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New Study - Southern Ocean winds strengthen, shrink south

by Sail-World Cruising round-up on 12 May 2014
Southern Ocean winds SW
If you have ambitions to sail the Southern Ocean and would like to be carried around by the Roaring Forties, you'd better not delay too long. According to some new results from a scientific group the winds are strengthening and moving south.

For the first time, Dr Nerilie Abram, Queen Elizabeth II Research Fellow, Research School of Earth Sciences has led a team which studied data since the year AD1000, and found that Southern Ocean winds are now stronger than at any time in the past 1000 years, and likely to increase.

In the words of Dr Abram to Sail-World Cruising, 'Rising greenhouse gases are causing the westerly winds that circle around Antarctica to get stronger and shift closer to Antarctica. Over the last 50 years the wind belt has shifted south by around 2-5 degrees of latitude, while also getting around 10-15% faster. This is a trend that looks set to continue as greenhouse gas levels continue to rise over the coming century.'

However, the changes will not occur evenly across the year. 'Strengthening of the westerly winds over the coming decades is expected to be most pronounced in the winter months. But during the summer season, a gradual recovery of the Antarctic ozone hole is expected to offset some of the changes being caused by rising greenhouse gases,' she added.

The study has been published in the latest issue of Nature Climate Change.

Antarctica has been warming relatively slowly compared with the rest of the world. The explanation seems to be that the winds spinning clockwise around the continent have been getting stronger, preventing warm air from entering.

In 2009, it seemed that the hole in the ozone layer above Antarctica was responsible for boosting the winds. Dr Abram and her colleagues have shown the ozone hole is only part of the story. Global warming is just as important.

The team reconstructed Antarctic temperatures over the past 1000 years, using an ice core from James Ross Island near the Antarctic Peninsula. The temperatures correlated with how strong and tight the winds are, so they could construct a record of wind strength.

They found that the current strength of the winds is unprecedented over the past millennium. But the surge in strength started in the 1940s, decades before the ozone hole.

So Abram's team simulated the last millennium using eight climate models, driven by actual greenhouse gas levels previously reconstructed from ice cores. All the models predicted that the winds would pick up by the 1940s, suggesting greenhouse gases were playing a role. That may be because the northern hemisphere is warming faster than the south – because it has more continents – creating a strong temperature gradient that boosts the winds.

However, the stronger winds experienced by sailors in the Southern Ocean is one of the milder side effects that scientists are worried about.

'The west Antarctic ice sheet [adjacent to the peninsula] is probably the bit of the Antarctic ice mass that we’ve be most concerned about for the longest time,' Matthew England, from the University of NSW’s Climate Change Research Centre, and a co-author of the paper, said recently.

'If it all melted, that ice sheet could lift global sea levels by 4 to 5 metres, he said.

The full study can be found in the most recent edition of Nature Climate Change
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