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Climate Assessment Table of Contents

Regional Climate Highlights - Australia

1) Northern and Northeastern Australia

Northern Australia (indicated by the boxed area in Fig. 62a) experiences a tropical climate with a well-defined rainy season that typically begins during October and ends in April (Fig. 62c). Much of the area receives more than 75% of its mean annual rainfall during this 7-month period, with portions of extreme northern Australia recording more than 90%. This annual cycle is modulated by the Southern Oscillation, with below- (above-) average rainfall often observed during Pacific warm (cold) episodes (Ropelewski and Halpert 1987, 1989).

Overall, the October 1998–April 1999 rainy season featured above-average rainfall across the northern half of northern Australia (Fig. 62a), with record totals observed in portions of northwestern Australia. In each of these regions, totals averaged more than 200 mm above-normal during the season, with the locally extreme totals in some areas leading to significant flooding. In northern Australia rainfall was above-average in every month of the season except January (Fig. 62c, green bars), with particularly excessive totals observed in November–December 1998 and in March–April 1999. The increased rainfall in December, March, and April was partly associated with a series of five tropical cyclones (TC), which included TC Thelma in December, TC Rona in February, TC Vance in March, TC Elaine in March, and TC Gwenda in April. These systems, with the exception of TC Rona, also produced copious rainfall totals over portions of western Australia.

At Darwin, Australia, indicated by the yellow dot in Fig. 62a, seasonal rainfall totals reached 2250 mm (Fig. 63a), which approached the record total of 2499 mm observed during the previous record 1997/98 wet season (Bell et al., 1999). The largest rainfall event during the 1998/99 season at this station was a 2-day total of 420 mm, in association with Category-5 TC Thelma in December 1998 (Fig. 63b). This single rainfall event contributed to more than half of the total seasonal anomaly of 750 mm. Darwin also recorded measurable rainfall nearly every day from January to mid-April, with several substantial rain events (totals reaching 50 mm each) observed during the period.

In northwestern Australia 1998/99 seasonal rainfall totals at Wittendom Gorge (Fig. 64), location indicated by the red dot in Fig. 62a, reached 850 mm (Fig. 64a). This total is 525 mm, and 260%, larger than the climatological mean. Most of this excess rainfall resulted from three major precipitation events (Fig. 64b), two of which occurred in February and one of which occurred in mid-March in association with TC Elaine.

2) Southeastern Australia

Significantly below-average rainfall was observed over southeastern Australia (indicated by the boxed region in Fig. 62b) from April 1999 through the end of the year (Fig. 62d). Rainfall was below-average during every month of this period (Fig. 62d), with the most significant deficits observed during June–December. Overall, totals during this period were generally more than 200 mm below average across nearly all of Victoria, which includes major cities such as Melbourne and Sydney. The state of Victoria has recorded significantly below-average rainfall since late 1996, with December 1996–December 1999 deficits exceeding 500 mm throughout the region. These long-term moisture deficits have led to severe drought over most of southeastern Australia.