11 Jan 2011

Australia floods: what causes such severe weather?

As more flooding devastates parts of Australia Channel 4 News asks what is causing such unusual weather. Some experts believe the extreme conditions could last into the summer.

The Australia floods have killed at least nine people (Reuters)

The volatile conditions in Australia are being blamed on a weather system called La Niña.

La Niña is a natural three to six year cycle which results in a cooling phase in the eastern Pacific just off the coast of Peru.

A drop in sea temperatures of only a degree or so can cause a dramatic change in weather patterns around the world.

This year the cooling phase has caused severe rain in parts of Australia which have suffered drought conditions for several years.

La Niña is the opposite of El Niño which causes a rise in sea temperatures in the same area.

It means the heat and energy that is found in the tropics is transferred away from eastern side of the Pacific to western side.

The Queensland floods are caused by what is one of the strongest, if not the strongest, La Niña events since our records began. Professor Neville Nicholls

Typically La Niña – sometimes described as the quiet sister – is not as severe as El Niño but scientists say we are currently experiencing a strong La Niña phase.

Professor Neville Nicholls is President of the Australian Meteorological and Oceanographic Society.

“The Queensland floods are caused by what is one of the strongest, if not the strongest, La Niña events since our records began in the late 19th century.

“The La Niña is also associated with record warm sea surface temperatures around Australia and these would have contributed to the heavy rains.”

The severe flooding in parts of Australia has left at least nine people dead and several more are missing.

Australia floods have killed at least 10 people

How long will it continue?

The last time Australia experienced such extreme weather was in the 1970s when La Niña lasted for two years.

This time there are different models relating to how long the current severe conditions will persist.

Dr Liz Bentley from the Royal Meteorological Society told Channel 4 News: “Looking at predictions most say it’s about to reach its peak this month or into February but there are a few models which say it could go into the summer.”

Environment experts in Australia say the ground is now so wet that any further rain could cause serious problems.

Tony Weber is a visiting fellow at the Australian National University: “With the whole catchment now in a state of complete saturation, any subsequent rainfalls, even though they may not be record rainfalls, will likely result in extreme runoff events as we have seen in Toowoomba and the Lockyer Valley.

“Until sufficient drying of the catchment occurs (likely to be in the order of several weeks with no further major rainfall), there will continue to be a very high risk of further major runoff events.”

Climate change

La Niña and El Niño are natural cycles which are not thought to be caused by climate change.

Scientists have been studying the weather systems for many years in a bid to find out what causes the change in sea temperatures.

Dr Liz Bentley said: “We could see the frequency or severity of La Niña and El Niño altered because of climate change but they in themselves are natural things that happen in the ocean and are not caused by climate change.”