El Nino And La Nina Phenomena


Sea surface temperatures play a major role in global weather which influence 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.

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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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Where as 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.

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EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily by cold Peruvian current or Humbolt 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.

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Normal Condition

 

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El Nino

What is La Nina?

Scientists refer to the event when exceptionally cook water lies off the coast of South America as La Nina or “the baby girl.” Strong La Nina events have been responsible for the opposite effects on climate as El Nino. For example, a major La Nina event in 1988 caused significant drought across North America.

La Nina is described as cooler-than-normal sea surface temperatures in the central and eastern Pacific Ocean, near the equator off the west coast of South America. El Nino is like La Nina’s brother, the totally opposite and attention grabbing brother. This is described as warmer-than-normal sea surface temperatures in the same area of the Pacific Ocean.

 

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Relationship of El Nino to Climate Change

As of this writing, El Nino and La Nina do not appear to be significantly related to climate change. As mentioned above, El Nino is a pattern that had been noticed for hundreds of years by South Americans. Climate change may make the effects of El Nino and La Nina stronger or more widespread, however. A similar pattern to El Nino was identified in the early 1900s and was called the Southern Oscillation. Today, the two patterns are known to be pretty much the same thing and so sometimes El Nino is known as El Nino/Southern Oscillation or ENSO.

 

What Causes La Nina and El Nino?

Simply put, easterly trade winds over the equatorial Pacific Ocean are partly to blame for both phenomenon. For La Nina, the easterly trade winds strengthen. This blows more warm water west, and allows cold water below the ocean’s surface to push towards the top near the South American coast to replace the warm water.

In an El Nino, the opposite occurs. The easterly trade winds become weaker, and can even reverse direction. The warm Pacific Ocean becomes nearly stationary or pushes eastward and gains heat. Besides affecting weather, El Nino has also been known to hurt fishing off the coast of Peru.

What Does this Mean for the Weather?

We’re already seeing affects of the building La Nina. A typical La Nina winter will feature drier and milder conditions across the South, much like what we’re seeing in the current South-east drought and elevated fire conditions. The Pacific Northwest will become wetter than normal, while the Northeast will have cold periods, but these are usually short lived.

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In an El Nino winter, we see what we had last season. The southern branch of the jet stream gets displaced across the Deep south, leading to wetter conditions from Los Angeles to the Southeast. The Northeast typically has stormy winters, which in the case of last season led to “Snowmageddon.” Finally the Northwest is typically milder.

 

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In other parts of world, La Nina and El Nino can affect Asia’s Monsoon’s and rainfall from Australia to Peru.

 

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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2 responses to “El Nino And La Nina Phenomena”

  1. esprit says :

    Nice explanation. Thanks

    Like

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