The El Niño Southern Oscillation (ENSO) cycle is an irregular fluctuation in winds and sea-surface temperatures across the tropical Pacific Ocean. It shifts between El Niño, neutral conditions and La Niña, affecting weather, ecosystems and societies worldwide.
How Does the ENSO Cycle Work?
This is HL-only content in IB ESS subtopic 2.4 Climate and biomes. ENSO couples the ocean and atmosphere through the east to west Walker circulation over the tropical Pacific.
Under neutral conditions, easterly trade winds push warm surface water westwards towards Indonesia and Australia. Near the western coast of South America, upwelling brings cold, nutrient-rich deep water to the surface, supporting productive fisheries.
| ENSO state | Ocean to atmosphere mechanism | Major consequences |
|---|---|---|
| Neutral | Normal trade winds move warm water west; strong upwelling occurs near Peru and Ecuador. | Wet western Pacific conditions and productive eastern Pacific fisheries. |
| El Niño | Trade winds weaken or reverse; warm water moves east; stratification increases and upwelling decreases. | Lower fish productivity near Peru; drought may affect Australia and Indonesia, while western South America may receive heavy rainfall. |
| La Niña | Trade winds and Walker circulation strengthen; warm water moves farther west and eastern Pacific upwelling intensifies. | Australia and Indonesia may become wetter; western South America may become cooler and drier. |
ENSO creates teleconnections, meaning regional ocean to atmosphere changes alter distant climates. Impacts differ according to communities' dependence on fisheries and agriculture, infrastructure and preparedness. For example, Peruvian fishing communities may lose income during strong El Niño events as nutrient supply and fish populations decline.
A common misconception is that ENSO is perfectly regular. It is an irregular fluctuation with neutral and transitional states, not a fixed annual sequence.
Exam Technique
For an explain question, show the causal chain: changed trade winds → altered warm-water distribution → changed stratification and upwelling → ecological and societal impacts. Distinguish El Niño from La Niña and use a named location such as Peru, Indonesia or Australia.
