El Niño and La Niña

Background and evolution

The IMD issues its long-range monsoon forecast in April and updates it in late May or June, explicitly considering the expected ENSO and IOD states. Since 2021 it has also given monthly forecasts and a multi-model ensemble approach. Global centres such as the US National Oceanic and Atmospheric Administration and Australia's Bureau of Meteorology publish regular ENSO outlooks, which are widely followed in India by farmers, traders and policymakers.

The name El Niño comes from Peruvian fishermen, who noticed that warm water sometimes appeared off their coast around Christmas, reducing fish catches. Scientists later linked this ocean warming with the Southern Oscillation, a pressure see-saw across the Pacific first described by Gilbert Walker, who was studying the Indian monsoon while head of the India Meteorological Department. The combined system is called ENSO.

In normal years, trade winds blow from east to west across the Pacific, piling warm water near Indonesia and allowing cold, nutrient-rich water to upwell off South America. During El Niño, the trade winds weaken, warm water spreads east, and rainfall shifts towards the central Pacific. During La Niña, the trade winds strengthen, the western Pacific becomes warmer and wetter, and the east becomes cooler. This page focuses on what these phases mean for India's farms, prices and disaster planning; the detailed physics of air masses is covered in the linked lesson.