Southern Ocean warming increases rainfall

Ocean warming has caused an eight per cent increase in rainfall over the Southern Ocean and South Pacific over the past 20 years, say Australian scientists in a new report published in Nature this week.

The research team from the Antarctic Co-operative Research Centre (ACRC) and CSIRO reports a systematic decrease of salinity or ‘freshening’ occurring in waters deeper than 500 metres in the Indian Ocean, the Tasman Sea and the North and South Pacific.

According to oceanographer Ms Annie Wong of the Antarctic CRC, Hobart, warmer ocean temperatures are intensifying the cycle of rainfall, introducing greater volumes of fresh water to the sub-polar regions.

Observations at the surface are too sparse to directly measure this effect, says Ms Wong, but confirmation of the process has been found in the deep ocean.

“This is the first evidence from deep ocean measurements for systematic increases of rainfall patterns over the polar oceans,” she says.

“The results are consistent with greenhouse warming in the Pacific and Southern Oceans, causing a rainfall increase of eight per cent over twenty two years,” says co-researcher Dr Bindoff.

Direct observations of rainfall and salinity over the Southern Ocean are sparse and unreliable, say the researchers, but the results are consistent with ice cores from Wilkes Land, Antarctica, which show that snow accumulation increased over a similar period by 23 per cent.

Dr Bindoff says the increase in Southern Ocean precipitation is about three times greater than that predicted by recent computer simulations of ocean responses to atmospheric warming for the same period as the observations.

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