Model Answer

El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO), characterised by abnormal warming of sea surface temperatures in the central and eastern equatorial Pacific Ocean. Occurring irregularly every 2–7 years, El Niño significantly influences global climate patterns and often contributes to record-breaking global temperatures.

Mechanism of El Niño Formation

Under normal conditions, strong easterly trade winds push warm surface waters toward the western Pacific, promoting cold water upwelling along the South American coast.

During El Niño:

• Trade winds weaken or reverse, allowing warm water to move eastward.
• The thermocline deepens in the eastern Pacific, suppressing nutrient-rich cold water upwelling.
• This oceanic warming alters atmospheric pressure patterns, weakening the Walker Circulation.
• The coupled ocean–atmosphere interaction reinforces warming through positive feedback, sustaining El Niño conditions.

Indicators of El Niño

• Sea Surface Temperature (SST) anomalies in Niño regions of the Pacific.
• Subsurface heat build-up at depths of 100–250 metres, acting as a precursor signal.
• Oceanic Niño Index (ONI) showing SST anomalies of ≥ +0.5°C for at least five consecutive overlapping three-month periods.
• Weakening of trade winds and disruption of Walker Circulation.

Global and Indian Impacts

• Global climate: El Niño years often rank among the hottest on record, amplifying heatwaves and altering precipitation patterns.
• South America: Heavy rainfall, flooding and coastal erosion along the Pacific coast.
• Australia and Southeast Asia: Droughts, water stress, heatwaves and increased wildfire risk.
• India: Greater likelihood of weakened southwest monsoon, droughts, reduced agricultural output and stress on water resources.
• Long-term global warming may intensify the frequency and impacts of El Niño by increasing ocean heat content.

El Niño is a powerful manifestation of ocean–atmosphere coupling with far-reaching climatic consequences. Early signals of its possible return underscore the need for robust climate monitoring, adaptive agriculture, and disaster preparedness, particularly for monsoon-dependent economies like India. Understanding its mechanisms and indicators remains crucial for climate resilience planning.