El Niosouthern Oscillation Effect On Australian Climate

Australian droughts generally accompany El Niño episodes (e.g., Allan, 1991). Figure 1 illustrates the relationship between widespread Australian drought and low values of the Southern Oscillation Index (the SOI, a simple measure of the ENSO, is the standardized difference in surface atmospheric pressure between Tahiti and Darwin), by comparing time series of the percentage of Australia with annual rainfall in the lowest decile with annual averages of the SOI. The figure also indicates that years with little of the country in drought tend to have large positive SOI values, ie., La Niña episodes.

Figure 1 only uses data from 1950, for clarity. The relationship between the SOI and drought, however, is evident in data throughout the twentieth century. Prior to the late nineteenth century there are insufficient data to allow a strict, quantitative comparison of widespread Australian droughts with the ENSO. Nicholls (1988) examined reports of the governors of the colony of New South Wales to the colonial secretary of the British Government in London for references to drought in the early

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

1950 1955 1960 1965 1970 1975 1980 1985 1990

Year

Figure 1 Annual mean SOI (full line) and the percentage of Australia with annual rainfall below the first decile, i.e., in drought, (broken line).

1950 1955 1960 1965 1970 1975 1980 1985 1990

Year

Figure 1 Annual mean SOI (full line) and the percentage of Australia with annual rainfall below the first decile, i.e., in drought, (broken line).

years of the colony and found that the coincidence of El Niño events and Australian droughts has existed from, at least, the start of European colonization in 1788.

The ENSO also enhances Australian rainfall variability, as it does wherever it impacts on climate (Nicholls et al., 1997). Also, many Australian droughts tend to last about a year because El Niño and La Niña episodes both tend to last about 12 months and this sets the time scale of Australian rainfall fluctuations (Nicholls, 1991). The link with the ENSO is most consistent with east and north Australian rainfall (e.g., Pittock, 1975; McBride and Nicholls, 1983; Ropelewski and Halpert, 1987, 1989).

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