Water Journal : Water Journal March 2011
opinion regular features 18 MARCH 2011 water are important sources of water for people and livestock, especially during droughts. The current flooding in eastern Australia will flush out salt accumulations following many years of drought and will stem the oxidation of wetland sediments to acid that has afflicted parts of the Murray- Darling Basin. Many pastoralists in inland Australia depend on beneficial floods to grow pasture for livestock, and iconic floodplain forests -- such as red gum, black box and Coolibah -- are benefitting. Further, the strong flows into estuaries and coastal waters will increase fish breeding and fish stocks. Wetland biodiversity will also benefit, despite some downsides, including discharge of sediment onto coral reefs. Important Lessons to be Learned In rebuilding after the floods in eastern Australia we have an important opportunity to learn the lessons from damage inflicted and adopt a broader range of options for better managing flood risk. We should focus on the 'soft-path' flood risk reduction options, which are cheaper and more flexible than dams and provide multiple benefits in terms of water, fish, timber, sand and gravel, agricultural production, recreation and nature conservation. Dr Andrew B Watkins, National Climate Centre, Australian Bureau of Meteorology Dr Andrew Watkins earned his PhD from the University of Melbourne, studying Antarctic sea ice and its interactions with high latitude weather and climate. This involved two trips south to experience the ice first hand. Subsequently he joined the Co-Operative Research Centre for Southern Hemisphere Meteorology, under Professor David Karoly, examining ENSO and ways to improve model outlooks. In 1999 he joined the Bureau of Meteorology's National Climate Centre, and became Head of Climate Prediction in 2010. Why Has This Year Been So Different? One of the main drivers of our Australian climate is the El Niño -- Southern Oscillation (ENSO). Most people know it by its two extremes: El Niño and La Niña. ENSO has been driving Australia's climate for many thousands of years, but only really became known to science when British mathematician and meteorologist Sir Gilbert Walker took an interest in why the Indian monsoon failed in 1899, leading to mass starvation and ruin. He proposed a circulation pattern that oscillated between the Indian and Pacific Oceans, bringing changes to rainfall and temperature over India and, indeed, tropical Australia. We now know that he was generally correct, with the Southern Oscillation being driven by swings in temperature of the Pacific Ocean. Central and eastern parts of the Pacific are generally cool in a La Niña phase and warm in an El Niño event. This shifting pattern of temperatures in turn drives the shifting pattern in convection and, hence, causes a change in where air rises and falls. We now term this atmospheric pattern the Walker Circulation. In La Niña events there is an enhancement of the normal pattern -- warmer waters in the western Pacific and cooler waters in the east, meaning more convection and cloudiness in Australian longitudes, with stronger onshore trade winds. In El Niño events, however, the pattern starts to break down as the central and eastern Pacific warms, and the Walker Circulation breaks into smaller cells. This means there is no longer as much moisture drawn over the Australian continent, and indeed we can have dry descending air dominating the continent. Clusters of Events Typically, ENSO oscillates on a three-to-seven-year cycle. While there have been a roughly equal number of El Niño and La Niña events over the past century, there have also been periods when there has been more of one phase than the other. For instance, the 1940s and 1990s had three El Niño events, while the 1930s had none. Likewise, the 1970s had a number of La Niña events. It is still unclear exactly why these events can sometimes cluster, athough it may have something to do with persistence of temperatures in the slow-moving deeper oceans. When it comes to climate change, we know that the central Pacific has warmed by about half a degree, meaning that technically El Niño events are more likely (if they are defined by ocean temperatures); however, modelling suggests that the oscillation between the two patterns may not change in frequency. There is still a lot more work to be done on this issue, but the projections for Australian rainfall remain fairly clear; an increase in aridity, particularly in winter and spring, but with more extreme rainfall events when rain does come. Causes and Effects As for the 2010--11 La Niña event, it arose out of the ashes of the 2009--10 El Niño. Flipping straight from an El Niño to a La Niña does occur about 40 per cent of the time, so this was not all that unusual. What was unusual was the strength of the event. The Southern Oscillation Index (a measure of the strength of the Walker Circulation) for the August to December period was +21.1, more than two standard deviations from the mean, and second only to 1917--18 (+24.6). Central Pacific Ocean temperatures were not as comparatively strong, ranking fifth, with the coolest being 1955; however, it is known that the central Pacific has warmed considerably since 1955. What is clear, however, is that the 2010--11 La Niña has been one of the strongest La Niña events since 1900, and has had significant impacts upon rainfall not only in Australia, but also in a number of other countries, with substantial flooding and loss of life in places as far afield as Sri Lanka and north-east Brazil. While I am sure Sir Gilbert would have been saddened by these impacts, just as he was with the 1899 Indian famine, he would be heartened to see that his reasoning has stood the test of time, ultimately enabling a degree of planning which has the potential to save countless lives and livelihoods. HAVE YOUR SAY If you'd like to comment on the dams debate, or air your views on any of topics covered in this issue, please write to: Letters to the Editor, Water Journal, AWA, PO Box 222, St Leonards, NSW 1590 or email firstname.lastname@example.org. Letters will published in the next available edition.
Water Journal April 2011