Why El Niño numbers differ between agencies: baselines explained simply

You may see El Niño described as +2.1°C in one report, +2.8°C in another and +3.7°C in a third. Nobody is making it up. The agencies are measuring different things.

Different yardsticks. The older Oceanic Niño Index (ONI) compares the Niño-3.4 patch of the Pacific with a fixed long-term average. The Relative Oceanic Niño Index (RONI) compares it with the average of all tropical oceans at the same time. IMD explains that RONI removes the influence of widespread tropical ocean warming, so it isolates El Niño itself. NOAA's Climate Prediction Center adopted RONI in February 2026, and Australia's Bureau of Meteorology moved to a relative index in September 2025.

Different time windows. NOAA's +2.1°C is a monthly value for September. The Bureau's +2.81°C is a single week ending 11 October. WMO's +3.7°C is a forecast for October–December, on its own baseline. A week can spike higher than a month, and a forecast is not a measurement.

So which is right? All of them, for what they measure. That is why we always label each figure with the agency and the period, and never place them side by side as if they were the same thing.

What it means for you. Do not get lost in decimals. Every agency agrees this is a strong El Niño, and IMD calls conditions over the Pacific strong.

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