The Figures Behind the Floods

2011 annual rainfall compared against historical rainfall records.

© Australian Bureau of Meteorology.

The past two years have been hard for Australia, with record floods following many years of drought across most of the country.  Last month the Australian Weather Bureau provided the statistics to go with the devastating stories:

The persistent rain of 2010 and 2011 was thanks to La Nina; a shift in ocean currents and temperatures that alters weather patterns across the Equatorial Pacific.  Usually, there’s warm water to the East of Indonesia and Australia, warming the air above, which rises to produce clouds and rain.  When La Nina takes hold, though, sea temperatures are much warmer than normal, creating more convection (rising warm air), and delivering much more rain. 

The effects of La Nina extend much further than the Pacific; the atmosphere’s global circulation is driven by convection at the equator, and changes here can affect the rest of the globe: jet streams shift; patterns of temperature and rainfall on other continents change; and the greatest impact is in the Northern Hemisphere, not the South.

2011 annual mean temperatures compared against historical

temperature records.  © Australian Bureau of Meteorology.

Back in the pacific, the damp conditions of La Nina are in contrast to those of its more famous sibling:  El Nino.  During El Nino years the warm pacific waters move west, sometimes stretching all the way to the west coast of South America, and the rain shifts with it.  But there’s another subtl difference: Unlike El Nino, which often lasts for around a year and then disappears, La Nina can come back.  That was the case in 2011.  The strong La Nina of 2010/2011 continued into the Southern Hemisphere’s autumn (March to May 2011), and was followed by another in the spring (September to November).  The result was one of the wettest years on record: an average of 699mm of rain in 2011.  The cloud and rain made 2011 cooler than the 30 year average¹ too - the only such year of the past 10.  There were other records too: Despite the overall cooling, the south of the country continued to host higher than average temperatures, and 2011 was the warmest year to start with La Nina.  Not that that’s much consolation if your farm’s still under water - those parts of Australia will be hoping for a dryer 2012.

1:  The thirty year average is calculated between 1961-1990.

A few more statistics from the Australian Weather Bureau’s AnnualAustralian Climate Statement 2011: 

Annual Average Rainfall:	699mm	234mm above the long term average, and the third wettest since records began in 1900.
Two Year (2010 & 2011) Average Rainfall Total:	1402mm	5mm less than the all-time two-year high, and the second wettest on record.
Average Mean Temperature:	21.67oC	0.14oC below the 30-year average (1961 – 1990) .
Autumn Mean Temperature:	1.15oC below average.
Maximum Recorded Temperature:	48.1oC	An all-time record for central South Australia (measured in Woomera).  (Several other locations measured close to 50oC in late December).
Warmest year on record for the south-west of Western Australia.	1.16oC above average.
Sea Surface Temperature Anomalies:	+0.39oC - January +0.43oC - November	Second and third warmest Sea Surface Temperature Anomalies on record.

Statistics and graphs are the copyright of the Australian Government's Bureau of Meteorology. Presented with permission of the Bureau under the provision for Secondary Distribution of Bureau of Meteorology Information.

Copyright on the remainder of the text: Christopher Lee, 2012.