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 Ocean. El Niño is its warm phase, while the Southern Oscillation refers to the associated east-west pressure changes that alter tropical atmospheric circulation.

How El Niño affects the monsoon

During El Niño, unusually warm waters in the central and eastern equatorial Pacific shift convection eastward and weaken the Walker circulation. The resulting circulation anomalies tend to suppress convection over parts of the Indo-Pacific and weaken the large-scale conditions supporting the southwest monsoon.

• El Niño generally raises the probability of below-normal June-September rainfall over South Asia.
• The cool phase, La Niña, generally favours a stronger southwest monsoon, although its effects are also not automatic.

Regional and seasonal variability

El Niño changes the probability and distribution of rainfall rather than producing an identical response across Pakistan, India, Nepal, Bangladesh and Sri Lanka. Its effects vary with topography, distance from moisture sources and the position of the monsoon trough.

• Seasonal rainfall may be deficient even when individual regions experience intense storms or floods.
• The northeast monsoon is a distinct circulation system, so October-December rainfall over southeastern India and Sri Lanka need not follow the southwest monsoon response.
• Variations in monsoon depressions and active and break spells create substantial differences within the same El Niño year.

Limits and disaster implications

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

• A weakened monsoon can contribute to meteorological, agricultural and hydrological drought, affecting crops, reservoirs and water supply.
• ENSO forecasts support preparedness, but regional rainfall forecasts and real-time monitoring remain necessary because local floods can occur during an overall deficient monsoon.