Indian Ocean Dipole (IOD)

The Indian Ocean Dipole, or IOD, is a swing in sea surface temperature between the western and eastern parts of the tropical Indian Ocean. In a positive IOD, the western Indian Ocean (near Africa and the Arabian Sea) becomes warmer than normal and the eastern part (near Indonesia) becomes cooler. In a negative IOD, the pattern flips.

"Dipole" simply means "two poles", here the two ends of the ocean that move in opposite directions. Because it works a bit like El Niño but in our own ocean, it is sometimes called the "Indian Niño".

Why does it matter?

Warm water heats the air above it, makes it rise and forms clouds and rain. So when one side of the Indian Ocean warms and the other cools, rainfall shifts towards the warm side. This changes rainfall in India, East Africa, Indonesia and Australia. For India, the IOD is one of the most important factors, after ENSO, that decides whether the monsoon is good or bad.

Where did it come from?

The IOD was described in 1999 in a paper in the journal Nature by N. H. Saji, B. N. Goswami, P. N. Vinayachandran and Toshio Yamagata. Before this, scientists mainly looked at the Pacific to explain the Indian monsoon. The discovery helped explain years like 1997, when a very strong El Niño did not cause an Indian drought.

How does a positive IOD work?

1. Winds along the equator in the Indian Ocean blow more strongly from east to west, from Indonesia towards Africa.
2. These winds push warm surface water westwards, towards East Africa and the Arabian Sea.
3. Near Sumatra and Java, cold water rises from below (upwelling), so the eastern Indian Ocean becomes cooler.
4. The warm west gets more rising air, more clouds and more rain. The cool east gets sinking, dry air.
5. As with El Niño, the steps reinforce each other: a cooler east makes the winds stronger, and stronger winds cool the east further.

How is it measured?

Scientists use the Dipole Mode Index (DMI). It is the temperature anomaly of a western box (50°E to 70°E, 10°S to 10°N) minus the anomaly of an eastern box (90°E to 110°E, 10°S to the equator).

• A positive DMI means the west is relatively warmer: a positive IOD.
• A negative DMI means the east is relatively warmer: a negative IOD.
• The Australian Bureau of Meteorology declares a positive IOD event when the index stays above +0.4°C for a sustained period, and a negative event when it stays below −0.4°C. A single warm week is not enough.

When does it happen?

The IOD follows the seasons closely. An event usually starts around May or June, becomes strongest around September to November, and breaks down by about December, when the monsoon winds over northern Australia and Indonesia change direction.

How does the IOD affect India and the world?

• Positive IOD: often helps the Indian southwest monsoon, because the warm Arabian Sea and western Indian Ocean feed moisture and rising air towards India. It also brings heavy rain and floods to East Africa (Kenya, Somalia), and dry weather, heat and bushfire risk to Indonesia and Australia. In 2019, a very strong positive IOD was linked to Australia's severe bushfire season and to floods in the Horn of Africa.
• Negative IOD: tends to weaken the Indian monsoon and bring more rain to Indonesia and Australia, with drought in East Africa.
• With El Niño: A positive IOD can partly cancel the drying effect of El Niño on India. This is why 1997 had a normal monsoon despite a very strong El Niño, and why 2023 rainfall ended close to normal. In 2019, a positive IOD helped India record above-normal monsoon rain.
• Strong positive IOD years include 1994, 1997, 2006 and 2019.

Commonly confused concepts

• IOD vs ENSO: The IOD happens in the Indian Ocean and works between its western and eastern halves. ENSO happens in the Pacific Ocean. They can occur together (often a positive IOD comes with El Niño) or separately.
• Positive IOD vs El Niño, for India: El Niño usually hurts the Indian monsoon. A positive IOD usually helps it. So the two can pull in opposite directions.
• IOD vs Indian Ocean Basin-wide warming: The IOD is a difference between west and east. Basin-wide warming means the whole Indian Ocean is warmer than normal, often in the months after an El Niño.
• IOD vs Madden-Julian Oscillation (MJO): The IOD is a slow pattern lasting months. The MJO is a band of clouds and rain that travels eastwards around the tropics over 30 to 60 days and causes short active or weak spells within one monsoon season.

Issues, criticism and the way forward

• Hard to forecast early. IOD forecasts made before May or June are not very reliable, which is a problem for the IMD's first monsoon forecast in April.
• Climate change effects. Several studies suggest that warming may make strong positive IOD events more frequent, but the evidence is still being studied.
• Limited ocean data. The Indian Ocean has fewer measuring buoys than the Pacific. India supports ocean observation through the Indian National Centre for Ocean Information Services (INCOIS) in Hyderabad, and through the RAMA buoy network with international partners. Experts call for more buoys and better coupled ocean-atmosphere models.

Concepts to Know

• Dipole: Something with two opposite poles or ends. In the IOD, the two "poles" are the western and eastern tropical Indian Ocean.
• Upwelling: Cold water rising from the deep sea to the surface, usually because winds push the surface water away.
• Dipole Mode Index (DMI): The number used to measure the IOD. It is the western box's temperature anomaly minus the eastern box's.
• Madden-Julian Oscillation (MJO): A moving band of clouds and rain that circles the tropics from west to east every 30 to 60 days, causing short wet and dry spells.