Friday, 12 June 2015

Geography Series 3

EVENTS AFFECTING SOUTH-WEST MONSOON


  1. Snow cover on Tibetan plateau
  • The plateau is often called the third pole because it hosts the world’s third-largest stock of ice.
  • Greater the snow cover, greater is the reflection of the incoming solar radiation, which reduces the heating capability of the plateau
  • Lesser is the intensity of south-west monsoon as the intensity of low pressure system over the plateau decreases


     2. El Niño and La Niña events

         To understand El Niño, one has to know about a few ocean currents in the South pacific region


 
  • Equatorial current (warm): Flows to the north and south of equator due to the action of trade winds. The north-east trades in the northern hemisphere, and the south-east trades in the southern hemisphere, drive the warm surface water from eastern to the western part of the ocean. 
  • Equatorial counter-current (warm) : Flows between the north and south equatorial currents in the opposite direction (western to eastern part of oceans), due to piling up of warm waters in the western part of the oceans due to equatorial currents.
  • Humboldt/ Peruvian current (cold) : It  flows north along the west coast of South America from the southern tip of Chile to northern Peru. It is cold due to ice-melt water from the South Pole. Cold water upwelling brings oxygen and nutrient-rich water to the surface.
 
What is El Niño?

             It is the unusual warming of the Eastern Tropical Pacific Ocean due to the dominance of the warm Equatorial counter-current over the Peruvian cold current, leading to an anomalous accumulation of warm water off the coast of Peru. This happens due to less intensity of the Trade winds and the consequent strengthening of the counter current.
 
        It causes ocean biological disaster, as increase in temperature leads to death of fishes (especially anchovies) and coral bleaching events. Lack of nutrient-rich water (otherwise available due to cold water upwelling) also affects phytoplankton population.
 
            At the same time it causes heavy rains in the Coastal Peru and Central America, and even in the interiors as far as up to the Gulf of Mexico, due to intense low pressure system. Otherwise this region has dry, stable and settled weather conditions due to high pressure and subsidence of air masses.
 
          High temperature and low pressure in the Eastern Tropical Pacific near Peru leads to a relatively low temperature and high pressure in the Western Tropical Pacific near the East coast of Australia. This leads to an atmospheric circulation known as the WALKER CIRCULATION.

 
          The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean, and a low pressure system over Indonesia. When the Walker circulation weakens or reverses, an El Niño results.
 
               Thus El Niño is an ocean-atmosphere phenomenon.
 
               It causes droughts in parts of Australia, Indonesia, Malaysia, Philippines and below-normal rains in the Indian sub-continent. El Niño event is associated with poor monsoons, like delayed onset, frequent breaks and early withdrawal, as the winds from mascarene high are diverted to the Peruvian low pressure system leaving the Sub-continent devoid of winds.
 
What is La Niña?

          It is the opposite of El Niño, extensive cooling of the central and eastern tropical Pacific Ocean. This occurs due to the dominance of the Peruvian cold current over the warm Equatorial counter-current, leading to intense cold water upwelling off the coast of Peru.
          
          Oxygen and nutrient rich water due to upwelling helps in thriving of planktons and fishes, while high pressure system leads to dry arid conditions in the land (Western coastal America)

          Low temperature and high pressure in the Eastern Tropical Pacific near Peru leads to a relatively high temperature and low pressure in the Western Tropical Pacific near the East coast of Australia. An especially strong Walker circulation causes a La Niña.


 
          It causes heavy rain in parts of Australia, Indonesia, Malaysia, Philippines and normal to above-normal rains in the Indian sub-continent.
 
          These 2 events together form the El Niño southern Oscillation (ENSO).  The "Southern Oscillation" refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean, with warming known as El Niño and cooling known as La Niña.
 
         The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti, French Polynesia and Darwin, Australia.


  
                           
 
  
   SOI = surface air pressure at Tahiti - surface air pressure at Darwin

El Niño = negative SOI    
La Niña = positive SOI




Alternate Events of El-Nino and La-Nina
  • El Niño occurs every 3-4 years, and is often, though not always, followed by La Niña
  • Teleconnection refers to climate anomalies being related to each other at large distances (typically thousands of kilometres). ENSO is an example.
  • Just to know: In Spanish, El Niño means Christ Child, while La Niña means little girl!
 


         3.  Indian ocean dipole (known as the Indian Niño)

     It is the irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean. The IOD also affects the strength of monsoons over the Indian subcontinent. It interacts with events like ENSO and that determines monsoon strength.

        Positive IOD = Greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, and a corresponding cooling of waters in the eastern Indian Ocean. It causes droughts in land areas of Indonesia and Australia.


     Negative IOD = It brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
 

         4. Other cyclonic activities in the region

         Cyclonic activitites in the region may affect the monsoon winds. The recently developing severe cyclonic storm Ashobaa not only disrupted the advance of the monsoon over India, but may also result in more rainfall deficiency in June than was earlier forecast. But the monsoon will strengthen once this cyclone weakens after landfall (expected to make landfall in Oman)



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