become weaker, some water slumps back east, and less cold water is pulled up from below, which causes the ocean temperature to rise. The warmer water then causes the winds to become weaker, causing temperatures to continue rising as the wind can push less and less water west.
During El Nino years, rainfall increases and temperatures decrease during the winter months in the southeastern U.S. and Peru. With easterly trade winds weakened, rainfall follows the warm water eastward. Southern California experiences more rain while the Pacific Northwest gets less of it. El Nino causes more storms, flooding and landslides in southern California, crop damage in California, flooding and damage in the southern U.S. and fewer salmon in the Pacific Northwest fisheries.
In western Canada and southern Alaska, temperatures rise while northern Canada gets cooler. Although some temperatures decline in some areas, overall snow levels are lower and reduce the risk of spring flooding. However, British Columbia has more landslides, storms and flooding in El Nino years. The warmer coastal water brings more sockeye salmon that head further north and causes more icebergs to impede travel on the eastern seaboard. In Hawaii, El Nino is responsible for some of the state's most severe droughts.
The rainfall that ends up in Peru and the southern U.S. during El Nino years would otherwise go to the West Pacific. As a result, drought and forest fires are more likely in Indonesia, including brush fires in Australia. A similar effect is felt in southern Africa, with countries such as Zimbabwe sometimes experiencing devastating droughts.