El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO), marked by abnormal warming of surface waters in the central and eastern equatorial Pacific. Through ocean-atmosphere interactions, it alters rainfall across distant regions (teleconnections).
Mechanism of El Niño
- Weakening of trade winds: Normally, easterly trade winds push warm surface water towards Indonesia and Australia. During El Niño, these winds weaken, allowing warm water to spread eastwards.
- Reduced upwelling: Warm surface waters suppress the rise of cold, nutrient-rich water along the Peru-Ecuador coast, increasing sea-surface temperatures.
- Shift in Walker circulation: Normally, moist air rises over the western Pacific and sinks over the eastern Pacific. El Niño shifts convection and ascending air eastwards, relocating rain-bearing clouds.
Influence on Regional Rainfall
- Peru and Ecuador: Warmer eastern Pacific waters favour heavy rainfall and flooding.
- Indonesia and Australia: Weakened convection over the western Pacific raises drought and wildfire risks.
- India: Altered atmospheric circulation can weaken the southwest monsoon, threatening agriculture and water security.
- Other regions: Rainfall belts shift across the tropics, creating contrasting wet and dry anomalies through teleconnections.
Conclusion
El Niño demonstrates the close coupling of oceans and atmosphere. However, its effects are not uniform or inevitable: the Indian Ocean Dipole, regional circulation and climate change also modify rainfall outcomes.
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