Thursday, April 24, 2014

Indian Ocean Dipole and Polar vortex

Indian Ocean Dipole: 

  • The Indian Ocean Dipole(IOD) also known as the Indian Nino is an 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 Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia
  • The IOD affects the climate of Australia and other countries that surround the Indian Ocean Basin, and is a significant contributor to rainfall variability in this region. 
  • Like ENSO, the change in temperature gradients across the Indian Ocean results in changes in the preferred regions of rising and descending moisture and air. 
  • In scientific terms, the IOD is a coupled ocean and atmosphere phenomenon, similar to ENSO but in the equatorial Indian Ocean. 
  • It is thought that the IOD has a link with ENSO events through an extension of the Walker Circulation to the west and associated Indonesian throughflow (the flow of warm tropical ocean water from the Pacific into the Indian Ocean). 
  • Hence, positive IOD events are often associated with El Niño and negative events with La Niña. 
  • When the IOD and ENSO are in phase the impacts of El Niño and La Niña events are often most extreme over Australia, while when they are out of phase the impacts of El Niño and La Niña events can be diminished.

The Indian Ocean Dipole

Positive event:
  • warmer sea surface temperatures in the western Indian Ocean relative to the east
  • easterly wind anomalies across the Indian Ocean and less cloudiness to Australia’s northwest
  • less rainfall over southern Australia and the Top End.

Negative event:
  • cooler sea surface temperatures in the western Indian Ocean relative to the east
  • winds become more westerly, bringing increased cloudiness to Australia’s northwest
  • more rainfall in the Top End and southern Australia.

Polar Vortex

  • A polar vortex (also known as an Arctic cyclone, sub-polar cyclone, and a circumpolar whirl) is a persistent, large-scale cyclone located near one or both of a planet’s geographical poles. 
  • On Earth, the polar vortices are located in the middle and upper troposphere and the stratosphere. They surround the polar highs and lie in the wake of the polar front. 
  • These cold-core low-pressure areas strengthen in the winter and weaken in the summer. 
  • They usually span 1,000–2,000 kilometers, in which the air is circulating in a counter-clockwise fashion (in the northern hemisphere). 
  • As with other cyclones, their rotation is caused by the Coriolis effect.
  • The Arctic vortex in the Northern Hemisphere has two centres, one near Baffin Island and the other over northeast Siberia. 
  • In the southern hemisphere, it tends to be located near the edge of the Ross Ice Shelf near 160 west longitude.
  • When the polar vortex is strong, the Westerlies increase in strength. 
  • When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. 
  • Ozone depletion occurs within the polar vortex, particularly over the Southern Hemisphere, which reaches a maximum in the spring.


  1. Thanks a lot........really very useful information........

  2. Thanks a lot........really very useful information........


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