When journalists ask why the 2019 monsoon was so much better than the terrible 2012 season despite similar ENSO conditions, the answer usually involves the Indian Ocean Dipole. It's the climate signal that most people haven't heard of, but if you care about Indian weather — particularly the monsoon — it deserves a proper explanation.
What the IOD Actually Is
Imagine the Indian Ocean split into two halves: the western basin (off the coast of East Africa and Arabia) and the eastern basin (near Indonesia and Sumatra). Normally, sea surface temperatures across these two zones are fairly similar. But every few years, one side gets warmer and the other cooler. That temperature difference — the dipole — changes how atmospheric convection and wind patterns organise themselves above the ocean, which in turn directly affects the winds that carry monsoon moisture to India.
A positive IOD means the western Indian Ocean is anomalously warm while the eastern side cools. The warm western waters drive intense rising air and convection there, strengthening the pressure gradient that pulls moisture-laden winds northward into the Indian subcontinent. In plain terms: a positive IOD tends to give India a better monsoon. It also dries out Indonesia and Australia significantly, which is why a positive IOD year in India is typically a drought year in southeastern Australia.
A negative IOD flips this. The eastern Indian Ocean warms, drawing convective energy toward Indonesia. The wind patterns weaken, and India often sees reduced or poorly distributed monsoon rainfall.
IOD Versus ENSO — Which One Wins?
This is where it gets genuinely interesting. The IOD and ENSO can work together or in opposite directions. If both are strongly negative in the same year — La Niña and positive IOD — you can get exceptional monsoon years. If El Niño and negative IOD align, the combination can be brutal for Indian rainfall.
The 2019 season is a great example of the IOD overriding ENSO effects. An El Niño that had developed through early 2019 would normally have suppressed the monsoon. But an unusually strong positive IOD kicked in from June onwards, effectively counterbalancing the El Niño's drying effect. The result was a late but ultimately above-normal monsoon. This is exactly why seasonal forecasters look at multiple drivers simultaneously rather than fixating on ENSO alone.
When Does the IOD Develop?
The Indian Ocean Dipole typically begins to develop in late May or June, peaks between August and October, and dissipates by December as the monsoon withdraws. This timing means it's often not fully established when IMD issues its first long-range monsoon forecast in April — which is one reason mid-season forecast updates (issued in June) can significantly change the picture.
Monitoring IOD status through sea surface temperature anomaly maps — available from NOAA and IMD — is now a routine part of seasonal outlook work. For anyone making agricultural or infrastructure planning decisions that depend on the monsoon, following both the ENSO and IOD together gives a meaningfully more complete picture than either one alone.
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Dr. Arun Sharma
Author & ResearcherPh.D. in Atmospheric Sciences, M.Sc. Meteorology
Dr. Arun Sharma has over 15 years of experience researching tropical meteorology, monsoon dynamics, and atmospheric modeling in South Asia. He oversees WeatherPulse's weather data verification standards and climate trend models.
