Indian Ocean Dipole, and its potential to limit El Nino effects

Contact Counsellor

Last Updated29/06/2023

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.

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 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.
The opposite happens during the La Nina
Both these conditions, together called El Nino Southern Oscillation or ENSO, affect weather events across the world.
Over India, the El Nino has the impact of suppressing monsoon rainfall.

IOD, sometimes referred to as the Indian Nino, is a similar phenomenon, playing out in the relatively smaller area of the Indian Ocean between the Indonesian and Malaysian coastline in the east and the African coastline near Somalia in the west.
- 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.

The air circulation in the Indian Ocean basin moves from west to east near the surface and in the opposite direction at the upper levels.
Thus, 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 support 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 causes more hot air to rise and the cycle reinforces itself.
A negative IOD is sometimes associated with La Nina.

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.
A positive IOD event is often seen developing at times of an El Nino
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.

While the El Nino is already firmly established in the Pacific Ocean this year, the IOD is still in the neutral phase.
All international climate models surveyed by the Bureau suggest a positive IOD event may develop in the coming months
IOD as a system has relatively limited impacts.
However, a positive IOD does have the potential to offset the impacts of El Nino to a small measure in neighbouring areas

In 2019, the IOD event developed during the late monsoon, but was so strong that it compensated for the deficit rainfall during the first month of the monsoon season
The deficit in June that year was also attributed to a developing El Nino but that fizzled out later. A similar incident can also be observed this year.