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El Nino "boy child" developing slowly

An El Nino weather pattern that could bring drought to Australia's farms and weaken Asia's monsoons, severely disrupting crop and livestock production, is developing slower, Australia's weather bureau said on Tuesday.

world Updated: Jul 07, 2009 11:46 IST

An El Nino weather pattern that could bring drought to Australia's farms and weaken Asia's monsoons, severely disrupting crop and livestock production, is developing slower, Australia's weather bureau said on Tuesday.

Here are some facts about the El Nino and La Nina weather patterns in the Pacific.

PACIFIC OCEAN

The Pacific Ocean is a huge mass of water which controls many climate features in its region. Its equatorial expanse, far larger than the Indian or Atlantic Oceans, is critical to the development of an El Nino.

In most years relatively cold water moves northward along the west coast of South America, an effect increased by upwelling or rising cold water along the Peruvian Coast. The cold water then flows westward along the equator and is heated by the tropical sun. These normal conditions make the western Pacific about 3-8 degrees Celsius warmer than the eastern Pacific.

EL NINO OR BOY CHILD

El Nino translates from Spanish as "the boy-child".

Peruvian fisherman originally used the term -- a reference to the Christ child -- to describe the appearance, around Christmas, of a warm ocean current off the South American coast.

El Nino now refers to the extensive warming of the central and eastern Pacific that leads to a major shift in weather patterns across the Pacific.

El Nino benefits traditional fisheries in Peru and Ecuador, with colder nutrient-rich water from the deeper ocean drawn to the surface near the coast, producing abundant plankton, food source of the anchovy. However, when the upwelling weakens in El Nino years and warmer low-nutrient water spreads along the coast, the anchovy harvest plummets. It was ruined in the four or five most severe El Nino events this century.

In Australia (particularly eastern Australia), El Nino events are associated with an increased probability of drier conditions and severe El Ninos have caused widespread drought, decimating Australia's crops and livestock, and bushfires.

Asia's monsoons are weakened by El Nino and can severely impact vital food production in India and Indonesia.

SOUTHERN OSCILLATION INDEX

The Southern Oscillation Index (SOI) gives a simple measure of the strength and phase of El Nino and La Nina.

The SOI is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti in the eastern Pacific and Darwin in northern Australia.

SOI close to zero indicates a normal Pacific weather pattern.

A sustained negative SOI often indicates an El Nino. A negative SOI means the seas around Australia cool, trade winds weaken and feed less moisture onto Australia and Asia.

A positive SOI reflects a La Nina episode. A positive SOI are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia, meaning a high chance that eastern and northern Australia will be wetter than normal.

LA NINA OR GIRL CHILD

La Nina translates from Spanish as "the girl-child" and is the meteorological label for the opposite of El Nino.

The term La Nina refers to the extensive cooling of the central and eastern Pacific Ocean. La Nina events are associated with increased probability of wetter conditions in the western Pacific, particularly in eastern Australia and Asia.