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Westerly influence determines southern rainfall?

Lindsay Smail Traditional weather forecasters and farmers have looked to the El Nino

For Kondinin Group Southern Oscillation as the main driver behind rainfall and drought. However a shift in thinking has revealed that Australian farmers are perhaps more at

Photo: Kondinin Group Kondinin Photo: mercy of the than any other weather pattern.

From the west: A south-easterly moving cloud The known major suspects and culprits models for predicting rainfall across the band generated by the Indian Ocean can often that bring about changes to ’s south-eastern states. Ian saw the existing produce rain in the cropping and grazing areas of weather and climate conditions include predictive models used by the Bureau of southern Australia. the El Nino Southern Oscillation (ENSO) Meteorology (the Bureau) relied too heavily in the eastern states, the Madden-Julian on the then recently discovered ENSO Oscillation (MJO) — mainly in northern effect, purported to bring drought to eastern areas, the Antarctic Oscillation (AAO) in the Australia in an El Nino and rainfall during south and the Indian Ocean Dipole (IOD) an La Nina. across the centre and south-east. The problem was that droughts in most For more 20 years it was suspected that the of SA and Victoria were only sometimes At a glance temperature fluctuations of the Indian Ocean associated with El Nino, and La Nina only to Australia’s west were responsible for the sometimes brought floods. For example El Nino Southern Oscillation development of the well-known north-west there is only a 60 per cent correlation (ENSO) in the eastern states, cloud bands. between El Nino and drought in southern the Madden-Julian Oscillation Victoria. Many El Nino years had average (MJO) — mainly in northern areas, These bands spread across rainfalls. And so forecasters believed the Antarctic Oscillation (AAO) in there had to be other factors involved the south and the Indian Ocean the continent mainly in rainfall variations across the south- Dipole (IOD) across the centre and during the cooler months eastern states. south-east all influence Australia’s One of these other factors is the Antarctic weather and climate. and result in growing Oscillation, which will be described in an rains. upcoming edition of Farming Ahead, and Some weather forecasters claim the other is what has become known as the Australia relies too heavily on Especially since the advent of satellite Indian Ocean Dipole (IOD). the influence of ENSO effect, images, with which the cloud bands could The Indian Ocean Dipole purported to bring drought to actually be seen forming, developing and eastern Australia in an El Nino and moving south-eastwards over Western Ian’s web site explains the of the rainfall during an La Nina. Australia, South Australia, Victoria and IOD: New South Wales, climate researchers “The IOD encompasses some of the During recent years the effect of have been working on ways to link the warmest ocean areas in the world, which the IOD on rainfall in southern strength of the Indian Ocean effect to evaporate large quantities of water into the Australia has been more closely seasonal rainfall. surrounding lower and middle atmosphere. examined and researchers now A private long-range forecaster, Ian This water vapour is a major source of claim it has more impact on Holton of Adelaide, had suspected the link potential heat energy ready to fuel the rainfall in southern Australia for a long time and several years ago southern Australian weather systems. than the ENSO. incorporated statistical data from the “The large area of warm and moist air Indian Ocean into his highly successful forms during the summer months. As

28 Farming Ahead November 2009 No. 214 www.farmingahead.com.au Climate and sustainability Weather forecasting

south-eastwards, ahead of short-wave frontal This generates winds that pick up Figure 1 Negative phase features. This south-eastward moving cloud moisture from the ocean and then sweep band (called a north-west cloud band) will down towards southern Australia to deliver

