Indian monsoon: El Nino southern oscillations


El Nino is a temporary warm ocean current which appears off the coast of Peru in December in some years. ‘El Nino’ in Spanish means the child Christ and it is named so as it appears around Christmas.

Sir Gilbert Walker in 1920 noticed that when the pressure was high over equatorial south Indian Ocean, it was low over the equatorial south Pacific and vice-versa.

This pressure variation gives rise to circulation along the equator known as Walker Circulation. The pressure variation is stated as Southern Oscillation Index (SOI) and it is measured as difference in pressure between Tahiti in French Polynesia, representing the southern Pacific Ocean and Port Darwin, in northern Australia, representing the Indian Ocean. 

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El Nino and Southern Oscillation


  • Positive: Tahiti pressure greater than that of Darwin
  1. Pressure high over east pacific and low over west pacific.
  2. Low rainfall over east pacific and prospects of good monsoon over Indian Ocean.
  • Negative: Darwin pressure exceeds that of Tahiti
  1. High pressure over western pacific and low over eastern pacific
  2. Low rainfall in west pacific and poor monsoon in India
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