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Monday, February 13, 2012

Drying trend in Australia still evident despite wettest two year period on record

Back-to-back La Niña events has produced the wettest two year period on record for Australia according to the Bureau of Meteorology (BOM). The two year rainfall total for 2010-2011 of 1409 mm, eclipsed the old record of 1407 mm set during the big wet of 1973-1974. But underlying this wet record is a strong drying trend in the southeast and southwest of the continent with a consistent reduction in Autumn and winter rainfall and streamflow which are important for both agriculture and water storage.

"Well above-average rainfall fell in November and December and was sufficient to push the Australian 2011 total to the second-highest value on record, with the state of Western Australia recording its highest annual rainfall on record." said the BOM statement.

The La Niña event in 2010 was one of the strongest on record, which was accompanied by elevated sea surface temperatures (SST) to the north of Australia and in the eastern Indian Ocean. Over the southern winter in 2011 La Niña declined but emerged again as a weaker event for the 2011 summer, with elevated SSTs. Assessments by the BOM and NOAA show La Niña conditions weakening through March into neutral conditions.

The big wet, commencing in December 2009, ended the driest 11-year, 10-year, 9-year, 8-year, and 7-year periods on record including a record sequence of dry years in parts of southern and eastern Australia, especially the southeast of Australia focussed on Victoria.

Heavy Summer rains and flooding brought by La Niña

Many state and continent rainfall records were broken including September 2010 being Australia's wettest in 111 years of rainfall records. NOAA later determined that 2010 was the wettest year globally and for Queensland. By January 2011 the big wet was responsible for disastrous flooding in Queensland and Victoria, which culminated in large areas of Brisbane being flooded and one third of Queensland declared a disaster area.

Rainfall deciles for the period 1st January 2010 to 31st December 2011. Source: Bureau of Meteorology Climate statement 38

The Rainfall record over the 2010/2011 summer includes:

  • September 2010 - wettest September on record for Australia, Queensland, NT
  • October 2010 - third wettest October on record for Australia, wettest for NT
  • Spring 2010 - wettest on record for Australia, Queensland, NSW, and the NT and the second-wettest for SA.
  • December 2010 - second wettest on record for Australia, wettest for Queensland. SOI highest on record for this month.
  • January 2011 - Victoria had its wettest January on record.
  • February 2011 - second-wettest February on record and SA and WA their wettest and second-wettest, respectively.
  • Summer 2011 - Australia and WA recorded second wettest summer on record. Victoria recorded its wettest summer and SA its third-wettest.
  • March 2011 - highest-on-record rainfall for the month in Australia, Queensland and the NT.

By autumn the La Niña event faded and rainfall returned to below average levels over the southern winter for Australia. According to BOM this continues a trend of drier than normal late autumn - early winters across this region.

La Niña returned, although weaker than the previous summer, bringing above average rain in October and November 2011.

Strong drying trend continues for Margaret River and the south west Western Australia

The south west of the continent is still facing a bleak climate future with a strong drying trend continuing. 2010 was the driest year on record for the region (an annual total of 395 mm, with the next-driest 1940 with 439 mm), despite the strong La Niña. During 2011 the region received only near-average rainfall. This is not sufficient to offset the extreme dry conditions that have affected this region for some decades, according to BOM.

Small areas of southwest Western Australia inland from Margaret River have seen the driest two-year period on record. For the southwest corner of Western Australia as a whole, the two-year period 2010-2011 was the fourth-driest two-year period on record, with the partially overlapping 2009-2010 period being the driest on record.

Autumn and Winter rains continue long term decline

While la Niña has brought substantial flooding to large parts of Australia during summer months, an important long term trend that has continued even through the Big Wet is the decline in autumn and winter rainfall. Winter rains have historically been important for agriculture and filling dams and water storages.

Perhaps the most statistically significant rainfall changes that have occurred are a 10-20% reduction in autumn and winter rainfall across the southeast since 1996, and a similar decline in winter rainfall across the southwest since around 1970. The April to November period is the main rainfall and runoff season for both these parts of Australia, with less rainfall falling in summer.

Rainfall deciles for the 15 year period 1st January 1997 to 31st December 2011. Source: Bureau of Meteorology Climate statement 38

So why is there less winter rainfall now across large parts of the continent? In July 2011 CSIRO climate scientist Dr Jorgen Frederiksen addressed the International Union of Geodesy and Geophysics conference in Melbourne on the long term trend for less rain across southern Australia. He said that increasing atmospheric carbon dioxide concentrations are increasing atmospheric temperatures changing the dynamic of the sub-tropical jet stream and thus the number and intensity of storms bringing autumn and winter rainfall to southern Australia.

Although the summer rains and flooding has increased water storage and ended the decade long drought in many areas, the long term trend still shows large areas of southeastern Australia and southwestern Australia at below-average and very-much-below-average rainfall according to BOM 15 year and 10 year rainfall decile mapping. This includes record-low 15-year totals for the coast in southwest WA, western Tasmania and parts of Victoria including near the catchments for Melbourne.

According to BOM "the recovery peaked in autumn 2011, with a return to deficits from that time on. In other words, the accumulated below-normal rainfall during the 'Big Dry' remains substantially greater than the extra spring and summer rainfall that has fallen during the past two years."

The CSIRO and Bureau of Meteorology have warned for several years that overall Australia is to get hotter, wetter, with more extreme weather and that south eastern and south western Australian regions are becoming drier.

Even in our wettest years on record, the long term drying trends continues. The Australian Bureau of Meteorology and the Climate Prediction Center of NOAA both say that Climate models are indicating a gradual decline in the strength of the La Niña over the coming months, with most models suggesting a return to neutral conditions during the southern autumn.

Next El Niño likely to step up the heat

According to the World Meteorological Organisation 2011 hottest year globally with a La Niña event, 10th warmest on record. The sun is just coming out of a period of low solar radiation, which, along with the effect of La Nina, has acted to dampen the rate of anthropogenic global warming. Even a major drop in solar activity was assessed by scientists at the Potsdam Institute for Climate Impact Research as having little effect on global warming.

The last strong El Niño year occurred in 1998 which was the hottest year of the 20th century. According to the NASA GISS temperature analysis the warmest years on record so far were 2005 and 2010, in a virtual tie.

With the next El Niño we are likely to experience another jump in the level of temperatures. "It's always dangerous to make predictions about El Niño, but it's safe to say we'll see one in the next three years," said NASA climatologist James Hansen, "It won't take a very strong El Niño to push temperatures above 2010."

BEST land-only surface temperature data (green) with linear trends applied to the timeframes 1973 to 1980, 1980 to 1988, 1988 to 1995, 1995 to 2001, 1998 to 2005, 2002 to 2010 (blue), and 1973 to 2010 (red). Source: Graph from Skeptical Science. (Free to reuse)


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