Supplementary Information: Why was the 2003 drought so bad?

The 2002–03 drought was one of the worst on record. As with much of the ACT’s climate, it was driven partly by the combined effect of ocean surface temperatures and atmospheric pressures in the Pacific Ocean.

Called El Niño by many Australians, the effect is represented by an index based on the difference between atmospheric pressure at sea level, measured at the same time in Darwin and Tahiti. It partly ‘explains’ the drought, but the effect is actually strongest during spring when it explains between 18% and 25% of the ACT’s rainfall v ariability.

El Niño events (warm sea surface temperatures in Peru) are associated with low rainfall over northern and eastern Australia. The reverse is true during La Niña events. Either one can last for about a year.

The El Niño event was not particularly strongly developed during 2002–03. Even so, the drought across almost all of the eastern half of Australia, and in parts of Western Australia, was amongst the most severe (in duration, extent and intensity) since 1900. The 2002 crop growing season (from March to October) was in the 10% of driest on record for almost three quarters of the Australian continent, according to the World Meteorological Organization.

The severity may be partly due to the influence of another phenomenon, the Pacific Decadal Oscillation (PDO), which modulates the impact of El Niño and La Niña on Australia’s rainfall. The PDO involves large-scale fluctuations of ocean surface temperature and winds across the North Pacific Ocean.

A PDO phase (positive or negative values) lasts for approximately 20 years.

Research has shown that during periods when the PDO is positive, El Niño and La Niña impacts on Australian rainfall tend to be weaker. When the PDO is negative those impacts are likely to be stronger. In other words, the PDO accentuated the low rainfall influence of the recent El Niño.

The PDO was defined as positive from 1977 to 1998–99; it appears that it has since changed phase to negative.

Records show there were several El Niño events during the most recent positive PDO phase. The 1982–83 drought was a very strong event with strong impacts in eastern Australia. A moderate El Niño event in 1987–88 did not result in a severe drought. The 1991–95 El Niño lasted for some time, but did not have particularly strong impacts. Most recently, the 1997–98 El Niño was a strong event, but had only moderate impacts.

The moderate El Niño event of 2002–03 was the first to occur during the current negative phase of the PDO and, as we experienced, the impacts were amongst the most severe in a century. While the PDO remains negative, it is possible that another El Niño event could bring the same severe results.

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