The two opposite phases of an ocean-atmosphere cycle in the equatorial Pacific, El Nino (warming) and La Nina (cooling), that together with the Southern Oscillation (ENSO) affect global and Indian weather.
- El Nino: abnormal warming of the central and eastern Pacific (off Peru) that weakens trade winds; generally associated with a weaker Indian south-west monsoon and drought risk.
- La Nina: abnormal cooling of the same region with stronger trade winds; generally associated with a good or above-normal Indian monsoon.
- The atmospheric component is the Southern Oscillation (a see-saw of air pressure between Tahiti and Darwin); together they form ENSO (El Nino Southern Oscillation).
- The Indian Ocean Dipole (IOD) is a separate but related sea-surface temperature pattern in the Indian Ocean that can offset El Nino's effect on the monsoon.
- Effects are tendencies, not certainties; verify the latest forecast (IMD and global agencies) for a given year.
The El Nino (warming, weak monsoon) versus La Nina (cooling, good monsoon) contrast and the ENSO/Southern Oscillation link are recurring climatology and current-affairs facts.
El Nino is the warm phase (tends to weaken the Indian monsoon); La Nina is the cool phase (tends to strengthen it). The Southern Oscillation is the atmospheric pressure see-saw.
Pacific ocean cycle: El Nino (warming, weak monsoon) versus La Nina (cooling, strong monsoon); together ENSO.