El Niño–Southern Oscillation (ENSO) and Climate Variability

7.1 Normal Pacific Circulation

Under normal (La Niña-like) conditions:

• Trade winds blow strongly westward across the tropical Pacific
• Warm water piles up in the western Pacific (Indonesia, Australia) — sea surface temperature (SST) 28–30°C
• Cold, nutrient-rich water upwells along Peru-Ecuador coast — supports world's richest fishery
• Walker Circulation: Air rises over warm western Pacific → moves eastward at altitude → descends over cool eastern Pacific → surface trades complete the cell
• Result: Heavy rainfall over Indonesia/Australia; dry Peru coast

7.2 El Niño ("The Child" in Spanish)

Occurs every 2–7 years, roughly around Christmas (hence the name "El Niño" — the Christ child):

• Trade winds weaken or reverse
• Warm water spreads eastward across central and eastern Pacific
• Eastern Pacific SST rises 2–5°C above normal
• Walker Circulation disrupted or reversed

Global Impacts of El Niño

Region	Impact
India	Weakened monsoon → drought (1987, 2002, 2009 El Niño years coincided with monsoon deficiency)
Australia	Drought; increased wildfire risk (SE Australia); reduced rainfall
Indonesia	Drought; forest fires; haze over SE Asia
Peru/Ecuador	Heavy rainfall; floods (normally arid coast becomes wet)
East Africa	Above-normal rainfall in Kenya, Tanzania
NE Brazil	Drought in the semi-arid Nordeste
Global temperature	El Niño years are typically warmest years globally (1997–98, 2015–16 — both record-breaking warmth)

7.3 La Niña ("The Girl")

Opposite of El Niño — trade winds strengthen, warm water piles even more in western Pacific, and eastern Pacific SST drops below normal.

Global Impacts of La Niña

• India: Stronger monsoon → above-average rainfall (good for agriculture)
• Australia: Severe flooding (2010–11 La Niña — Queensland floods)
• East Africa: Drought
• Global: Slightly cooler than El Niño years

Southern Oscillation Index (SOI): Measures atmospheric pressure difference between Tahiti (eastern Pacific) and Darwin, Australia (western Pacific). Positive SOI = La Niña; Negative SOI = El Niño.

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