Key Terms & Concepts — UPSC Mains
El Nino-Southern Oscillation (ENSO)
"A periodic warming and cooling cycle of the central and eastern equatorial Pacific Ocean that is one of the most significant natural drivers of year-to-year global climate variability."
The El Nino-Southern Oscillation is a coupled ocean-atmosphere phenomenon involving periodic fluctuations in sea-surface temperature and air pressure across the central and eastern equatorial Pacific Ocean. It has three phases: El Nino, the warm phase, in which Pacific surface waters warm and disrupt normal atmospheric circulation; La Nina, the cool phase, with the opposite pattern; and Neutral, in which neither signal dominates and other regional drivers, such as the Indian Ocean Dipole, play a proportionately larger role. ENSO is one of the most significant natural drivers of year-to-year climate variability worldwide, with well-documented but probabilistic, not deterministic, effects on the Indian monsoon: El Nino years are historically associated with below-normal or delayed monsoon rainfall, while La Nina years are associated with above-normal rainfall. The correlation is not fixed, however, since the Indian Ocean Dipole can offset or reinforce ENSO's effect, meaning several past El Nino years have still delivered near-normal Indian monsoon rainfall. The World Meteorological Organization issues periodic Global Seasonal Climate Updates tracking ENSO's evolving strength, and the India Meteorological Department factors both ENSO and IOD signals into its own seasonal monsoon forecasts rather than relying on either driver alone.
A foundational GS1 climatology concept and a recurring GS3 agriculture/food-security driver, tested both as a standalone Prelims fact and as an analytical frame in Mains answers on monsoon variability and its downstream effects on food inflation.
- 1 Three phases: El Nino (warm), La Nina (cool), Neutral.
- 2 El Nino: warming of central/eastern equatorial Pacific surface waters.
- 3 El Nino historically linked to below-normal Indian monsoon rainfall; La Nina to above-normal.
- 4 The relationship is probabilistic, not deterministic, and is modulated by the Indian Ocean Dipole (IOD).
- 5 WMO issues periodic Global Seasonal Climate Updates tracking ENSO strength.
- 6 IMD factors both ENSO and IOD into its own seasonal forecasts rather than using ENSO alone.
- 7 2026: a strong El Nino was forecast to strengthen through July-September, with the IOD remaining neutral into September.
The WMO's July-September 2026 forecast of a strong El Nino carried direct implications for India's monsoon tail and rabi outlook, though IMD cautioned that the offsetting Indian Ocean Dipole was expected to turn positive, if at all, only after the peak grain-filling window had passed.
