A pressure gradient metric capturing planetary-scale influences on eastern Australian rainfall

The IPCC 4th assessment report summarises the uncertainty associated with projections for Australian summer rainfall by stating ‘more broadly, across the continent summer rainfall projections vary substantially from model to model, reducing confidence in their reliability'. Considerable attention is now being given to the possible impacts of anthropogenic climate change on eastern Australian rainfall following the observed abrupt decline in annual rainfall across much of central-eastern Australia since the year 2000, and further south (in the state of Victoria) since 1996.

The current protracted drought across eastern Australia has severely impacted agricultural productivity, depleted water storages to critical levels, and resulted in serious reductions in environmental river flows. Five year inflows to the vast Murray Darling Basin (MDB), which accounts for the majority of irrigated agricultural production in Australia, were the lowest on record (since 1891) during the period 2001 to 2006. The recent abrupt rainfall decline is contextualized by historical river observations which show abrupt shifts repeatedly occurring between multidecadal drought and flood dominated periods along the east Australian coast since records were first kept in the 1790's. However, multidecadal drought dominated periods are poorly understood with causes of the shifts between pluvial and drought phases remaining obscure.

Variability in summer easterly zonal wind flow and associated moisture advection from the warm waters of the Pacific Ocean and Tasman Sea are one possible mechanism producing observed regional decadal rainfall variability. To investigate this relationship a measure of the easterly geostrophic wind flow across eastern Australia was developed.

The Gayndah-Deniliquin index (GDI), a measure of the north-south atmospheric pressure gradient across eastern Australia, is presented. The 113 year long GDI record reveals strong interannual to decadal scale variability in zonal geostrophic wind flow across eastern Australia. The GDI, as a measure of easterly geostrophic wind strength and associated moisture transport from the Pacific Ocean, is shown to be significantly correlated with summer rainfall over vast areas of the Australian continent, especially over the Murray Darling Basin and the state of New South Wales.

The latest abrupt decline in the GDI, which commenced around 2001, corresponded with the onset of a severe prolonged drought across eastern Australia. We demonstrate that the northern and southern poles of the MSLP derived GDI are differentially influenced by El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Understanding the effects of these interactions between SAM and ENSO on moisture transport to eastern Australia could have important implications for future Australian climate variability and climate change.