Indian Ocean Dipole and its Potential to Limit El Nino Effects

Indian Ocean Dipole and its Potential to Limit El Nino Effects

Subject :Geography

Section: Climatology

Context:

• With the El-Nino phenomenon almost certain to affect the Indian monsoon this year, high hopes are pinned on the development of a positive Indian Ocean Dipole (IOD) and its ability to counterbalance the El-Nino effect.
• While the El-Nino has been established in the Pacific Ocean this year, IOD is still in the neutral phase.

Indian Ocean Dipole (IOD):

• The IOD was identified as an independent system only in 1999.
• The IOD, sometimes referred to as the Indian Nino, is an ocean-atmosphere interaction very similar to the El Nino fluctuations in the Pacific Ocean that occur in the Indian Ocean between the Indonesian and Malaysian coastline in the east and the African coastline near Somalia in the west. One side of the ocean, along the equator, gets warmer than the other.
• Mechanism:
  • The air circulation in the Indian Ocean basin moves from west to east, that is from the African coast towards the Indonesian islands, near the surface, and in the opposite direction at the upper levels.
  • That means the surface waters in the Indian Ocean get pushed from west to east.
  • In a normal year, warmer waters in the western Pacific near Indonesia cross over into the Indian Ocean and make that part of the Indian Ocean slightly warmer. That causes the air to rise and helps the prevailing air circulation.
  • In the years when the air circulation becomes stronger, more warm surface waters from the African coast are pushed towards the Indonesian islands, making that region warmer than usual. This caused more hot air to rise and the cycle reinforces itself. This is the state of negative IOD.
  • The opposite case involves air circulation becoming slightly weaker than normal.
    • In some rare cases, the air circulation even reverses direction. The consequence is that the African coast becomes warmer while the Indonesian coastline gets cooler.

• IOD impacts:
  • A positive IOD helps rainfall along the African coastline and also over the Indian sub-continent while suppressing rainfall over Indonesia, Southeast Asia and Australia.
  • The impacts are opposite during a negative IOD event.
  • Compared with ENSO events, the impacts of IODs are much weaker.

El-Nino Southern Oscillation (ENSO cycle):

• In a normal year, the eastern side of the Pacific Ocean, near the northwestern coast of South America, is cooler than the western side near the islands of the Philippines and Indonesia.
• This happens because the prevailing wind systems that move from east to west sweep the warmer surface waters towards the Indonesian coast.
• The relatively cooler waters from below come up to replace the displaced water.
• An El-Nino event is the result of a weakening of wind systems that leads to lesser displacement of warmer waters.
• This results in the eastern side of the Pacific becoming warmer than usual.
• During La Nina, the opposite happens.
  • The wind system becomes stronger, displacing greater amounts of warm surface waters towards the Indonesian coastline, because of which the ocean near the South American coast gets cooler than usual.
• Both these conditions, together called El Nino Southern Oscillation (ENSO), affect weather events across the world. Over India, the El-Nino has the impact of suppressing monsoon rainfall.

ENSO and IOD:

• A positive IOD event is often seen developing at times of an El Nino, while a negative IOD is sometimes associated with La Nina.
• During El Nino, the Pacific side of Indonesia is cooler than normal because of which the Indian Ocean side also gets cooler. That helps the development of a positive IOD.
• Many studies suggest that IOD events are actually induced by ENSO.
• However, IOD is relatively a much weaker system than El Nino and thus has relatively limited impacts.
  • IOD is said to be positive when the western side of the Indian Ocean, near the Somalia coast, becomes warmer than the eastern Indian Ocean.
  • It is negative when the western Indian Ocean is cooler.
• A positive IOD has the potential to offset the impacts of El Nino to a small measure in neighbouring areas.