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A temperature anomaly in the Pacific Ocean influences rainfall over India. The relationship is real, well documented, and considerably less deterministic than commonly presented.
Monalisa DuttaFew subjects in Indian meteorology attract as much attention, or as much over-interpretation, as El Niño. The announcement of developing conditions in the Pacific reliably produces forecasts of drought, and those forecasts are frequently more confident than the underlying science supports.
The relationship is genuine and well established. It is also a tendency across many seasons rather than a rule governing any individual one, and that distinction deserves to be understood clearly.
El Niño–Southern Oscillation, generally abbreviated to ENSO, refers to a coupled oscillation of ocean temperature and atmospheric pressure across the tropical Pacific. It has three phases.
Neutral conditions. Trade winds blow from east to west across the Pacific, driving warm surface water towards Indonesia and Australia. Cool water upwells along the South American coast. Deep convection is concentrated over the western Pacific.
El Niño. The trade winds weaken. Warm water spreads eastward across the central and eastern Pacific. The zone of deep convection migrates east with it.
La Niña. The trade winds strengthen beyond normal. Warm water is confined further west and upwelling of cool water intensifies in the east. Convection over the western Pacific is enhanced.
The connection operates through the Walker circulation — an east-west overturning circulation along the equator, with rising air over the warm western Pacific and Maritime Continent, and descending air over the cooler eastern Pacific.
During El Niño, the warm pool shifts east and the rising branch of the Walker circulation shifts with it. This displacement alters the entire tropical circulation. A descending branch may become established over the Indian Ocean and the subcontinent, and descending air suppresses convection. Reduced convection means reduced rainfall.
During La Niña the effect is broadly reversed, with the strengthened Walker circulation tending to favour enhanced convection over the Indian region.
The relationship is one of the strongest known seasonal predictors for Indian rainfall, and it is meaningful. Historically, El Niño seasons have shown a clear tendency towards below-normal monsoon rainfall, and a substantial proportion of India's significant drought years have coincided with El Niño conditions.
Several factors modify the ENSO influence, and any of them may offset it.
The Indian Ocean Dipole. A positive IOD tends to favour Indian monsoon rainfall and can substantially counteract a concurrent El Niño. The two operate independently and their combination determines the net effect.
The location of the Pacific warming. Warming concentrated in the central Pacific — sometimes described as Modoki — appears to affect the Indian monsoon differently from warming concentrated in the eastern Pacific. The distinction is an area of ongoing research.
Timing and evolution. An El Niño which develops late in the season, after the monsoon is well established, exerts less influence than one already mature at onset.
Intra-seasonal variability. The Madden–Julian Oscillation and the internal active-break rhythm of the monsoon operate on timescales of weeks and may dominate the outcome of any particular period regardless of the seasonal background state.
A weakening association. Some research indicates that the strength of the ENSO–monsoon relationship has varied over the decades, and that it may have weakened in recent periods. This remains under active investigation.
I would suggest the following approach to ENSO information.
Treat it as a shift in probability, not as a forecast. The presence of El Niño raises the likelihood of a deficient season. It does not establish one. Planning should account for increased risk rather than assume a particular outcome.
Consider it alongside the IOD. Neither should be examined in isolation, as their combined state is more informative than either alone.
Attend to distribution as well as total. As discussed in our guide to break periods, the timing of rainfall frequently matters more than its aggregate quantity. ENSO offers limited guidance on distribution.
Rely upon official seasonal outlooks. The IMD issues seasonal forecasts which incorporate ENSO alongside numerous other predictors within statistical and dynamical modelling frameworks. Those outlooks represent a considerably more complete assessment than any single index.
El Niño is a genuine and valuable signal. It is one of the relatively few tools available which offers any skill at seasonal range, and its identification represented a significant advance in climate science.
It is not, however, a mechanism by which the coming monsoon may be read off in advance. The atmosphere over India responds to a great many influences, and the Pacific is only one of them — important, well studied, and by no means decisive on its own.
Meteorology consultant and former Squadron Leader in the Indian Air Force, with over eleven years of experience in operational aviation meteorology, weather forecasting and flight safety support. She now advises technology organisations on AI-driven weather forecasting and decision-support systems. Consultancy services →