Understanding drought
Information sheet

The 1991-95 Australian drought was as long as any in living memory. Depending on whom you speak to, the cause was either a single persistent El Niño, or a series of El Niño events, each following on the heels of its predecessor.

Ocean surface temperature image

Typical air movements and changes to ocean surface temperatures during an
El Niño and La Niña event.

What is El Niño?

At Christmas a warm current usually appears in the Pacific off the coast of Peru. This warm water displaces nutrients that normally sustain enormous fish stocks. Because of the time when the fishing season is temporarily disrupted, the Peruvians refer to the start of the warm current as El Niño, Spanish for Christ child.

Now we use the term El Niño for a far more widespread event, one that arises on average at 2–7 year intervals. El Niño often affects weather patterns across much of the planet.

Normally, Pacific Ocean trade winds propel surface water in a westerly direction along the equator. This permits warm water to accumulate in the western equatorial Pacific, to the north-east of Australia, heating air in contact with it. The warm, moist air produces clouds and rain.

However, from time to time this process is disrupted. Trade winds and tropical Pacific currents weaken. Warm water in the western Pacific off Australia is displaced to the central Pacific. Clouds disappear and much of Australia is plunged into drought. Simultaneously, in parts of north and south America, heavy rain begins to fall. An El Niño event has begun.

Measuring El Niño

The strength of El Niño is assessed in part by a measure known as the Southern Oscillation Index (SOI). The index is determined by the difference in barometric pressure between Tahiti and Darwin. Pressure see-saws between the two locations, normally being higher in Tahiti than in Darwin. During El Niño the opposite condition prevails, taking the SOI to negative values.

The latest research

The CSIRO Climate Variability and Impacts Multi-Divisional Program is conducting research into the causes and impacts on Australia of climate variability. The Program involves scientists from fourteen CSIRO Divisions, the Bureau of Meteorology, the Bureau of Resource Sciences and other agencies.

By Paul Holper
August 1996

Site updated 1st October 2003

Modified: 3 April, 2008

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