Decrease in southeastern Australian water availability linked to ongoing Hadley cell expansion

Abstract

Southeastern Australia experienced the worst drought of the instrumental record from 1997 to 2009, which was broken by Australia's wettest 2 year period on record (2010/2011). This drought was primarily a cool season (April to October) phenomenon. In contrast, the breaking of the drought was a warm season (November to March) phenomenon. This reduction in winter rainfall along with an absence of very wet months led to a greater reduction in streamflow across the region than would be anticipated based on the 12% reduction in mean annual rainfall alone. The results presented in this article have linked the extent, duration, and severity of this drought to the ongoing observed expansion of the Southern Hemisphere Hadley cell at a rate of 0.5 ∘ /decade. This expansion has intensified the subtropical ridge over southern Australia, pushing cool season midlatitude storm tracks further south, leading to a reduction in winter rainfall and runoff across the region. Changes in Climate Observed Changes in ClimateThe mean annual rainfall of 512 mm for southeastern Australia (defined by the black rectangle in Figure 1) over the period 1997-2009 (the Millennium drought) was 12% below the 1900-2010 long-term mean annual rainfall. The Millennium drought in southeastern Australia occurred at a time when the Australian continent as a whole was experiencing above-average rainfall, which was primarily due to high summer rainfall over the northwest of the continent (Figure 1). In contrast, the two major previous droughts across southeastern Australia, the Federation (1896-1905) and World War 2 (1936-1945 droughts, occurred when at least the eastern half of the continent was in drought [Verdon-Kidd and Kiem, 2009a]. Seasonally, the Millennium drought was far more pronounced in autumn, whereas the World War 2 drought was associated with rainfall deficits in all seasons and the Federation drought showed a larger reduction in rainfall in summer and spring [Verdon-Kidd and Kiem, 2009a;Timbal et al., 2010]. In addition to the large reductions in cool season rainfall, the Millennium drought was also remarkable for its lack of very wet months. Figure 2 illustrates this, where none of the 180 consecutive months of the Millennium drought recorded a rainfall above the 90th percentile.The breaking of the Millennium drought was primarily a warm season phenomenon with 10 of the 11 very wet months across southeastern Australia in 2010-2012 occurring during the warm season as seen in Figure 2. Several factors contributed to these record-breaking rainfalls across the southeast of the continent [Timbal and Fawcett, 2013]. These included:(i) the strongest La Niña event of the past 50 years, (ii) a strongly negative Indian Ocean Dipole, (iii) the warmest sea-surface temperatures on record to the northeast and northwest of the Australian continent, and (iv) the largest positive value of the Southern Annular Mode since 1960.A return to Pacific Decadal Oscillation negative values may also have contributed to the record wet conditions; however, the ...

Preliminary December–January inflow and streamflow reconstructions from tree rings for western Tasmania, southeastern Australia

“…Large‐scale ocean‐atmosphere processes impact on seasonal water availability in the SEA region generally, and also Tasmania specifically [ Cai and Cowan , ; Risbey et al ., ; Pook et al ., ; Ummenhofer et al ., ; Timbal and Drosdowsky , ; Post et al ., , among many others]. Typical seasonal and regional signatures of these processes vary, as described by Risbey et al .…”

“…For western and southwestern Tasmania, Risbey et al [2009] single out SAM as the dominant process affecting precipitation in the summer months. The intensification of the subtropical ridge (STR) has also been linked to recent declines in precipitation over SEA generally [Timbal and Drosdowsky, 2012;Post et al, 2014].…”

“…Over the past 20 years in particular, this decline has negatively impacted urban water supplies, agricultural food production, river discharge required to sustain the aquatic environment, and capacity to generate hydroelectricity. To make matters worse, various modeling studies indicate that drying of southern southeastern Australia is likely to increase over the course of the 21st century [CSIRO, 2008;Timbal and Jones, 2008;Viney et al, 2009;Chiew et al, 2011Chiew et al, , 2014Post et al, 2014]. Predictions also suggest that the seasonal distribution of precipitation across the region is expected to change Chiew et al, 2011].…”

Climate change and water‐related ecosystem services: impacts of drought in california, usa

“…Decrease in water availability in the last decade is associated with the expansion of the Hadley cell, which dampens winter precipitation in southern Australia (Post et al. ). In southwest Western Australia, a 13.6% decline in mean annual rainfall is projected to result in 36% decline in annual streamflow in the mid‐21st century under the A2 emission scenario.…”

“…In southern Australia, climate change will reduce not only streamflow but also recharge, although specific impacts of climate change at a local scale will vary depending on topographic and soil conditions (Dawes et al 2012). Decrease in water availability in the last decade is associated with the expansion of the Hadley cell, which dampens winter precipitation in southern Australia (Post et al 2014). In southwest Western Australia, a 13.6% decline in mean annual rainfall is projected to result in 36% decline in annual streamflow in the mid-21st century under the A2 emission scenario.…”