The Stable Ruler

For decades, climate scientists debated whether the relationship between ENSO and the Indian summer monsoon was weakening. Papers documented declining correlations. Others found the relationship had become nonstationary. The consensus drifted toward instability: the strongest climate coupling in the tropics was falling apart.

Sharma, Tandon, Chopra, Sujith, and Goswami (arXiv:2603.16346) show the relationship was stable the entire time. The correlation is approximately 0.8 and has been for the full historical record. The apparent weakening was an artifact of the measurement instrument, not the climate.

Standard ENSO indices are derived from Pacific sea surface temperatures — specifically, from regions like Niño 3.4 in the central-eastern Pacific. But ENSO is not a Pacific-only phenomenon. It modulates ocean dynamics across all three tropical basins. The Pacific index captures some of this influence but is contaminated by regional climate noise that has no connection to the monsoon.

The fix: use the depth of the 20°C isotherm across all three tropical ocean basins as a subsurface predictor. This “Global-ENSO” framework integrates the coupled dynamics that Pacific surface indices only partially reflect. The correlation with monsoon rainfall becomes strong, stable, and predictive at 18-month lead — long enough to be genuinely useful for forecasting.

The paradox was generated by the index, not by the climate. A measurement framework tuned to one ocean basin treated multi-basin coupling as noise. When the coupling strengthened in the Indian and Atlantic basins (as it did in recent decades), the Pacific index underestimated ENSO’s monsoon influence — making a stable relationship appear to weaken.

The relationship didn’t change. The ruler did. What looked like a dying signal was a healthy signal viewed through a narrowing aperture.