The ocean and the air share heat and water
The sea and the air are always giving things to each other. This is called ocean–atmosphere interaction.
- Heat: the sun heats the sea surface. Warm sea heats the air above it.
- Water: warm sea water evaporates (turns into vapour). The vapour rises, cools, and makes clouds and rain.
- Push: wind drags the sea surface and makes currents.
So warm sea means more vapour, more rising air and more rain. Cold sea means less vapour, sinking air and dry weather.
A normal year in the Pacific
Near the equator the trade winds blow from east to west. They push the warm top layer of the sea towards the west. The warm water piles up near Asia and Australia. Here the sea can be about 29 °C.
In the east, near the coast of South America, the warm layer is thin. Cold, rich water rises from the deep. This is upwelling. It brings food for fish, so this coast has great fishing. The sea there is only about 23 °C.
Warm sea in the west = rising air = clouds and rain. Cold sea in the east = sinking air = dry coast. The air moving round this loop is called the Walker circulation.
El Niño: when the wind weakens
Every few years (about every 2 to 7 years) the trade winds become weak. Nothing holds the warm water in the west, so it slides back east. This is El Niño.
- The sea in the east and middle Pacific becomes warmer than normal.
- The rain clouds move east with the warm water.
- Upwelling stops or becomes weak, so fish leave and fishing falls. Warm sea can also bleach coral.
- Places far away feel it: heavy rain and floods on the Peru coast; drought risk in Indonesia, Australia and often India (weaker monsoon).
Scientists call it El Niño when the sea is at least 0.5 °C warmer than average for several months.
La Niña: when the wind gets stronger
La Niña is the opposite. The trade winds blow harder than normal. Even more warm water piles up in the west, and even more cold water rises in the east. The east Pacific becomes cooler than normal.
- More rain over Indonesia and Australia; often floods there.
- Dry weather and drought on the Pacific coast of South America.
- In India La Niña years often give a good monsoon but can mean a colder winter.
El Niño, La Niña and the normal state together are called ENSO (El Niño–Southern Oscillation).
Try it: the tray and the breath
Fill a flat tray with a little warm water. Gently blow along it from one end. Watch the water gather at the far end. Stop blowing: the water slides back. Now blow harder: the heap is bigger. Blow = trade wind, far end = west Pacific. Match what you see with the 3D slider.
Key formulas and definitions
- Normal year: strong trade winds → warm pool in the west, upwelling in the east
- El Niño: weak trade winds → warm water moves east, upwelling weak
- La Niña: stronger trade winds → bigger warm pool west, strong upwelling east
- El Niño if the sea is ≥ +0.5 °C above average for several months
- ENSO = El Niño–Southern Oscillation
Worked examples
1. In a normal year the west Pacific sea is 29 °C and the east is 23 °C. What is the difference?
29 − 23 = 6 °C. The west is warmer because the trade winds pile warm water there.
2. The trade winds weaken and the east sea warms by 3 °C. What is the new east temperature and the new east–west difference?
East = 23 + 3 = 26 °C. Difference = 29 − 26 = 3 °C, half of the normal 6 °C. The warm water has moved east.
3. Peru fishermen find very few fish and the sea feels warm. Which condition is this and why?
El Niño. The winds are weak, so cold nutrient-rich water is not rising (no upwelling). Without it, fish food is scarce.
4. Explain in order how a strong trade wind causes rain over Indonesia.
1) Wind pushes warm surface water west. 2) The warm sea there evaporates a lot. 3) Moist air rises, cools and makes clouds. 4) The vapour turns to rain.
5. The sea temperature of the east Pacific is 0.8 °C above average for six months. Is it El Niño?
Yes. It is above the +0.5 °C limit and lasts for several months.
Common mistakes
- Thinking El Niño is a storm. It is a slow warm-water change that lasts months.
- Mixing up the two: El Niño = warm east and weak winds; La Niña = cold east and strong winds.
- Saying warm water is created in the east. It is moved there from the west when the wind weakens.
- Believing every El Niño gives the same weather everywhere. Effects vary from place to place and year to year.