Context: Recently, the Indian Meteorological Department’s monthly bulletin stated that the Indian Ocean dipole is currently neutral but international climate models suggested that it could turn positive in the coming months.

• The Indian Ocean Dipole is an important phenomenon for India as it helps counterbalance the effects of El Nino, which is likely to affect the Indian monsoon this year.

• El Nino is a weather phenomenon tinvolving the anomalous warming of surface waters in the eastern tropical Pacific Ocean.

Indian Nino

• The Indian Oean Dipole was identified as an independent system in 1999 and is sometimes known as the Indian Nino.

• It takes place between the eastern (Indonesian and Malaysian coasts) and western (eastern coast of Africa) parts of the Indian Ocean.

• The IOD involves one side of the ocean along the equator becoming warmer than the other side.

There are three phases of the IOD

• Positive: Here, the western side of the Indian Ocean near Somalia is warmer than the eastern side.

• Negative: In this phase, the western Indian Ocean is cooler than the eastern side.

• Neutral: In the neutral phase, water from the Pacific Ocean flows through the Indonesian islands, warming the oceans to Australia’s north-west.

Air Circulation in the Indian Ocean

• The air circulation in the Indian Ocean basin near the surface exists in a west-to-east direction i.e. from the African coast to the Indonesian islands while it exists in the east-to-west direction at the upper atmospheric levels.

• Consequently, the surface waters of the Indian Ocean are pushed from west to east.

• In a normal year, the warmer waters of the western Pacific Ocean near Indonesia cross over into the Indian Ocean.

• As a result, the eastern Indian Ocean is slightly warmer than usual which causes the air to rise and aids the prevailing air circulation.

Negative IOD

• In the negative IOD phase, the air circulation strengthens, causing more warm surface waters from the African coast to be pushed towards the Indonesian islands, making the eastern region warmer than usual. This also causes more hot air to rise and reinforces the cycle.

Positive IOD

• In the positive phase, air circulation becomes slightly weaker than normal and in rare cases, the air circulation may even reverse direction.

• As a consequence, the African coast becomes warmer while the Indonesian coast becomes cooler.

El Nino Southern Oscillation

• In a normal year, the prevailing wind systems in the Pacific Ocean which move from east to west push the warm surface water towards the west i.e. the Indonesian coast.

• Cooler water from below the surface rises up to replace the displaced warm surface water.

• As a result, the eastern Pacific Ocean near the northwestern coast of South America is cooler than the western Pacific near the Philippines and Indonesia.

• During an El Nino year, the wind systems are weakened leading to lesser displacement of warm water. As a result, the eastern Pacific Ocean is much warmer than normal.

• During a La Nina year, the El Nino conditions are reversed.

• The El Nino Southern Oscillation or ENSO, which is the combination of the El Nino and La Nina conditions, has a significant effect on weather events across the world.

How is the IOD affected by El Nino?

• A positive IOD event is often observed to develop alongside 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 leading the Indian Ocean side to get cooler, which aids in the development of a positive IOD.

• When both the IOD and ENSO are strong, their circulation patterns can impact each other.

• While most studies suggest that IOD events are induced by ENSO, a study by Saudi Arabia’s King Abdullah University of Science and Technology claims that the IOD is an independent event.

• Although external factors like ENSO can trigger the IOD, it is usually caused by local circulations or subsurface processes within the equatorial Indian Ocean.

• IOD events largely develop and mature through internal dynamics even when triggered externally.

Impact of IOD

• A positive IOD encourages rainfall along the African coastline and the Indian sub-continent while suppressing rainfall over Indonesia, Southeast Asia, and Australia.

• The impacts are reversed during a negative IOD event.

• The impact of IOD events is much weaker than that of ENSO events but the influence of ENSO is usually countered by a positive IOD index resulting in enhanced monsoon rainfall even in an El Nino year.

• A negative IOD supports El Nino and causes a severe drought.

• A positive IOD is a factor behind the increased occurrence of cyclones in the Arabian Sea while a negative IOD contributes to more intense cyclogenesis in the Bay of Bengal and reduced cyclogenesis in the Arabian Sea.

• IOD events usually originate in the months of April and May but strong IOD events develop late as well.

• In 2019, the IOD event developed during late monsoon but was so strong that it compensated for the deficit rainfall during the first month of the monsoon season which was caused by a developing El Nino.

Conclusion

• The El Nino is already firmly established in the Pacific Ocean but the IOD is still in the neutral phase.

• The IMD has forecasted that the probability for positive IOD conditions is around 80% and 15% for a neutral IOD between June and August 2023 season.

• Therefore, the Indian monsoon is very likely to be adversely impacted by El Nino. In such a case, the occurrence of a positive Indian Ocean dipole will be beneficial for India as it has an increased possibility of monsoon rainfall.