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The El Nino Theory

20-NOV-2015 16:06

    El Nino is a name given to the periodic development of a warm ocean current along the coast of Peru as a temporary replacement of the cold Peruvian current. Sea surface temperatures play a major role in global weather which influences two extreme phases of a naturally occurring climate cycle. I.e. El Niño/Southern Oscillation and La Nina. Both terms refer to large-scale changes in sea-surface temperature across the eastern tropical Pacific.

    Before moving on to the discussion, firstly know that, what is an Inter Tropical Convergence Zone (ITCZ) and southern oscillation? then we get clear picture of these two extreme phases of a naturally occurring climate cycle. The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator where trade winds converge, and so, it is a zone where air tends to ascend. In July, the ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called the monsoon trough. This monsoon trough encourages the development of thermal low over north and northwest India. Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 60°E longitudes and start blowing from southwest to northeast due to the Coriolis force. It becomes southwest monsoon. In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place. They are called north-east monsoons.

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    Whereas Southern Oscillation, in oceanography and climatology, a coherent inter annual fluctuation of atmospheric pressure over the tropical Indo-Pacific region. The Southern Oscillation is the atmospheric component of a single large-scale coupled interaction called the El Niño/Southern Oscillation (ENSO). The phase of the Southern Oscillation at a given point in time may be understood by comparing the difference in atmospheric pressure over Australia and Indonesia with that of the eastern South Pacific.

    Equatorial circulation undergoes variations following the irregular periods of roughly three to eight years in response to changes in atmospheric pressure over the tropical Indo-Pacific region. Weakening of the east-to-west wind during a phase of the Southern Oscillation allows warm water in the western margin to slip back to the east by increasing the flow of the Equatorial Counter Current. Surface water temperatures and sea level decrease in the west and increase in the east, producing an event called El Niño. The combined ENSO effect has received much attention because it is associated with global-scale climatic variability.

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    What is El Nino?

    The word EI-Nino means ‘Child Christ’ because this current appears around Christmas in December. December is a summer month in Peru (Southern Hemisphere).It is a complex weather system that appears once every three to seven years, bringing drought, floods and other weather extremes to different parts of the world. The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the Eastern Pacific. EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily by cold Peruvian current or Humboldt Current. This current increases the temperature of water on the Peruvian coast by 10°C.This results in:

    • the distortion of equatorial atmospheric circulation;

    • irregularities in the evaporation of sea water;

    • reduction in the amount of planktons which further reduces the number of fish in the sea.

    Effects of El Nino on Indian Monsoon

    • It has also been notice that changes in the pressure conditions over the southern oceans also affect the monsoons.

    • Normally when the tropical eastern South Pacific Ocean experiences high pressure (El Nino Phenomena), the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. The difference in pressure over Tahiti (Pacific Ocean, 18°S/149°W) and Darwin in northern Australia (Indian Ocean, 12°30’S/131°E) is computed to predict the intensity of the monsoons. If the pressure differences were negative, it would mean below average and late monsoons.

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