1. Normal Conditions
The trade winds move warm surface water towards the western Pacific.
Cold water wells up along the west coast of South America (the Peru or Humboldt current)

2. La Nina The easterly trade winds are more intense than normal.
The trade winds move warm surface water even more towards the western Pacific.
Air pressure is unusually high over the west coast of South America and low over northern Australia.
Cold water wells up along the west coast of South America

3. El Nino
El Nino Year
Air pressure over West coast of S. America becomes low and air over west Australia high pressure
The normal east to west trade winds over pacific are disrupted and warm water ‘sloshes’ eastwards
No upwelling on S.American coast

4. THEORIES ON THE CAUSE OF EL NINO
Rossby wave –
wave lowers thermocline in W. Pacific reducing the upwelling of water. Winds then blow towards warmer water and event starts.
2. Random occurrence
3. Tropical storms –
if storms are strong enough they can blow water eastwards and start the event.
4. Heat from sea floor –
warming by sea floor volcanoes (no evidence)

5. El Nino contributes to a more hazardous world because...
Rainfall reduced in SE Asia, New Zealand, Australia and India leading to crop failure, wildfires and drought.
Heavy rain in California, Mexico and the coasts of Peru and Ecuador can result in flooding and mudslides.
Suppression of the cold current in the east Pacific devastates the fishing off the west coast of S America.
Strong winds in the Atlantic prevent convection currents developing reducing the number of hurricanes.
Tornadoes in the USA are reduced.
More cyclones in Hawaii but less in northern Australia.
Southern Africa may have droughts whereas east Africa may have floods.

6. La Nina contributes to a more hazardous world because…
Rainfall is higher than usual in Indonesia and the Philippines and lower on the West coast of South America.
Southern Africa and SE Australia may have floods.
Eastern Africa, California and South America may experience drought.
There are more hurricanes in the Caribbean and USA.

7. Problems caused elsewhere
Due to the teleconnections of the earth’s climate problems are caused elsewhere! El-nino can disrupt passage of jet streams!

9. El Nino Southern Oscillation.
The Pacific Ocean contains huge amounts of circulation warm and cold water.
El Nino and La Nina are changes in this circulation, linked with changing in atmospheric processes. They are part of a continually oscillating climatic pattern called El Nino Southern Oscillation. (ENSO)
These events occur every 2-7 years and last for 1-2 years.