Warming Indian and Pacific Oceans Reshaped a Century of Monsoon Rainfall

The Indian summer monsoon is one of the most consequential climate phenomena on Earth, delivering the rain that sustains agriculture, water supplies and economic activity for more than a billion people. A new study published in the journal Climate Dynamics by V. Krishnamurthy and J. Shukla of George Mason University now offers the most detailed account yet of how two ocean basins, warming at different rates, have jointly steered the fate of that rainfall over the past 121 years. Drawing on daily gridded rainfall records from the India Meteorological Department stretching back to 1901, the researchers show that the seasonal monsoon is governed by the interplay of two distinct, ocean-driven patterns, and that the balance between them shifted dramatically around 1960 as the Indian Ocean began to warm faster than the Pacific.

The two patterns at the heart of the study are the monsoon’s fingerprints of the El Niño-Southern Oscillation, or ENSO, in the Pacific, and the Indian Ocean Dipole, or IOD, in the Indian Ocean. El Niño events, marked by unusually warm sea surface temperatures in the equatorial eastern Pacific, have long been known to suppress Indian rainfall, while La Niña conditions tend to enhance it. The IOD, a see-saw of sea surface temperatures between the western and eastern Indian Ocean, similarly modulates the monsoon, with positive dipole events generally favouring above-normal rain. What has remained poorly understood is how these two influences combine, particularly during the recent decades of accelerating ocean warming.

To disentangle the two signals without imposing any preconceived filter on the data, the team applied a data-adaptive statistical technique called multi-channel singular spectrum analysis, or MSSA. The method decomposes daily rainfall and satellite-measured outgoing longwave radiation into space-time patterns, extracting oscillations and persisting structures directly from the observations. Because the rainfall data cover only India’s land area, the researchers first identified the seasonally persisting modes in outgoing longwave radiation, which spans both land and ocean, and then projected the century-long rainfall record onto those modes. This procedure yielded two clean components, dubbed M-ENSO and M-IOD, each retaining the same sign of anomaly through most of the June-to-September season but varying from year to year.

The results are striking. The sum of the two modes correlates at 0.83 with the observed seasonal rainfall anomaly across the full 1901 to 2021 period, meaning that the combined influence of the Pacific and Indian oceans explains the bulk of the year-to-year variation in India’s summer rain. Individually, the ENSO-related mode correlates at 0.55 and the IOD-related mode at 0.6 with the total anomaly. Crucially, the two modes sometimes reinforce one another and sometimes cancel out. In 1983, both modes were positive and the season ended with a strong rainfall anomaly of 0.89 millimetres per day; in 1972, both were negative, producing a weak monsoon at minus 1.62 millimetres per day. In 1989 and 1997, the modes opposed each other and the seasonal rainfall came out near normal.

The year 1997 stands out as a dramatic illustration of this destructive interference. That year brought the strongest El Niño of the twentieth century, an event that on its own would likely have produced severe drought across India. Instead, a coincident positive IOD event counteracted the Pacific influence, and the seasonal rainfall anomaly finished at just minus 0.08 millimetres per day, essentially normal. Over the full record, the two modes were jointly positive in 32 years, yielding an average anomaly of 0.48 millimetres per day, and jointly negative in 37 years, averaging minus 0.68 millimetres per day. Years of opposing phases, 52 in total, produced near-normal seasons regardless of which mode dominated.

Beneath this interannual drama, the study uncovers a longer-term reorganisation of the monsoon system. Splitting the record into two epochs, 1901 to 1960 and 1961 to 2021, the researchers found that sea surface temperatures in both basins showed little or no warming trend in the first epoch but steep warming in the second. The Indian Ocean warmed faster than the Pacific, consistent with the assessment of the Intergovernmental Panel on Climate Change that the Indian Ocean and western Pacific have warmed faster than the global average. The variance of the Niño-3.4 index, a standard measure of ENSO, increased from 0.29 to 0.42 square kelvin between the epochs, while the variance of the dipole mode index rose from 0.08 to 0.12 square kelvin, signalling more energetic variability in both oceans.

The rainfall itself followed suit. Nonlinear trend analysis, performed with ensemble empirical mode decomposition and independently confirmed by the MSSA method, reveals that the all-India monsoon rainfall was dominated by strong positive anomalies before 1960 and by predominantly negative anomalies with an almost flat trend after 1960. The two ocean-driven modes shifted in opposite directions: the ENSO-related rainfall mode, which had an increasing trend in the first epoch, settled into a near-flat but positive regime in the second, while the IOD-related mode flipped from positive anomalies in the first epoch to a pronounced negative trend in the second. The relative strength of the two modes, rather than the behaviour of either alone, therefore largely determines the long-term character of the monsoon.

The spatial fingerprints of the two epochs also differ in revealing ways. Correlation maps show that the ENSO-monsoon relationship has remained essentially stable across the century, with the familiar Pacific pattern of sea surface temperature anomalies linked to Indian rainfall in both epochs. The IOD-monsoon relationship, by contrast, was weak and ill-defined before 1960 but became stronger and better organised after 1960, particularly when the IOD and ENSO influences opposed each other. In the second epoch, positive rainfall anomalies became associated with a clear positive dipole pattern in the Indian Ocean, whereas in the first epoch the corresponding dipole signature was weakly negative. The authors attribute this reorganisation to the anomalous warming of the Indian Ocean, which has significantly altered the intensity and phases of the IOD mode and its associated rainfall over the past six decades.

The implications reach well beyond the archive of historical gauges. India has issued seasonal monsoon forecasts for more than a century, first with statistical regression models that ultimately showed little skill and more recently with dynamical global models that still perform poorly. The authors argue that a central reason for this shortfall is the inadequate representation of the oceans in prediction systems. Their findings suggest that reliable seasonal forecasts and long-term projections demand coupled ocean-atmosphere models capable of correctly simulating sea surface temperatures over both the Pacific and the Indian Ocean, including the Arabian Sea, and of capturing the constructive and destructive interference between the two basins’ influences.

The study also leaves open a compelling question for future research: why did the epochal shift occur around 1960, and how will the monsoon respond as the differential warming of the two oceans continues through the twenty-first century? With ocean heat content expected to keep rising, and with the livelihoods of more than a billion people hanging on the answer, the framework developed by Krishnamurthy and Shukla provides a unified way to read the monsoon’s past and a sharper lens for anticipating its future. What emerges is a monsoon not governed by a single oceanic puppeteer, but by a delicate duet between two warming basins, one whose harmony, and discord, may now be changing with the climate itself.

Subject of Research: Long-term variability of Indian summer monsoon rainfall linked to ENSO and the Indian Ocean Dipole under differential ocean warming

Article Title: Long-term variability of Indian monsoon rainfall related to warming of Indian and Pacific oceans

Article References: Krishnamurthy, V., & Shukla, J. (2026). Long-term variability of Indian monsoon rainfall related to warming of Indian and Pacific oceans. Climate Dynamics, 64(11), Article 451. https://doi.org/10.1007/s00382-026-08409-4

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08409-4

Keywords: Indian monsoon, ENSO, Indian Ocean Dipole, sea surface temperature, ocean warming, climate variability, seasonal prediction, Climate Dynamics, monsoon rainfall, Pacific Ocean, Indian Ocean, climate change