El Niño–Southern Oscillation

The El Niño-Southern Oscillation (ENSO) is a recurring coupled fluctuation of ocean temperatures and atmospheric pressure over the tropical Pacific. El Niño is its warm phase, La Niña its cool phase, and the Southern Oscillation is the associated east-west atmospheric pressure variation. Through atmospheric teleconnections, ENSO influences rainfall and weather far beyond the Pacific, including the Indian monsoon.

Coupled ocean-atmosphere mechanism

ENSO phases arise because equatorial Pacific winds, sea-surface temperatures, upwelling and atmospheric convection influence one another.

• Under neutral conditions, easterly trade winds push warm surface water westward, maintain a deeper western thermocline and promote cold-water upwelling in the east.
• The Walker circulation is an east-west overturning circulation. Air rises over the warm western Pacific, flows eastward aloft, sinks over the cooler eastern Pacific and returns westward near the surface as trade winds.
• During El Niño, weakened trade winds allow warm water and convection to shift eastward. The thermocline becomes flatter, eastern upwelling weakens and linked pressure changes further weaken the winds, reinforcing warming.
• During La Niña, stronger trade winds intensify westward water movement, thermocline tilt and eastern Pacific upwelling. Delayed changes in subsurface ocean heat help terminate events and permit transitions between phases.

Walker circulation and the Indian monsoon

The southwest monsoon depends on strong convection over South Asia and surrounding seas, supported by moisture-bearing winds and rising air. El Niño reorganizes this tropical circulation rather than directly removing all monsoon rainfall.

• As warm water and the Walker circulation's ascending branch shift toward the central and eastern Pacific, ascent over the western Pacific and adjoining Asian monsoon region tends to weaken. Compensating subsidence suppresses cloud formation and organized convection.
• This eastward redistribution of convection can weaken the circulation supporting moisture transport towards India and increase monsoon breaks. El Niño therefore generally raises the probability of below-normal June-September rainfall over South Asia.
• The response is not automatic because the monsoon also depends on land-sea thermal contrast, the position of the monsoon trough, monsoon depressions, intraseasonal oscillations and Indian Ocean conditions.
• Impacts vary across Pakistan, India, Nepal, Bangladesh and Sri Lanka with topography, moisture sources and the position of the monsoon trough.
• Changes in monsoon depressions and active and break spells can produce intense storms or floods in some regions even when seasonal rainfall is deficient.
• The northeast monsoon is distinct, so October-December rainfall over southeastern India and Sri Lanka need not follow the southwest monsoon response.

Forecast limits and disaster implications

The ENSO-monsoon relationship is a probabilistic teleconnection, not a one-to-one rule. The Indian Ocean Dipole and intraseasonal atmospheric variability may reinforce or offset ENSO's influence.

• A weakened monsoon can contribute to meteorological, agricultural and hydrological drought, affecting crops, reservoirs and water supply.
• La Niña generally favours a stronger southwest monsoon, but it does not guarantee excess rainfall in every region.
• ENSO forecasts aid preparedness, but regional forecasts and real-time monitoring remain necessary because local floods can accompany an overall deficient monsoon.