Clarification of results of GRL paper

Ummenhofer et al. (2009) What causes Southeast Australia's worst droughts? Geophysical Research Letters, 36, L04706, doi:10.1029/2008GL036801

The implications of our work (Ummenhofer et al. 2009) have been misunderstood in some media commentary, with some reporters asserting we have discovered that south-eastern Australia’s recent “Big Dry” is not related to climate change. This is not correct.

Our research investigates iconic droughts of the 20th Century for south-eastern Australia, including the Federation Drought (1895-1902) and World War II Drought (1937-1945), as well as the recent “Big Dry”. The research shifts the focus from Pacific Ocean to Indian Ocean variability, in particular the absence of negative Indian Ocean Dipole (IOD) events. Specifically, our results show that extended multi-year droughts in the Victorian region are associated with a lack of negative IOD events (see Figure 1, reproduced from Ummenhofer et al. 2009).

Negative IOD events consistently result in very wet conditions in the south-east. The prolonged absence of a negative IOD deprives the region of its normal rainfall quota. Without the occurrence of these wet negative IOD years, the south-east only experiences dry or average years for a decade or more. This is conducive to drought.

When taken in the context of other historic droughts over the past 120 years, the “Big Dry” is still exceptional in its severity. The last negative IOD event occurred in 1992. This is the longest period on record over the past 120 years without a single negative IOD event. Furthermore, the severity of the “Big Dry” has been exacerbated by recent warmer air temperatures over the past few decades (Figure 1). Warmer air temperatures lead to increased evaporation, which further reduces soil moisture and worsens the drought. While this work does not explicitly focus on the link between changing IOD frequency and recent regional and global warming, it does send a stark message: in a warmer world, the severity of droughts would likely become far worse.

In short, our paper does not discount climate change as playing a role in this most recent drought, the “Big Dry”. In fact, there are indications that climate change has worsened this recent drought. Refer to Abram et al. (2008) and Cai et al. (2009) and Nicholls (2004) who investigate changes in IOD frequency and south-eastern Australian drought due to climate change.

Fig. 1: Historical record of IOD and ENSO years and mean climatic conditions over southeast Australia. (a) Years of positive/negative IOD (red) and El Niño/La Niña (blue) years. Timeseries of anomalous (b) precipitation (mm month-1), with 5-year running mean superimposed in red (c) 5-yrrunning mean of temperature (°C), with a 15-year running mean superimposed in green, and (d) 5-yr running mean of Palmer Drought Severity Index (PDSI) over southeastern Australia during June-October. The grey shaded bars highlight periods of below average precipitation when the 5-yearrunning mean falls below one standard deviation. The duration of three major droughts has been indicated in (d) with horizontal black bars. (Taken from Ummenhofer et al. 2009).

References
Abram, N. et al. (2008) Recent intensification of tropical climate variability in the Indian Ocean, Nature Geoscience, 1, 849, doi:10.1038/ngeo357.

Cai, W. et al. (2009), Recent unprecedented skewness towards positive Indian Ocean Dipole occurrences and their impact on Australian rainfall, Geophys. Res. Lett., doi:10.1029/2009GL037604, in press.

Nicholls, N. (2004) The changing nature of Australian droughts, Clim. Change, 63, 323-336.

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