Queensland flooding - Jan 2011

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Dr Nick Klingaman, a Walker Institute researcher funded by the Queensland government discusses the recent Queensland flooding.

Summary

Record rains across eastern Queensland have fallen on ground already saturated by the wettest spring (September through November) on record in the state, causing levels of flooding not seen in nearly 100 years.  Cool Pacific Ocean temperatures, called La Nina, have intensified the monsoon rains.  La Nina is at its strongest since 1973-1974, when Queensland also experienced severe flooding.  The current weather pattern, with easterly winds along the Queensland coast bringing humid oceanic air onshore, has persisted since late December and is conducive to further heavy coastal rains.  With La Nina expected to continue through the end of the Australian monsoon season in March, the current seasonal forecasts predict above-normal rainfall for southern Queensland and northern New South Wales for the next several months.

While we expect intense rain events to become more frequent in a warmer world, it is impossible to blame any single event wholly on climate change. Over the last 100 years in Queensland, decades of drought have alternated with decades of flood. Much of Queensland experienced below-normal rainfall during 1998-2007, a pattern which has since reversed to bring several years of wet conditions, culminating in the recent floods. These decadal variations are natural, resulting from long-term fluctuations in local weather patterns and Pacific Ocean temperatures. It is unclear whether climate change will exaggerate these fluctuations in Queensland, but the Walker Institute is working with the Queensland Government to understand the causes of variations in rainfall and improve predictions for the next several decades.

This animation shows daily rainfall totals over Australia. Particularly heavy rain fell on the 27th December. You can also see the heavy rainfall from 10th to 12th January around Brisbane.

Recent flooding in eastern Queensland

The recent flooding apears to have resulted from a combination of factors:

• several years of plentiful rains
• an anomalously wet spring (Sept - November) driven by cool La Nina conditions in the tropical Pacific
• remnants of Tropical Cyclone Tasha, which made landfall near Cairns on Christmas Day
• a weather pattern that produced enhanced onshore winds and moisture transports

Heavy December rains followed wet September to November

On 27 December, several rainfall stations southwest of Rockhampton received their highest daily rainfall on record: Carnavon (273.6 mm), Wharton Creek (257.0 mm), and Wyseby (247.2 mm).  The data records at these stations extend back to 1924, 1946 and 1948, respectively.

The remnants of Tropical Cyclone Tasha, which made landfall near Cairns on Christmas Day and subsequently weakened, brought moisture inland, which combined with a trough moving northeast across Queensland to produce the heavy rainfall. There were other, weaker areas of low pressure off-shore that brought moist air over Queensland.

The area around Rockhampton and Emerald received rainfall of 200-500 millimetres above normal in September-November, before the heavy December rains even started.  

Eastern areas of Queensland were anomalously wet throughout southern spring (September, October, November). The state as a whole experienced its wettest September on record.  

Following on from several normal and above-normal years since the early 2000s drought, it is likely that the soils were saturated and the rivers were running at normal to above-normal prior to the heavy rains.

Cool La Nina conditions in tropical Pacific linked to Queensland flooding

Wet conditions over Queensland, like those seen over the last few months, are often linked to cool sea surface temperatures over the tropical Pacific (known as La Nina). The tropical Pacific moved into strong La Nina conditions over June/July 2010.

The heavy rainfall In December was linked to low pressure and stronger onshore winds.

Rainfall anomalies (difference from long term mean) for September, October, November 2010.