What is El Nino?

'El Nino' is a phenomenon caused by unusual warming of normally cool near-surface waters off the west coast of South America. It was also named by the inhabitants of northern Peru in reference to the Christ-child, because it typically appears as an enhancement to the annual onset of a warm, southward settling current that happens around Christmas season. Such event lasts about a year. The effects are not felt only in the Pacific; all ocean areas at trade wind latitudes in both hemispheres can be affected.

However, the phenomena of the Southern Oscillation and El Nino are coupled, so the terms are often combined to form the acronym ENSO, for El Nino/Southern Oscillation. Here, Southern Oscillation (SO) is a term used to describe a reversal in the usual westward flow of air between the normally stable low-pressure area over the western Pacific north of Australia, and the normally high-pressure area over the eastern Pacific near Easter Island. This reversal in the distribution of atmospheric pressure between the eastern and western Pacific causes the trade winds to weaken or reverse. The trade winds normally drag huge quantities of water westward along the ocean's surface near the equator, but without the winds these equatorial currents stop at once. Warm water that has accumulated at the western side of the Pacific can return east along the equator toward the coast of Central and South America. Beside ENSO, other name includes 'Pacific warm episode'.

What is 'La Nina'?

It is a catastrophic event which is followed by El Nino, in which the waters of the Pacific Ocean cool down, while the tropical Atlantic heats up, producing conditions favorable for the development of frequent and strong tropical cyclones and hurricanes. Therefore, El Nino and La Nina are opposite phases of the El Nino-Southern Oscillation (ENSO) cycle, with La Nina sometimes referred to as the cold phase of ENSO and El Nino as the warm phase of ENSO.