Deviations from normal temperature patterns of the southern Pacific Ocean, between Australia and South America, result in the phenomenon called El Nino. Under normal conditions, eastern trade wind blows across the Pacific. These drive the sun-warmed surface water from the central Pacific to the coast of northern Australia. When clouds form above this area of warm water and move over Indonesia, Papua New Guinea, and Australia, they bring rain with them.

    Every two to seven years, however, this pattern is interrupted by the El Nino event. During El Nino, the Pacific Ocean of Australia does not warm as much as it normally does. Instead, it becomes warmer right up to the coast of Peru in South America. At the same time, the easterly trade winds that blow across the Pacific reverse their direction. This causes high-pressure systems to build up to the north of and across the Australian Continent, preventing moist tropical air reaching the continent. These condition in turn result in storms, and in rain falling in the eastern Pacific Ocean and in South America instead of in Australia, Papua New Guinea, and Indonesia, which suffer drought conditions.

    While the · effect of El Nino are sometimes weak, at the other times they are very strong. During a severe El Nino period, extreme drought conditions prevail, as in 1 982-1983 and 1 997-1 998. In contrast, heavy rainfall and flooding occured in parts of North and South America. In 1 997, there were severe storms and floods in Mexico and further north along the west coast of the United States.

    The converse of the El Nino effect is the La Nina effect, which is an exaggeration of normal conditions. This takes place when trade winds blow strongly and consistently across the Pacific towards Australia. This pushes the warm waters from the central Pacific, off the northern Australian coast, to build up into a mass that is bigger than normal. Thus, much more cloud develops than usual, and this brings considerably more rain to Australia and neighboring countries.

(taken from Geographica's Pocket World Reference, 2007)

What is the difference between El Nino and La Nina?