🌊 Walker Circulation — The Normal State

To understand El Niño and La Niña, we must first understand the Walker Circulation — the normal atmospheric circulation pattern over the tropical Pacific Ocean.

• In normal conditions, trade winds blow from east to west across the tropical Pacific (from South America toward Asia/Australia)
• These winds push warm surface water westward, piling it up in the western Pacific (near Indonesia/Australia)
• The western Pacific becomes warm and wet — warm water evaporates, rises, forms clouds and rainfall
• The eastern Pacific (near South America) becomes cool and dry — cold water upwells from the deep ocean (Humboldt Current)
• This east-west circulation of air is called the Walker Circulation (named after Sir Gilbert Walker)
• The warm western Pacific drives the Indian monsoon — warm moist air flows toward India

🔥 El Niño — The Warm Phase

El Niño (Spanish for “The Boy Child” or “The Christ Child” — named by Peruvian fishermen because it typically appears around Christmas) is the warm phase of ENSO.

• During El Niño, the trade winds weaken or even reverse direction
• Warm water that was piled up in the western Pacific sloshes back eastward toward South America
• The eastern Pacific becomes abnormally warm (sea surface temperatures rise by 0.5°C or more above average)
• The warm water suppresses the cold upwelling off South America → collapse of fisheries (anchovies disappear)
• Rainfall shifts from the western Pacific to the central/eastern Pacific
• The western Pacific (Indonesia, Australia) becomes drier → droughts, wildfires
• South America (Peru, Ecuador) gets heavy rainfall and floods

❄️ La Niña — The Cool Phase

La Niña (Spanish for “The Girl Child”) is the cool phase of ENSO — essentially the opposite of El Niño.

• During La Niña, the trade winds strengthen beyond normal
• More warm water is pushed westward → western Pacific becomes even warmer than usual
• The eastern Pacific becomes cooler than normal — enhanced cold upwelling
• The western Pacific (Indonesia, Australia) gets more rainfall → floods
• South America (Peru) gets drier conditions
• La Niña typically strengthens the Indian monsoon → above-normal rainfall in India
• La Niña is associated with good agricultural seasons in India

🔄 ENSO — El Niño Southern Oscillation

• ENSO = El Niño Southern Oscillation = the combined ocean-atmosphere phenomenon in the tropical Pacific
• The Southern Oscillation is the atmospheric component — the see-saw of air pressure between the eastern Pacific (Tahiti) and western Pacific (Darwin, Australia)
• Measured by the Southern Oscillation Index (SOI):
  • Negative SOI = El Niño (low pressure in eastern Pacific, high in western)
  • Positive SOI = La Niña (high pressure in eastern Pacific, low in western)
• ENSO cycles occur every 2–7 years and last 9–12 months on average
• El Niño and La Niña are the two extremes; neutral conditions exist in between
• ENSO is the most important year-to-year climate variability on Earth — affects weather globally

🌊 Indian Ocean Dipole (IOD)

The Indian Ocean Dipole (IOD) is an ocean-atmosphere interaction in the Indian Ocean, similar to ENSO but in the Indian Ocean. It significantly influences Indian monsoon rainfall.

• IOD = difference in sea surface temperature between the western Indian Ocean (Arabian Sea) and eastern Indian Ocean (Bay of Bengal/Indonesia)
• Positive IOD: western Indian Ocean warmer than eastern → warm moist air flows toward India → above-normal monsoon rainfall in India; drought in Indonesia/Australia
• Negative IOD: eastern Indian Ocean warmer than western → moisture flows away from India → below-normal monsoon rainfall in India; floods in Indonesia/Australia
• IOD can counteract or amplify ENSO effects on Indian monsoon:
  • El Niño + Positive IOD = effects partially cancel → moderate monsoon
  • El Niño + Negative IOD = both weaken monsoon → severe drought risk
  • La Niña + Positive IOD = both strengthen monsoon → very good rainfall
• IOD was discovered in 1999 by Saji et al. — relatively recent discovery

🌀 Madden-Julian Oscillation (MJO)

• The MJO is a large-scale pattern of tropical rainfall and atmospheric circulation that moves eastward around the globe over 30–60 days
• It is the dominant mode of intra-seasonal variability in the tropics (shorter timescale than ENSO)
• When the active phase of MJO is over the Indian Ocean, it enhances monsoon rainfall in India
• When the suppressed phase is over India, it can cause monsoon breaks (dry spells)
• MJO interacts with ENSO — during El Niño, MJO tends to be weaker and less frequent
• Important for extended range weather forecasting (2–4 weeks ahead)

🌍 Global Impacts of ENSO

✅ Revision Checklist — El Niño, La Niña & ENSO