Indian Ocean Dipole’s Grip on Monsoon Rainfall Flips Dramatically Around 1985

The Indian summer monsoon is the most consequential weather system on Earth for more than a billion people, delivering the rain that fills reservoirs, feeds fields and sets the rhythm of agricultural life across South Asia. For decades, scientists have searched for reliable oceanic signals that could tip forecasters off months in advance about whether the coming monsoon season will be generous or stingy. One of the most celebrated of these signals is the Indian Ocean Dipole, a see-saw of sea surface temperatures between the western Arabian Sea and the eastern equatorial Indian Ocean off Sumatra. Now a new study published in the journal Climate Dynamics has delivered a sobering and fascinating verdict on how dependable that signal really is, tracing the dipole-monsoon relationship across more than a century of observations and finding that it is anything but stable.

The research, led by Alok Kumar Mishra, Suneet Dwivedi, Safal Saxena and Mudit of the Banerjee Center of Atmospheric and Ocean Studies at the University of Allahabad, examined the multi-decadal relationship between the Indian Ocean Dipole and Indian summer monsoon rainfall over the period 1901 to 2023. Their central conclusion is that while the connection between the two phenomena is statistically robust when viewed across the full 123-year record, it is emphatically non-stationary. In plain terms, the strength and even the sign of the link swings back and forth over the decades, meaning that a forecasting rule of thumb that worked brilliantly in one era can quietly fail in the next. This kind of non-stationarity is one of the most unsettling findings in climate science, because it undermines the assumption that past behavior is a trustworthy guide to future outcomes.

The most striking discovery in the study is what the authors describe as a first-of-its-kind rapid shift in the dipole-monsoon correlation around the year 1985. Before that transition, during the epoch roughly spanning 1968 to 1982, the correlation between the two was significantly negative, meaning that a positive dipole event, with warm water in the west and cool water in the east, tended to accompany weaker monsoon rainfall. After the shift, during the epoch from about 1992 to 2006, the relationship flipped to significantly positive, so that the same dipole configuration became associated with stronger rainfall. A reversal of this magnitude and speed in a relationship that underpins operational seasonal forecasting is remarkable, and the researchers emphasize that no comparable abrupt sign change has been documented before in this particular pairing of climate phenomena.

What could drive such a dramatic about-face? The authors argue that the answer lies in the changing background state of the tropical Indian Ocean itself, specifically in the interplay between tropospheric temperature anomalies, sea surface temperatures and the large-scale atmospheric circulation that connects them. The monsoon is fundamentally a heat engine: summer solar heating of the Asian landmass relative to the surrounding oceans creates a tropospheric temperature gradient that draws moist maritime air inland and releases it as rain. Any factor that perturbs the vertical and horizontal distribution of temperature in the troposphere, or that alters the sea surface temperature patterns that feed convection, can modulate how strongly the dipole’s fingerprint appears in the rainfall record. During the negative-correlation epoch, the dipole’s influence apparently worked against the monsoon-favoring circulation, while in the positive-correlation epoch the same oceanic pattern reinforced it.

Methodologically, the team leaned on a suite of the most authoritative observational and reanalysis datasets available. Sea surface temperatures came from the COBE-SST2 analysis maintained by NOAA, the Met Office Hadley Centre’s HadISST product, and NOAA’s Extended Reconstructed Sea Surface Temperature version 5. Atmospheric fields were drawn from the ERA5 reanalysis produced by the Copernicus Climate Change Service and from the NOAA-CIRES-DOE Twentieth Century Reanalysis version 3, which extends atmospheric reconstructions back into the nineteenth century by assimilating historical surface observations into a modern numerical model. Rainfall over India was characterized using the high-resolution daily gridded dataset developed by the India Meteorological Department, which covers the country at a quarter-degree spacing from 1901 onward. To probe how the coherence between dipole and monsoon evolved through time, the researchers employed wavelet-based techniques, including cross wavelet transforms and wavelet coherence analysis, tools that are specifically designed to detect time-varying periodic relationships in non-stationary geophysical data.

