Everything on this page follows from one question: which way are the Pacific trade winds blowing, and how hard? Get that straight and you can work out the sea temperatures, the upwelling, the fisheries and the weather on both sides of the ocean without memorising a single list.
📘 What you need to know
ENSO is a natural climate phenomenon: fluctuations in wind patterns and sea surface temperatures across the tropical Pacific.
It has two extreme phases — El Niño (warming) and La Niña (cooling) — with neutral conditions in between.
Events are irregular and hard to predict, usually occurring every 2 to 7 years.
Normally, trade winds push warm surface water west, and cold nutrient-rich water rises off South America through upwelling.
El Niño: trade winds weaken or reverse, upwelling is reduced, marine productivity falls.
La Niña: trade winds strengthen, upwelling increases, marine productivity rises.
Effects reach far beyond the Pacific, hitting agriculture, health and economies.
Normal conditions first
Under neutral conditions the trade winds blow from east to west across the tropical Pacific. They push warm surface water towards the western Pacific, near Asia and Australia, where it piles up and the warm rising air brings heavy rain.
Because that surface water has been dragged away from South America, colder water rises from below to replace it. This is upwelling, and the water it brings up is rich in nutrients — which is why the coast of Peru and Ecuador supports one of the world’s most productive fisheries.
Read down any column and each line causes the one below it. Read across and you can see that La Niña is essentially normal conditions turned up, while El Niño is normal conditions switched off.
Learn the normal column properly and you barely need the other two. La Niña is normal but stronger; El Niño is normal but reversed. That saves you memorising three separate lists under exam pressure.
The El Niño phase
El Niño occurs when the east-to-west trade winds weaken or reverse, caused by a weakening or reversal of the Walker circulation.
The consequences follow in order:
Warmer-than-usual sea surface temperatures develop in the central and eastern tropical Pacific.
Upwelling of cold, nutrient-rich water off north-western South America is reduced, particularly affecting Peru and Ecuador.
Reduced upwelling decreases marine productivity, because the nutrients that feed plankton no longer reach the surface.
Fewer plankton means fewer fish, which damages local fisheries and the food chains that depend on them.
Global weather impacts of El Niño
Drier conditions in Australia and Southeast Asia, leading to droughts and wildfires
Heavy rainfall and flooding on the western coast of the Americas, particularly Peru and parts of the USA
Warmer winter temperatures in Canada and the northern USA
Decreased monsoon rains in India and parts of Southeast Asia
The La Niña phase
La Niña is the opposite. It occurs when the Walker circulation — the east-west atmospheric circulation — strengthens.
Stronger trade winds push warm water even further west.
Cooler water dominates the eastern and central Pacific.
Upwelling of cold, nutrient-rich water off South America increases.
Marine productivity rises in those areas.
Global weather impacts of La Niña
Increased rainfall in Australia and Southeast Asia, leading to floods
Drier conditions in South America and parts of the USA
Harsher winter conditions in Canada and the northern USA
Impacts on marine ecosystems
Both phases significantly affect marine productivity. During El Niño the lack of nutrient-rich upwelling causes a decline in plankton, leading to fewer fish and affecting entire marine food chains. La Niña promotes nutrient upwelling, boosting marine life and improving fisheries.
The cycle also affects coral reefs. Warmer waters during El Niño lead to coral bleaching events, damaging ecosystems in places such as the Great Barrier Reef in Australia.
The chain worth writing out. Weaker trade winds → less upwelling → fewer nutrients at the surface → less plankton → fewer fish → fisheries collapse. Every link in that sequence is a mark.
Secondary impacts
Area
Impact
Agriculture
Crops damaged by droughts during El Niño or flooding during La Niña, leading to food shortages
Health
Climate changes can increase the spread of diseases such as malaria in tropical regions, where wetter conditions give mosquitoes breeding grounds
Economy
Natural disasters caused by ENSO events cause economic losses, especially in countries that depend heavily on agriculture and fishing
WORKED EXAMPLE
Explain why anchovy catches off the coast of Peru fall sharply during an El Niño year.
Step 1: Start with the windstrade winds weaken or reverseStep 2: Effect on the waterWarm water is no longer pushed west, so it stays over the eastern Pacific and upwelling is reducedStep 3: Follow the food chainfewer nutrients at surface → less plankton → fewer fishReduced upwelling cuts primary productivity, so fish stocks fall
WORKED EXAMPLE
Australia experiences severe drought and bushfires in a particular year, while Peru suffers flooding. Identify the likely ENSO phase and justify your answer.
Step 1: Locate the rainfallWet on the eastern Pacific side, dry on the western sideStep 2: Work out where the warm water iswarm water and rising air have shifted eastStep 3: Name the phaseThis is the reverse of normal conditions, so the trade winds must have weakenedEl NinoLa Nina would give the opposite – floods in Australia, drought in Peru
💡 Exam tip
Always start from the trade winds. Every other effect follows from them.
Use the word upwelling and say what it brings up: cold, nutrient-rich water.
Name the regions. Peru, Ecuador, Australia and Southeast Asia all appear in mark schemes.
Give the frequency: every 2 to 7 years, irregular and hard to predict.
Link productivity changes to food chains, not just to fish numbers.
⚠ Common mix-up
El Niño and La Niña swapped. El Niño is the warming phase with weakened winds.
Calling ENSO a result of climate change. It is a natural cycle, though climate change may affect its intensity.
Saying upwelling stops completely. During El Niño it is reduced, not switched off.
Assuming effects are limited to the Pacific. ENSO affects weather across the globe.
Confusing coral bleaching with acidification. Bleaching here is caused by warmer water, not lower pH.
Up next: Tropical Cyclones in a Warming World — how these storms form, and what a warmer ocean does to them.
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