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El Nino link to southern ocean currents

Australian scientists have found that El Niño is primarily responsible for determining the strength of the Antarctic Circumpolar Wave.

The Antarctic Circumpolar Wave occurs in the Circumpolar Current around Antarctica. The Current takes eight years to complete a full rotation of the earth, and influences Australia's weather. Scientists suspect the current also affects other southern hemisphere countries.

The Wave alternates large regions of slightly warmer and cooler water.

"During El Niño events, there is a massive build up of warm water in the equatorial central Pacific Ocean," says Dr Peter Baines from CSIRO Atmospheric Research, a division of Australia's national science agency.

"This warm water causes atmospheric linkages, known as 'teleconnections', that set up a large region of high atmospheric pressure in the South Pacific near the Ross Sea, off Antarctica", says Dr Baines.

Associated with this high atmospheric pressure, warm, northerly winds heat the surface waters of the Southern Ocean nearby, slowly raising the temperature by up to 1ºC over a six-month period. Cold southerly winds also cool a similarly large region adjacent to it.

These areas of warm and cold water are then slowly transported eastward, approaching Australia approximately three years later.

"When the warm patch of water passes to the south and west of Australia, it appears to produce higher rainfall over the south of the continent. However, we are still investigating the actual impact of the Antarctic Circumpolar Wave on Australia's rainfall," says Dr Baines.

"From the 1980s until the mid-1990s, El Niño events occurred approximately every four years. This regular appearance strengthened the Antarctic Circumpolar Wave. Over the past few years, though, the pattern has been disrupted, with El Niños becoming irregular. As a result, the Antarctic Circumpolar Wave has weakened. We may expect the next big El Niño to fire it up again," says Dr Baines.

During the opposite phase of El Niño, known as La Niña, there is a build-up of cooler water near the Ross Sea, producing the cooler phase of the Circumpolar Wave, which is then transported around Antarctica.

A graphic showing the Antarctic Circumpolar Wave is available on request.

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