Cool waters Warm waters rise ahead of the front. The cloud band will wet conditions. In Cool waters c then thicken and often produce good rains re In its positive phase, the pattern of ocean a Timor Sea D s e Timor Sea e in the croppingc and grazing areas of d re temperatures is reversed, weakening the w Pacific Ocean a Pacific Ocean a southern Australia.”se r d winds and reducing the amount of moisture m Warm w a m rm o watersIan goes on to explain that the generation picked up and transported across Australia. i m st oi a of these cloud bands stis dependent on sea- So the south-east misses out on its usual Indian Ocean ir air surfaceIndian Ocean temperatures and gradients, and to quota of rain.” prepare forecasting models he uses The study notes that the IOD has mostly Decreased measurements over the Indian Ocean and Increased rainfall rainfall Decreased rainfall been in its positive or neutral phase since northern and western Australia as input 1992 (see Figures 1 and 2), the longest Cool Indian Ocean water drives moist warm air variables. period of its kind since records began during and brings average rainfall His pioneering research has only recently the late 19th Century. Source: UNSW Faculty of Science been verified and extended by other Dr Ummenhofer explained: climatologists. Furthering IOD research “An IOD event usually Figure 2 Positive phase A team of Australian scientists has starts about May or June, detailed for the first time how the IOD — a peaks between August and Cool waters Warm waters variable and irregular cycle of warming and In Cool waters c cooling of ocean water, and made visible by re October and then rapidly a Timor Sea D s e Timor Sea north-west cloud bands — determines e c d re w Pacific Ocean a Pacific Ocean decays.” a se whether moisture-bearing winds are carried r d m Warm w a across southern Australia. m rm o waters i m “The ramifications of drought for this st ois The new study explains the recent long a t a region are dire, with acute water shortages Indian Ocean ir ir drought in south-eastern Australia and Indian Ocean for rural and metropolitan areas, record further shows why a series of La Nina events agricultural losses, the drying-out of two of Decreased in the Pacific Ocean — which usually bring Increased rainfall rainfall Decreased rainfall Australia’s major river systems and far- rain — had failed to break it. reaching ecosystem damage”. Warm Indian Ocean water leads to weaker, Dr Caroline Ummenhofer and Professor drier winds and less rainfall Their study confirmed that the state of Matthew England of the University of NSW the Indian Ocean is highly important for Source: UNSW Faculty of Science Climate Change Research Centre, who led rainfall and, subsequently droughts in the study, also revealed that the IOD was the south-east Australia. More important cause of other extreme droughts in than the variability associated with the Australia’s history, notably the World War II El Nino/La Nina cycle in the Pacific Ocean, Drought of 1937–1945 and the Federation autumn approaches, (the) westerly wind- the Indian Ocean Dipole is the key factor Drought of 1895–1902. belt moves north-ward and starts to react for driving major south-east Australian They explained the influence of the IOD with the (Indian Ocean) moisture to the droughts during the past 120 years on southern Australia: (See Figure 3). west of the Australian continent. “When the IOD is in its negative phase, a Better times ahead “If a seasonal long wave trough is just to pattern occurs with cool Indian Ocean water the west of WA, the north-westerly wind west of Australia and warm Timor Sea water But on the bright side Ian believes that ahead of the trough will feed this moisture to the north. more strongly negative IOD figures leading to increased rainfall may be just around the corner. His research shows the strong Figure 3 The phases of Indian Ocean Dipole from 1880–2000 likelihood of a return to a wetter growing season rainfall cycle for Australia’s south- east in the next few years. 1880 1900 1920 1940 1960 1980 2000 He qualifies this conclusion by stating: “These forecasts do not mean that every -1OD More year will be wet in a wetter forecast cycle, or rain that every year will be drier in a dry forecast cycle — just that the majority of years in a +1OD wetter cycle will have average to above Less average growing season rainfall in SA, VIC rain and NSW. Each individual year will be moderated by 20 More the Dipole-Nino and other ocean effects for rain that particular year.” 0 ABOUT THE AUTHOR Lindsay Smail is the Millimetres -20 Less Federation World War II Big Dry rain director of Geelong Weather Services, a private drought drought operational meteorology and consultancy -40 business. Contact Lindsay Smail *Average rain declines in positive phase. Grey bars indicate drought. Bottom graph shows rainfall anomalies (03) 5241 5332 [email protected] Source: UNSW Faculty of Science www.geelongweatherservices.com

30 Farming Ahead November 2009 No. 214 www.farmingahead.com.au