El Nino Southern Oscillation (ENSO)

Definition

The El Nino Southern Oscillation (ENSO) is the coupled ocean-atmosphere phenomenon over the tropical Pacific in which sea-surface temperatures (the El Nino and La Nina ocean part) and the air-pressure seesaw between the eastern and western Pacific (the Southern Oscillation, the atmospheric part) swing together and affect weather worldwide.

Key points

• In a normal year, trade winds push warm surface water westward toward Indonesia, leaving cooler, nutrient-rich upwelling water off Peru (the Walker circulation).
• El Nino (the warm phase) occurs when the trade winds weaken or reverse, warm water spreads to the eastern Pacific off South America, upwelling and fisheries off Peru collapse, and the Indian monsoon tends to be weaker or deficient.
• La Nina (the cool phase) is the opposite: stronger trade winds, colder eastern Pacific, and often a normal-to-good Indian monsoon.
• The Southern Oscillation is measured by the Southern Oscillation Index (SOI), the air-pressure difference between Tahiti (east) and Darwin (west); El Nino and the SOI are two sides of the same ENSO system.
• ENSO is a quasi-periodic cycle (roughly every two to seven years), not annual, and is a major driver of global climate variability, droughts, and floods.

Why it matters for CAPF

That ENSO couples the ocean (El Nino, La Nina) with the atmosphere (Southern Oscillation), the link of El Nino to a weak Indian monsoon, the Walker circulation, and the Tahiti-Darwin index are recurring climatology facts.

Common confusion

ENSO is the combined system; El Nino and La Nina are its ocean phases, while the Southern Oscillation is its pressure-seesaw atmospheric part; El Nino usually weakens the Indian monsoon, La Nina usually strengthens it; it is the Pacific phenomenon, distinct from the Indian Ocean Dipole.

Parent note

oceanography