ENSO couples ocean temperatures and atmospheric circulation. It shifts the main tropical Pacific rain zone rather than warming every place equally.
During a typical neutral phase, easterly trade winds push warm surface water towards the western tropical Pacific. Warm water piles up near Indonesia, where moist air rises and heavy rain is common. Cooler deep water rises in the eastern Pacific. The thermocline, the layer where temperature drops rapidly with depth, is deeper in the west and shallower in the east.
This east-west circulation is the Walker circulation: surface air moves westwards, rises over the warm western Pacific, returns eastwards aloft and descends over cooler eastern waters. It is different from the north-south Hadley circulation.
During El Nino, the trade winds weaken and the warm pool spreads eastwards. The eastern thermocline deepens, reducing the cooling effect of upwelling. Deep convection shifts towards the central and eastern Pacific. Indonesia and parts of the western Pacific often become drier, while parts of the central and eastern Pacific become wetter.
During La Nina, stronger trades reinforce the westward warm-water accumulation. The eastern thermocline is shallower, upwelling cooling strengthens, and western Pacific convection is often enhanced. These ocean and atmospheric changes reinforce one another. Rainfall effects vary by season, location and event; ENSO changes probabilities, not the outcome of every storm.
Step by step
Wind changes
Trade-wind strength redistributes warm surface water.
Ocean responds
Thermocline depth and upwelling alter sea-surface temperature.
Rain zone shifts
Convection follows the changed pattern of warm water, affecting winds again.
Worked example: Reading a Pacific comparison
Weaker trades, a deeper eastern thermocline and eastward-shifted convection together support an El Nino interpretation. A single warm reading near Indonesia does not: warm western Pacific water also occurs during neutral conditions.
Watch out for this
El Nino means above-average rainfall across the whole tropics.
Its rainfall anomalies have a spatial pattern. Some regions become wetter while others become drier.
Check your understanding
Which set of changes is most consistent with La Nina?
- Stronger easterly trades, cooler eastern Pacific, stronger western convection.
- Weaker trades and an eastward shift of the main warm pool.
- No winds, no upwelling and equal rainfall everywhere.