Sunday, May 25, 2014

Australia becoming drier, Antarctica colder due to Westerly winds speeding up



The Southern Annular Mode consists of Westerly winds that blow continuously over the southern ocean, helping to insulate the southern continent of Antarctica. In recent decades these winds have sped up and moved further south as a result of climate change. This results in drawing rain clouds to the south away from Australia particularly affecting Autumn and winter rainfall.

The Bureau of Meteorology has observed a long term trend for south-eastern and south western Australia becoming more dry. New research has identified reasons for this and why Antarctica has not experienced a warming trend like the rest of the planet.

Western Australia has suffered a 20 per cent loss in winter rainfall since the 1960s. South eastern Australia has suffered a 10 per cent reduction in winter rainfall from the 1990s.

Even during the back to back La Ninas in 2011 and 2012, the Drying trend in Australia still evident despite being the wettest two year period on record. But we have known for some time that Less rain across southern Australia is a long term climate trend due primarily to increasing atmospheric carbon dioxide concentrations increasing atmospheric temperatures thus changing the dynamic of the sub-tropical jet stream and thus the number and intensity of storms bringing autumn and winter rainfall to southern Australia.




Rising carbon dioxide in the atmosphere is causing the Southern Ocean winds to strengthen according to Lead researcher Nerilie Abram, from the ANU Research School of Earth Sciences. She said the findings explained the mystery over why Antarctica was not warming as much as the Arctic, and why Australia faces more droughts.

"With greenhouse warming, Antarctica is actually stealing more of Australia's rainfall. It's not good news - as greenhouse gases continue to rise we'll get fewer storms chased up into Australia," Dr Abram said. "As the westerly winds are getting tighter they're actually trapping more of the cold air over Antarctica. This is why Antarctica has bucked the trend. Every other continent is warming, and the Arctic is warming fastest of anywhere on earth."

While most of Antarctica is becoming colder the Antarctic Peninsula has seen substantial warming with rapid increases in summer ice melt, glacier retreat and ice shelf collapse. The strong Westerly winds passing through Drake Passage are making the climate of the peninsula warm exceptionally quickly.

This new study published in Nature Climate Change - Evolution of the Southern Annular Mode during the past millennium (abstract) - extends the 60 year observational record of the Westerly winds back more than 1000 years through the use of climate proxies including ice cores from Antarctica, and tree rings and lake sediments in South America.

"The Southern Ocean winds are now stronger than at any other time in the past 1,000 years," Abram said.

"The strengthening of these winds has been particularly prominent over the past 70 years, and by combining our observations with climate models we can clearly link this to rising greenhouse gas levels."

Study co-authors Dr Robert Mulvaney and Professor Matthew England said the study answered key questions about climate change in Antarctica.

"Strengthening of these westerly winds helps us to explain why large parts of the Antarctic continent are not yet showing evidence of climate warming," said Dr Mulvaney, from the British Antarctic Survey.

"This new research suggests that climate models do a good job of capturing how the westerly winds respond to increasing greenhouse gases," added Professor England, from the Climate Change Research Centre at UNSW.

Previous scientific research from Mayewski et al (2013) - West Antarctica's sensitivity to natural and human-forced climate change over the Holocene (abstract) - also highlights changes to the austral westerly winds and it's impact on southern hemisphere precipitation and heat transport. Mayewski's 2013 research paper concludes: “our investigations point to the likelihood that the drying trend will continue in these regions [southern areas of the Africa, Australia, New Zealand and South America] and to the possibility that there will be in the future abrupt transitions in the continued poleward migration of the westerlies.”



Video: Animation of Antarctic Westerlies 12 months to November 2013

Matt Owens in reviewing this research in 2013 says that "this poses numerous serious risks to people, including for example, significant changes in agricultural output, fisheries production, water supplies, and wildfire."

While we are coming to terms understanding the greater links and impacts of climate change on southern hemisphere weather and climate dynamics, the Abbott Government is proceeding to slash climate science funding, climate mitigation and clean energy programs in the 2014 Federal budget, including abolition of the National Water Commission to advise and co-ordinate national water policy.


Sources:
  • Australian National University News, 12 May 2014, Ocean winds keep Australia dry, Antarctica cold
  • Nerilie J. Abram, Robert Mulvaney, Françoise Vimeux, Steven J. Phipps, John Turner & Matthew H. England. Evolution of the Southern Annular Mode during the past millennium (abstract)
  • Matt Owens, Fairfax Climate Watch, 20 November 2013 - The Rise and Fall of the Westerlies. Abrupt Change
  • Mayewski, P. A.; Maasch, K. A.; Dixon, D.; Sneed, S. B.; Oglesby, R.; Korotkikh, E.; Potocki, M.; Grigholm, B.; Kreutz, K.; Kurbatov, A. V.; Spaulding, N.; Stager, J. C.; Taylor, K. C.; Steig, E. J.; White, J.; Bertler, N. A. N.; Goodwin, I.; Simões, J. C.; Jaña, R.; Kraus, S. West Antarctica's sensitivity to natural and human-forced climate change over the Holocene. (abstract) Journal of Quaternary Science. Jan2013, Vol. 28 Issue 1, p40-48. 9p. DOI: 10.1002/jqs.2593.