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Weather officials to study possible emergence of El Nino
Feb 16, 2017

In news:

Meteorologists are likely to review the threat to the Indian monsoon from a possible El Nino. Scientists from the India Meteorological Department, Indian Institute of Tropical Meteorology and the Ministry of Earth Sciences are expected to meet in to analyse a range of forecasts from international climate models and their own-that suggest waters are likely to warm and change wind patterns enough to El Nino-like conditions.

  • Meteorologists say  it’s too early to be sure of an El Nino and its impact on the monsoon, as predictions made from a climate model before March could dramatically differ from that in April or later.
  • IMD scientists say that an El Nino by itself isn’t enough to disrupt a monsoon. How it influences the sea around India has to be studied and there could be local factors that may be stronger.
  • One significant influencer, the Indian Ocean Dipole (IOD), doesn’t seem to be giving good indications. An IMD report in January suggested negative IOD conditions from AMJ (April, May, June) onwards. A positive IOD is traditionally known to bolster monsoon rains in India

EL Nino

El Nino is oceanic phenomena observed in the southern Pacific ocean which emerges after a gap of 3 to 7 years and is associated with abnormally high rainfall in the otherwise dry condition along the coast of Peru. This phenomena result in floods in Peru while Pacific coast of Australia and Indonesia have abnormally dry conditions that forces collapse of Agriculture with instances of Forest fire. During El Niño there are on average fewer hurricanes over the Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico. In the central Pacific Ocean El Niño brings more typhoons, both north and south of the equator. Their more easterly genesis makes that fewer of these tropical cyclones reach Australia. In the northern Pacific Ocean the area with typhoons also shifts east. There are no effects on the number of cyclones over the Indian Ocean. The phenomenon is not well understood but it is related to Ocean current and trade winds. In this Sea Surface Temperature (SST) increases above normal by .5 degree.

In normal years the trade winds are fairly consistent and strong. The equatorial current of Pacific are well developed which leads to development of warm Western Pacific Pool  resulting in a zone of low pressure off Australian and Indonesian Coast with rising limb of air causing rain.



At the same time Peruvian coast have cold current (Humboldt current) and upwelling which brings cold current along coast of Peru resulting in High Pressure with descending air current. This is favorable to Peruvian economy as Upwelling brought up nutrients at surface which flourishes fishing population. This low Pressure at Australian Coast and High Pressure at Peruvian coast form a vertical cell called as Walker cell. (This cell is exception to otherwise general pattern of Air circulation e.g. Trade winds, westerlies and Polar circulation and tricellular meridional circulation. Here  East-West Zonal Pattern is found).

During El-Nino years, for reasons not yet known the trade winds of Pacific weakens, resulting in weak equatorial current. The Warm Pacific Pool waters flows back strengthening the Counter Equatorial current. This reverses water flows southward towards Peruvian coast and this current is Known as El Nino current. (It is warm surface current appear at the coast of Peru during December flowing from equator towards Pole) which can stop the upwelling Process.



This result in reversing the pressure cells (Now Peruvian coast have low pressure and Australian-Indonesian coast having relative High Pressure) with this Walker Cell too is reversed i.e. rising limb of air at Peruvian coast and descending limb of air along Australian coast which reverses the climatic condition along these coast which results to Peruvian coast having High rainfall and West Pacific coast have dry condition).

The atmospheric component of El Nino/La Nina is called Southern Oscillation. This component is an oscillation in surface air pressure between the tropical western and the eastern Pacific Ocean waters. In context of Monsoon the two cumulatively called as ENSO. The strength of it (Southern Oscillation) is measured by the Southern Oscillation Index (SOI). It is computed by taking air pressure Ocean difference between Tahiti in Eastern Pacific Ocean and Darwin in Western Pacific.  An El Nino year will have negative value of SOI that means eastern pacific have below normal pressure over Tahiti and above normal pressure over Darwin.

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