14 January 2011 Last updated at 16:29

How La Nina triggered floods in Australia

Scientists are blaming La Nina for the recent devastating torrential rainfall in Australia and the Philippines. This weather pattern is one part of a natural cycle called the El Nino Southern Oscillation (ENSO). Find out how it forms and affects global rainfall.

La Nina occurs when surface temperatures are cooler than normal in the eastern Pacific and warmer than normal in the western Pacific. Cold water rises up from the ocean's depths and collects off the west coast of South America.

Pulled by strong easterly winds, the cool water surges westwards across the Pacific in the form of a 'cold tongue'. Warm water builds up and high pressure is trapped along the east coast of Australia and South-East Asia resulting in unusually heavy rainfall.

La Nina's effects are wide-reaching and varied and this season's La Nina has been described as one of the strongest in decades. Powerful easterly winds may have intensified the usual monsoon in Sri Lanka - but meteorologists say it is unlikely to have led to recent flooding and landslides near Rio de Janeiro.

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