El Nino Southern Oscillation (enso)

• The warm water of the central Pacific Ocean slowly drifts towards South American coast and replaces the cool Peruvian current. Such appearance of warm water off the coast of Peru is known as the El Nino.
• The El Nino event is closely associated with the pressure changes in the Central Pacific and Australia. This change in pressure condition over Pacific is known as the southern oscillation. It is thus a type of East-West Walker Cell as opposed to the meridional (N-S) circulations of Hadley Cell.
• The combined phenomenon of southern oscillation and El Nino is known as ENSO.
• The SOI (Southern Oscillation Index) calculated on the basis of atmospheric pressure differences between Tahiti and Darwin; gives an indication of development of El Nino or La Nina.
• Sustained positive values of SOI are indicative of La Nina conditions while negative values suggest El Nino conditions.
• In the years when the ENSO is strong, large-scale variations in weather occur over the world. The arid west coast of South America receives heavy rainfall; drought occurs in Australia and sometimes in India and flood in China.

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