ARTICLE – LA NIÑA AND THE FLOODS OF 2010–11

One of the strongest La Niña events on record occurred in the second half of 2010 and the early months of 2011. Sea surface temperatures in the central and eastern tropical Pacific were more than 2°C below normal in places, and the Southern Oscillation Index (SOI), an indicator of air pressure differences between Darwin and Tahiti, was above +20 for several months; only 1917 has previously seen such high values sustained over such a period of time.

If conditions in the Pacific Ocean were not favourable enough for Australian rainfall, elsewhere the environment was taking perhaps the most favourable alignment possible for heavy rain in Australia. The waters of the Indian Ocean south of Indonesia were warmer than normal, as were those off the north coast of the Australian continent, providing an ample source of moisture for rain-bearing systems.

Unusual heavy rains occurred in northern Australia during the cooler months of 2010 – normally the dry season in the tropics – but it was in September that widespread heavy rain really began. September 2010 was Australia’s wettest September on record, starting a sequence of seven months that were all much wetter than average. March 2011 was also the wettest March on record, December 2010 and February 2011 both ranked second, and January 2011 was the only one of the seven months to miss the top ten wettest months. It was also the wettest spring on record for Australia (and for Queensland, New South Wales and the Northern Territory), and the second wettest summer (with Victoria having its wettest). The seven-month period from September 2010 to March 2011 saw almost exactly double the normal rainfall for Australia, and fell just short of November 1973–May 1974 as the country’s wettest seven-month period on record (map S1.1). Every mainland state and territory ranked either first or second for this period, with most also experiencing about double their usual rainfall, except in South Australia where it was closer to three times normal. The only parts of the country that had below-normal rainfall for the period were the south-west of Western Australia (where a severe drought was in progress) and parts of coastal New South Wales between Sydney and Port Macquarie.

The heavy rains brought with them widespread and extensive flooding throughout northern, central and eastern Australia. These began with floods in northern Victoria and southern New South Wales in September and October, but it was from late November onwards that the most significant floods occurred. Probably the worst-hit areas were the Fitzroy and Brisbane River catchments in eastern Queensland, and the Loddon and Wimmera catchments in northern Victoria. However, almost every significant river system in inland Victoria, New South Wales and southern Queensland, as well as numerous coastal rivers in those states (and in Tasmania), saw substantial flooding at some stage between late November 2010 and early February 2011. Brisbane itself experienced its worst floods since 1974.

In February and March 2011, the focus of most flooding shifted to the tropics and central Australia, with parts of the Kimberley being especially hard hit. March also saw a number of exceptional localised flash floods. There were extensive floods in central Australia, particularly in the far west of Queensland, although Lake Eyre did not quite reach its levels of the previous year as one of its main feeders, the Cooper Creek, missed out on the most extreme flooding. Conditions over most of the continent returned to near normal from April 2011 onwards.

The heavy rains of 2010–11 largely ended the long-term drought that had affected large parts of eastern Australia for most of the preceding decade. Many water storages that had been far below capacity for several years, especially in the southern half of the Murray-Darling Basin, were full or overflowing by mid 2011. Murray-Darling basin storages rose from around 25% of capacity in February 2010 to near 80% by year’s end, and further to 86% by spring 2011.

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