Atmosphere Contents

5. General Circulation

El-Niño is caused by warming and cooling of the Pacific Ocean

The El-Niño Southern Oscillation (ENSO) is caused by the cyclic warming and cooling of the surface ocean of the central and eastern Pacific.

  • Normal Conditions: The atmospheric low pressure (high rainfall) over the Indonesian Warm Pool (zones of high sea surface temperatures (SST)) and the high pressure over the eastern Pacific lead to an east-west pressure gradient driving the equatorial surface/trade winds and the Equatorial Ocean Current. Along the coast of Peru is a strong region of cold water upwelling.
  • El-Niño Conditions: The strength of the east-west pressure gradient is reduced leading to a shift of the rainfall over Indonesian Warm Pool into the Pacific and a weakening of the Peruvian cold coastal upwelling. The results in more uniform spread of SSTs over the Pacific and a weakening of the trade winds.
  • La-Niña Conditions: The opposite to El-Niño conditions occur, where there is an increase in the east-west pressure gradient and the east-west SST dipole.

The ENSO cycle flips between the El-Niño La-Niña phases every 2-7 years and can be seen in the ENSO index shown below. El-Niño phases result in heavy rainfall in South America and severe droughts in eastern Australia. In La-Niña phases, it causes droughts in South America and heavy rainfall in Australia.

The ENSO Index timeseries

What is El-Niño and what is La-Niña?