El Niño is a natural climate concept that is the warm phase of the El Niño-Southern Oscillation (ENSO), a global phenomenon involving variations in winds and sea surface temperatures over the tropical Pacific Ocean. The term, which means "The Little Boy" or "Christ Child" in Spanish, was first used by fishermen off the coast of Peru centuries ago to describe the appearance of unusually warm Pacific waters around the Christmas season.
The mechanism of El Niño involves a breakdown of the normal atmospheric and oceanic circulation in the Pacific. Normally, easterly trade winds push warm surface water toward the western Pacific (near Asia and Australia), causing cold, nutrient-rich water to rise (upwelling) off the coast of South America. During an El Niño event, these easterly trade winds weaken or even reverse, allowing the warm water to surge eastward toward the central and eastern tropical Pacific. This warming of the sea surface temperature is coupled with an atmospheric change called the Southern Oscillation, which is a shift in surface air pressure between the eastern and western Pacific. The opposite, or cold phase, of this cycle is called La Niña, where the eastern Pacific waters are cooler than average and the trade winds are stronger.
El Niño is connected to global weather patterns through "teleconnections," and its most significant link for India is its adverse impact on the Southwest Monsoon. The weakening of the Pacific trade winds during El Niño is connected to the moisture-laden monsoon winds over India, which typically dampens the monsoon and reduces rainfall over the Indian subcontinent. Historically, nearly 70% of El Niño years since 1980 have coincided with weak rainfall during the crucial June–September season. The relationship between El Niño and the Indian monsoon has changed recently, becoming stronger over north and south India, but weakening over the central Indian region in recent decades.