Wavelet coherence is particularly well suited to this problem because it reveals not just whether two signals are correlated, but when in time that correlation was strong, weak, positive or negative. Applied to the dipole and monsoon records, it exposed the alternating epochs of coupling and decoupling, and pinpointed the mid-1980s as the moment when the phase of the relationship pivoted. The authors also placed their findings in the context of two other celebrated monsoon teleconnections that have themselves been weakening. The link between the El Nino Southern Oscillation, the great Pacific climate oscillation, and Indian rainfall famously degraded in recent decades, and the relationship between the tropospheric temperature gradient and monsoon strength has also shown signs of erosion. Paradoxically, the new study suggests that as these other pillars of monsoon predictability weakened, the dipole-monsoon relationship grew more prominent, as if the dipole stepped in to fill the predictive vacuum left behind.

That apparent compensation, however, comes with a warning. The analysis indicates that the Indian Ocean Dipole is no longer a potential predictable driver of Indian summer monsoon rainfall in recent decades. This is a subtle but crucial distinction: the dipole may still co-vary with the monsoon, but if the dipole itself has become harder to forecast, or if its influence on rainfall has become contingent on background conditions that are shifting under greenhouse warming, then its practical value for seasonal prediction diminishes. Previous modeling work has suggested that prolonged greenhouse warming may reduce the variability of the dipole, and the rapid Indian Ocean warming observed over the past half century has already altered the basin’s mean state, compressing the land-sea thermal contrast that powers the monsoon. The new findings add a temporal dimension to that concern, showing that the dipole’s monsoon influence is not a fixed property of the climate system but a moving target.

The implications for the roughly 1.4 billion people who depend on the monsoon are considerable. Indian agriculture employs nearly half the workforce, and even modest deviations from normal seasonal rainfall translate into measurable swings in crop yields, food prices and rural incomes. Seasonal forecasting agencies, including the India Meteorological Department, have long woven sea surface temperature predictors, including dipole indices, into their statistical and dynamical forecast models. A predictor whose sign flips without warning is a predictor that can silently degrade a forecast system, and the 1985 transition documented in this study is a vivid illustration of that hazard. The authors’ demonstration that the relationship is robust only in a long-term, averaged sense, while unstable in any given multi-decadal window, argues for forecast frameworks that explicitly account for time-varying teleconnections rather than assuming eternal stationarity.

The study also contributes to a broader scientific conversation about how climate change reshapes the architecture of tropical climate variability. The dipole does not operate in isolation; it interacts with the Pacific through ENSO, with the Atlantic through cross-basin teleconnections, and with the monsoon circulation itself, which can in turn force oceanic responses during dipole events. Understanding how these coupled modes reorganize as the planet warms is one of the central challenges of climate science, and evidence that a major teleconnection can reverse sign within a few years suggests that the reorganization may be more abrupt and less gradual than many models assume. The Allahabad team’s work, grounded in more than a century of carefully curated observations, provides a template for detecting such reversals in other basins and other teleconnection pairs.

For now, the message for monsoon watchers is one of cautious humility. The Indian Ocean Dipole remains a genuine and physically meaningful component of the climate system, capable of shaping rainfall, drought and flood risk across the Indian Ocean rim. But its partnership with the Indian summer monsoon, once treated as a dependable lever for prediction, has proven to be a shifting alliance, negative in one generation and positive in the next, with a dramatic pivot point around 1985 marking the change. As the tropical Indian Ocean continues to warm and the global climate continues to evolve, the study’s authors suggest that scientists and forecasters alike must treat teleconnection relationships as living, breathing features of the climate system, subject to renewal, decay and, occasionally, complete reversal, rather than as fixed constants etched into the physics of the atmosphere.

Subject of Research: The multi-decadal, non-stationary relationship between the Indian Ocean Dipole and Indian summer monsoon rainfall from 1901 to 2023.

Article Title: Investigating the multi-decadal relationship between Indian ocean dipole and Indian summer monsoon rainfall

Article References: Mishra, A. K., Dwivedi, S., Saxena, S., & Mudit (2026). Investigating the multi-decadal relationship between Indian ocean dipole and Indian summer monsoon rainfall. Climate Dynamics, 64(10), Article 430. https://doi.org/10.1007/s00382-026-08389-5

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08389-5

Keywords: Indian Ocean Dipole, Indian summer monsoon rainfall, ENSO, teleconnection, non-stationarity, sea surface temperature, tropospheric temperature gradient, wavelet coherence, climate change, monsoon prediction, Climate Dynamics, tropical Indian Ocean