India’s monsoon forecasters watch the ocean as closely as the sky. Two indicators, IOD and OMT, tell them what the season ahead is likely to bring.
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MCQ QuestionsIndGeo0135 · IOD & OMT
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OMT’s real 26°C isotherm depth, SW Indian Ocean, Jan-Mar
Not the 129m figure often quoted — verified against a direct write-up of IITM Pune’s own research.
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OMT’s monsoon-prediction success rate, per IITM Pune
IITM Pune
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Plain SST’s success rate, for comparison
IITM Pune
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What IOD actually compares — Indian Ocean, not Pacific
NOAA Climate.gov
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Season when OMT readings are taken for forecasting
IITM Pune
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Body that issues India’s official monsoon forecast
India Meteorological Dept.
✊ Must Know
What IOD and OMT Actually Measure
- The Indian Ocean Dipole (IOD) tracks the sea-surface temperature difference between the western and eastern tropical Indian Ocean, not the Pacific.
- A positive IOD means the western Indian Ocean runs warmer than normal, and the eastern side runs cooler.
- Ocean Mean Temperature (OMT) measures heat down to the 26°C isotherm depth, roughly 59 metres in the south-western Indian Ocean during January-March.
- Both indicators feed into India's seasonal monsoon forecast, issued by the India Meteorological Department (IMD).
📘 Good to Know
Why Two Different Indicators Matter
- IOD can modulate El Niño's usual impact on the Indian monsoon, sometimes offsetting a drought signal from the Pacific.
- OMT reflects a deeper, more stable slice of ocean heat than plain sea surface temperature (SST), which is limited to the top few millimetres.
- IITM Pune reports around 80% predictive success for OMT, against roughly 60% for SST alone.
- January-March OMT readings help forecasters judge whether the coming monsoon is likely to bring above- or below-average rainfall.
Test Yourself
🌟 Great to Know
A Common Sourcing Trap
- Some circulated answer keys give the January-March OMT isotherm depth as 129 metres. A direct write-up of IITM's own published research instead gives 59 metres for that same figure.
- This is a real example of why a fact needs checking against original research, not just a repeated number.
- IOD and OMT are separate, independent indicators. A forecaster reads both together, not one in place of the other.
📝 Previous Year Questions
Indian Ocean Dipole: What It Measures
- Statement 1 IOD tracks the SST difference between the Western Indian Ocean and the Eastern Indian Ocean, not the Eastern Pacific as the statement claims. This statement is false.
- Statement 2 IOD genuinely can modulate El Niño's impact on the monsoon. This statement is true.
- Answer: (b) 2 only.
- View this question on the full 2017 paper →
Ocean Mean Temperature: A Figure Worth Double-Checking
- Statement 1 The question's own 129-metre figure for the January-March 26°C isotherm depth doesn't match a direct write-up of IITM's published research, which gives 59 metres instead. This statement is false as worded.
- Statement 2 OMT readings genuinely help assess whether the coming monsoon will bring more or less rainfall than the long-term average. This statement is true.
- Answer: (b) 2 only.
- View this question on the full 2020 paper →
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