Climate change

July 17, 2009

Climate update – ongoing decline in South-East Australian rainfall

Filed under: Climate Change, Global Warming, Rainfall Decline — Barry Brook @ 12:01 pm

searainfalldeclineSouth-East Australia is suffering under an extended drought of unprecedented character. Although local rain events have relieved the worst of the conditions in places like Adelaide, the region as a whole remains parched and tinder dry, and parts of the Murray-Darling river system, such as the spectacular Coorong lakes system, is perilously close to ecological collapse:

“The Murray-Darling Basin is experiencing the worst drought since records began in 1891. Record low inflows to the River Murray through drought and over-allocation are having a significant social, cultural, economic and environmental impact on the Lower Lakes and Coorong region. The unprecedented situation now facing South Australia is that the quality of the State’s water supply could be at risk because of increasing water salinity levels and acidification from exposed acid sulfate soils on the drying lakebeds and wetlands.”

The state of Victoria is experiencing a particularly steep decline in average autumn rainfall, with a 40 per cent decline recorded since 1950 (25 % in South-East Australia as a whole). CSIRO climate scientist Dr Wenju Cai and colleagues have reported that this seems to be connected to rising sea level atmopheric pressure and a a decrease in alternating high and low pressure systems — with connections to climate change.

Now a new paper has been written by Dr. Bertrand Timbal, of the Bureau of Meteorology’s Centre for Australian Weather and Climate Research (CAWCR), as part of the CAWCR Research Letters series, entitled “The continuing decline in South-East Australian rainfall — Update to May 2009“. It details the circumstances surrounding “the worst rainfall deficit in the region within more than a century long instrumental record.”

In brief, the mean annual (autumn) rainfall over the twelve-and-a-half year period Oct 96 — May 09 was 504 (100) mm, which is drier than extreme drought of Jan 35 — Aug 47, which averaged 512 (121) mm. For reference, the average over the entire 1900 — 2009 period is 567 (132) mm. Aside from the autumn decline noted above, a drying trend in spring is now emerging, with spring rainfall below the long term average 6 times during the last 7 years (2002-2008). There is a strong association of these conditions with the intensification of the sub-tropical ridge (STR), which may in turn be linked to the wetting trend and polewards expansion of tropical weather systems and Hadley circulation — a phenomenon predicted by global climate models to occur at a far slower rate than is being observed.

The conclusion to the Timbal 2009 paper sums up the bleak situation (some bolding highlighting and italic expansions by me):

The long-term rainfall deficiency since October 1996 across South Eastern Australia (SEA, south of 33.5ºS and east of 135.5ºE) documented by MT08 [Murphy, B. and B. Timbal, 2008: A review of recent climate variability and climate change in south-eastern Australia, Int J Climatol, 28(7), 859-879] was described as being severe but not unprecedented in the instrumental record. With an additional 3 years of below average rainfall, that statement is no longer true. The recent 12 year, 8 month period is the driest in the 110 years long record, surpassing the previous driest period during WWII [World War II]. The spatial extent of the deficiency covers most of the south-western part of eastern Australia and extends along significant orographic features eastward and northward. The seasonal signature of the rainfall decline has also evolved. It remains dominated by a strong and highly significant autumn rainfall decline, but has been supplemented by recent declines in spring, particularly after 2002. The spring decline is the dominant feature of the very dry 2006-2008 period.

This change in the relative contributions by the autumn and spring seasons now more closely resembles the picture provided by climate model simulations of future changes due to enhanced greenhouse gases. However, the growing magnitude of the rainfall decline is far more severe than any of the IPCC-AR4 model projections except for the lowest deciles from the model uncertainty range, forced with the highest emission scenarios occurring later in the 21st century (2050 to 2070) (CSIRO and Bureau of Meteorology, 2007).

The most important characteristics of the ongoing rainfall decline (spatial extension, intensification and change in seasonality) are well aligned with the recent evolution of the STR and its known influence on SEA rainfall. Other largescale influences were briefly evaluated. It appears unlikely that the ENSO [El Niño Southern Oscillation] mode of variability has contributed to the worsening of the rainfall decline in the last 3 years. On the contrary, it appears likely that the Indian Ocean mode of variability (with three positive IODs [Indian Ocean Dipole] in a row) may be linked to the strong spring signal in 2006-2008. However, that does not change the fact that the IOD is unlikely to be responsible for the largest component of the rainfall decline (the autumn part) and based on the limited evidence provided here, it is unclear whether the IOD is a contributor, or simply a covarying response to other factors. Finally, the long-term evolution of the SAM [Southern Annular Mode] remains unlikely to explain the longterm decline in SEA due to the seasonal nature of the influence of SAM on SEA rainfall but its role (both positive or negative) is visible while updating month by month anomalies.

You can download the full paper as a PDF here. It’s 8 pages long, and includes some highly informative statistics, analysis and revealing colour figures. I strongly suggest you take the time to read through it, if you wish to have a well-grounded scientific understanding of the worrying rainfall deficits that this part of the world is now experiencing.

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