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Indian Ocean Dipole

  Jul 24, 2020

Indian Ocean Dipole

What is IOD?

The Indian Ocean Dipole refers to a pattern of sea surface temperature variations in the tropical Indian Ocean that varies from one year to another. It is the difference in sea-surface temperatures between the western and eastern part of the Indian Ocean. The Indian Ocean Dipole influences weather and extreme events across the Indian Ocean. The Dipole fluctuates between 'positive', 'neutral' and 'negative' phases approximately every three to eight years.

What is positive phase of IOD and its impact on global weather?

At its positive phase, the eastern Indian Ocean (off Sumatra-Java) is cooler than normal whereas the west is warmer than normal. These changes alter the atmospheric circulation, enhancing convection (hot and moist air rising) and rainfall over the west, suppressing convection and rainfall in the east Indian Ocean and surrounding countries including Australia.

The change in convection induces easterlies along the equator and alongshore Sumatra-Java, which causes upwelling (subsurface cold water to surface), further enhancing the sea surface temperature contrast between the west and the east.

Impact on global weather:

The positive phase is characterized by greater-than-average sea-surface temperatures, more rain in the warmer western Indian Ocean region and cooler waters in the eastern Indian Ocean. It tends to cause droughts in East Asia and Australia, and flooding in parts of the Indian subcontinent and East Africa.

Positive Dipole activity has, to date, preconditioned major wildfires in southeast Australia, caused coral reef death across western Sumatra, and exacerbated malaria outbreaks in East Africa.

Why IOD in news?

IOD created atmosphere of widespread warm and dry descending air, higher pressure, hot and dry air, nil clouds/rain to the East, the near Australian coast during the second half of 2019 primed the Australian landscape for bushfire weather and heat waves this summer. This led to very severe Bush fire in Australia this year. The monsoon onset, which typically occurs over Darwin during the last week of the year, has also been delayed in 2019-20 likely due to the influence of a positive Indian Ocean Dipole (IOD). 

What is ENSO?

El NiƱo Southern Oscillation (ENSO) affects weather patterns across the Pacific Ocean. ENSO fluctuates between 'El Nino', 'neutral' and 'La Nina' phases approximately 3-8 years.