Ground Cover North : Ground Cover 071 November-December 2007 - North
NOVEMBER -- DECEMBER 2007 GROUND COVER 29 HARVEST RADIO www.grdc.com.au/director/events/ onlineservices/harvestradio.cfm Weather Weak winds wobble ENSO values A famous indicator used to help predict El Niño years has changed so much in the past 30 years that ENSO mean values may need to be revised BY REBECCA THYER n One of the world’s largest and most important wind systems – the Walker circulation – has weakened to a 30-year low, which could mean new definitions of El Niño Southern Oscillation (ENSO) values are needed. The Walker circulation is a loop of winds over the Pacific Ocean that influences much of the region’s climate. It is used by meteorologists to detect an El Niño event, or forecast how long it will last. A weakening system indicates an El Niño event, increasing the risk of drought in Australia. Bureau of Meteorology and CSIRO climate scientists investigating changes in El Niño, tropical climate and the Walker circulation have found that since 1977 changes in the tropical atmosphere and ocean reached record levels. The Southern Oscillation Index (SOI) has never been lower, trade winds have never been weaker and tropical ocean surface temperatures and air pressure recorded over northern Australia have never been higher in the observed climate record. The bureau’s Dr Scott Power and CSIRO’s Dr Ian Smith, who together examined changes in 30-year averages in sea surface temperature, air pressure and wind-stress records, suggest that the SOI has actually shifted to lower mean values. Dr Power says: “While we appear to have just lived through the most El Niño- dominated 30-year period on record, part of this dominance might instead reflect a long-lived weakening of the Walker circulation in response to global warming.” However, he says this does not imply that the drought in eastern Australia is locked in. “In fact there is no reason why we couldn’t see a higher frequency of rain-bearing big La Niña events in the future,” Dr Power says. “It’s just that the El Niño and La Niña cycle might now be driving year-to-year variability in a generally weaker Walker circulation.” Dr Power says it is useful to tie this study into the recently released CSIRO/Bureau of Meteorology Report on Climate Change in Australia. “This landmark report indicates that while human-induced global warming is expected to drive a decline in mean rainfall over decades, it does not change the frequency of El Niño or La Niña events,” he says. “So the recent dominance of El Niño could very well be a cruel, random, naturally occurring event, exacerbated to some extent by global warming. While concerning, taking global warming into account has the very real potential to unlock more accurate seasonal climate forecasts.” Late La Niña These findings coincide with the bureau’s latest ENSO data, which predicts 2007 is likely to be considered a La Niña year, even though, by historical standards, it has been late to develop. With the exception of the SOI, all ENSO indicators showed an intensifying La Niña during September and computer models forecast the La Niña to last until early 2008. In the past, most notable La Niña events were established by winter’s end, with widespread above-average rain falling over Australia’s eastern half. With a late- developing La Niña, this typical rainfall response is not as likely as in past episodes. The bureau also says that the unusual state of the oceans to the north, and particularly north-west, of the continent may continue, at least in the short term, to influence Australia’s climate. These oceans have been cooling since June, when they would have been expected to warm as the La Niña evolved in the Pacific. These cooler than normal waters inhibit north-west cloud bands from forming, which are a major source of winter and spring rain for central and south-eastern Australia during La Niña years. NEW RADAR BOOSTS RESEARCH POTENTIAL Warmer sea Cooler sea Typical summer positions of high pressure systems Surface winds Atmospheric winds Typical Walker circulation pattern Air circulation in a vertical plane at the equator INDIAN OCEAN H H L L PACIFIC OCEAN Tahiti Trade winds H The Walker circulation Darwin Chance of exceeding the median maximum temperature October to December 2007 Chance of exceeding the median rainfall October to December 2007 80% 75% 70% 65% 60% 55% 50% 45% 40% 35% 30% 25% 20% 80% 75% 70% 65% 60% 55% 50% 45% 40% 35% 30% 25% 20% 75% 75% 75% 75% 65% 65% 65% 55% 65% 65% 65% 65% 55% 55% 55% 50% 70% 70% 70% 60% 60% 60% 60% 60% 80% 80% 80% 80% For that area of WA within the thick line, October to December rainfall is commonly low and contributes only a small fraction of the annual total. SOURCE: THE NATIONAL CLIMATE CENTRE, AUSTRALIAN BUREAU OF METEOROLOGY Forecasting that predicts how much rain a passing cloud could bring is just one of the long-term benefits expected from a new weather radar. The Bureau of Meteorology's new research radar, at Redbank Plains near Ipswich in Queensland's south-east, will be online this summer. Called a CP2 radar, it will support research into storms affecting Brisbane, the Gold and Sunshine Coasts, Toowoomba and the eastern Darling Downs, by undertaking detailed storm measurements, includ- ing more accurate rainfall and hail measurements using Doppler, polarimetric and dual-frequency methods. Jim Davidson, regional director, Queensland, of the Bureau of Meteorology, says the research will be applicable nationwide. "The main benefits to growers will be in improved short-range forecasts of six to 12 hours," he says. "We will be able to provide more informa- tion on approaching storms or rain bands and how much rain or hail to expect." The new radar is Queensland's second. When combined with the Mt Stapylton radar, 30 kilometres away, its research and data capabilities will give unprecedented detail on the evolution and structure of storms in the Brisbane area. "It is the first time we have had the opportunity to study clouds using such a sophisticated Doppler radar network," Mr Davidson says. It will take about three to four years for results to lead to better model- ling and forecasting. In its first year, the facility will be used for validating satellite-based rainfall measurements undertaken by NASA as part of the Tropical Rainfall Measuring Mission. The data collected will also help with cloud-seeding experiments undertaken by the Queensland Department of Resources and Water. The CP2 radar was acquired by the bureau under an agreement with the University Corporation for Atmospheric Research, Earth Observing Laboratory in the US. -- REBECCA THYER Warm and wet for the west A warmer than average end to the year is predicted across Australia, although rainfall trends vary from east to west, with higher seasonal falls expected in western WA and a drier than average season more probable in the far south-east of the country (see maps). Both temperature and rainfall odds are a result of continuing higher than average temperatures over parts of the south tropical Pacific Ocean, and also in parts of the tropical and sub-tropical Indian Ocean. For western to central WA, the chances of exceeding the median rainfall are between 60 and 70 per cent. A small part of north- central Queensland also has a 60 per cent chance of a wetter than average season. In contrast, much of Tasmania and the far south-eastern coastal fringe of the mainland have a 30 to 40 per cent chance of exceeding the three-month median rainfall. This means that below-normal falls have a 60 to 70 per cent chance of occurring. Over most remaining parts of the country the chances of accumulating at least average rain for the December quarter are between 50 and 60 per cent. More information: Dr Scott Power, email@example.com The Walker circulation is a loop of winds over the Pacific Ocean that influences much of the region's climate.
Ground Cover 070 September-October 2007 - North
Ground Cover 072 January-February 2008 - North