Queensland floods driven by strongest La Nina since 1974

Record amounts of rainfall have inundated the northeastern Australian state of Queensland over the past four months, culminating in the most damaging floods in the state’s history.  September-November 2010 was the wettest spring on record in Queensland, with most of the central and eastern parts of the state receiving three or four times their normal amount of precipitation, or about 300 millimetres of additional rain.  Spring is not ordinarily a wet season in northern Australia, where most of the rain falls during the December-March monsoon.  The heavy, prolonged spring rains saturated the soil and filled rivers throughout the state, setting the stage for the floods in December and January once the monsoon season began.

Australian rainfall in September-November 2010, expressed as a percentage of normal (1961-1990 climatology). Image courtesy of the Australian Bureau of Meteorology.

December 2010 floods in central Queensland

The first severe floods occurred between Christmas and New Year’s Day in central Queensland.  Heavy rain fell throughout the week, but particularly on 27 December, when daily rainfall records fell throughout the Carnarvon range of hills to the west of Rockhampton (the purple area on the rainfall image below).  There were two synoptic factors behind this rainfall: the remnants of Tropical Cyclone Tasha, which made landfall near Cairns on Christmas Day before drifting south and west across the interior of Queensland; and a moist easterly, onshore flow across much of the state, driven by monsoonal low-pressure systems to the north and high pressure in the Tasman Sea between Australia and New Zealand.  The synoptic chart below shows this pressure pattern, which can occur in any season and frequently leads to heavy rains along the Queensland coast.  I experienced the effects of this synoptic pattern in April 2009 while in Brisbane, where 200 millimetres of rain fell in two days!

Accumulated rainfall on 27 December 2010, when 24-hour rainfall records fell in the Carnarvon range, to the west of Rockhampton. Image courtesy of the Australian Bureau of Meteorology.
Bureau of Meteorology synoptic chart for 28 December 2010 at 0Z

Falling on saturated ground, the water from these rains quickly ran off into the rivers, which burst their banks and flooded the towns of Rockhampton and Emerald, among others.  At one point, the flooded area of central Queensland was larger than the combined area of France and Germany!  While this part of Queensland is sparsely populated, it is a major agricultural (sugar cane and wheat) and cattle-grazing region.  Queensland also supplies more than half of the world’s coking coal; many mines were flooded, halting operations.  The floods have thus caused a spike in global food and energy prices.

December 2010 ended as the wettest December in Queensland on record.

Toowoomba and Brisbane floods of January 2011

Just as the floodwaters were beginning to recede near Rockhampton, flash flooding struck the town of Toowoomba, about an hour’s drive west of the state capital Brisbane.  Between 100 and 300 millimetres of rain fell in the Brisbane area on 10 January, again driven by moist, onshore northeasterly winds associated with a cut-off area of low pressure over the ocean.  Heavy rains had already pounded the region over the previous few days, with most stations reporting 50 millimetres or more per day.  As in the Rockhampton and Emerald floods, the soils were saturated and the rivers were at capacity, so there was simply nowhere else for the water to go.  Residents of Toowoomba described an “inland tsunami”, as water rushed down the hills and swept away houses, cars and, most sadly, a dozen people.

Accumulated rainfall for 10 January 2011, when flash flooding struck several towns near Brisbane
Bureau of Meteorology synoptic chart for 10 January 2011 at 0Z

In downtown Brisbane, the Brisbane River peaked at 4.4 metres on Thursday 13 January.  This was the highest level since 1974, when the River reached 5.4 metres.  This year’s floods were less severe because the waters were held back by the Wivenhoe Dam, which was constructed after the 1974 disaster.  Still, the financial cost of the 2011 floods is expected to be higher than for the 1974 event, because the Brisbane metropolitan area has grown considerably over the past four decades.  During the economic boom of the early 2000s, an average of 2000 people moved to southeastern Queensland each week!

The connection to La Nina

Both the record spring rains and the exceptionally strong start to the summer monsoon in Queensland can be connected to the current La Nina conditions.  La Nina refers to a pattern of abnormally cool equatorial Pacific Ocean temperatures; it is the opposite of the warm El Nino.  This year’s La Nina has been the strongest since 1974, when there were also severe floods in Queensland, particularly in Brisbane as noted above.  La Nina began in June and, as is normal, grew through the Northern Hemisphere summer and autumn months before peaking in early December.

Sea-surface temperature anomalies from mid-October 2010 through mid-January, showing La Nina in the central and eastern Pacific. Image courtesy of the National Oceanic and Atmospheric Administration.

La Nina intensifies rainfall in Australia by shifting the poles of the Walker Circulation, which is the east-west overturning circulation in the Pacific.  Typically, air rises over the warm waters of the central and western Pacific Ocean, causing convection and rainfall there.  To balance this rising air, air at the surface must rush in from both east and west.  In turn, this surface convergence causes air to move downward elsewhere, which suppresses rainfall; normally, “elsewhere” is the eastern Indian Ocean and the west coast of South America.  During La Nina, however, the cool Pacific Ocean temperatures stabilize the atmosphere there, reducing upward motion and rainfall.  Instead, air rises over the islands of the Maritime Continent and over Australia, where ocean temperatures are warmer and there is more moisture available.  Thus, the entire Walker Circulation shifts west, producing heavy rain over eastern and northern Australia, Indonesia, and the Phillippines; these countries have all seen flooding this year.  The opposite happens during El Nino, which typically produces drought in Queensland.

How La Nina and El Nino affect the Walker Circulation, the east-west overturning flow over the Pacific. Red (blue) hatching indicates warm (cold) ocean temperatures. Image courtesy Queensland Climate Change Centre of Excellence.

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