Droughts and Floods in Australia

The Big Dry and other iconic droughts

At the turn of the millennium, a large region of Australia was gripped by the most severe drought in living memory, the “Big Dry.” The ramifications for affected regions were dire, with acute water shortages for rural and metropolitan areas, record agricultural losses, the drying-out of two of Australia’s major river systems and far-reaching ecosystem damage. Yet the drought’s origins had remained elusive.

For Southeast Australia, Ummenhofer et al. (2009) show that the “Big Dry” and other iconic 20th Century droughts are driven by tropical Indian Ocean variability, not by Pacific Ocean conditions as traditionally assumed.

In particular, a conspicuous absence of characteristic Indian Ocean temperature conditions that are conducive to enhanced tropical moisture transport has deprived southeastern Australia of its normal rainfall quota. In the case of the “Big Dry”, its unprecedented intensity is also related to recent above-average temperatures.

Pacific Ocean variability is dominated by the well-known El Niño-Southern Oscillation (ENSO) phenomenon. Similarly, the leading mode of tropical Indian Ocean variability, the Indian Ocean Dipole (IOD), is a coupled ocean-atmosphere mode characterized by sea surface temperature anomalies of opposite sign in the east and west of the basin with an associated large-scale atmospheric re-organization.

To assess the relative importance of the IOD and ENSO for Southeast Australian drought, all years for the period 1889 to 2006 are classified as to the state of the Indian and Pacific Ocean. The “Big Dry” is apparent post-1995, characterized by severe drought conditions. Since 1995, four La Niña events failed to break the drought. El Niño and positive IOD events are associated with drier conditions. However, neither pattern occurred more frequently than normal. In contrast, there has been a complete absence of negative IOD events throughout this latest drought period, with the last negative IOD occurring in 1992. Negative IOD events bring anomalously wet conditions to the Southeast and their absence deprives the region of its normal rainfall quota.

Ummenhofer et al. (2011) found a distinct asymmetry in the impacts of the opposite phases of both ENSO and IOD on Australian rainfall and soil moisture. There are significant differences between the dominant drivers of drought at interannual and decadal time scales. On interannual time scales, both ENSO and the IOD modify southeastern Australian soil moisture, with the driest (wettest) conditions over the southeast and more broadly over large parts of Australia occurring during years when an El Niño and a positive IOD event (La Niña and a negative IOD event) co-occur.

Lower-frequency variability over southeastern Australia, however, including multiyear drought periods, seems to be more robustly related to Indian Ocean temperatures than Pacific conditions. The frequencies of both positive and negative IOD events are significantly different during periods of prolonged drought compared to extended periods of ‘‘normal’’ rainfall. In contrast, the frequency of ENSO events remains largely unchanged during prolonged dry and wet periods.