e. Australia

Northern Australia (indicated by the boxed area in Fig. 61a) experiences a tropical climate with a well-defined rainy season that typically begins during October and ends in April (Fig. 61c ). 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 strongly modulated by the Southern Oscillation, with below- (above-) normal rainfall often observed during Pacific warm (cold) episodes (Ropelewski and Halpert 1987, 1989).

Overall, the 1997-98 rainy season featured below-normal rainfall across the southern and central sections of this region, and above-normal rainfall in the vicinity of Darwin and in southern portions of the Cape York Peninsula (Fig. 61a). For the region as a whole, the above-normal rainfall occurred primarily during December 1997 and January 1998 (Fig. 61c), with substantially below-normal totals observed during October-November 1997 and February-March 1998.

In the extreme north, a series of intense rainfall episodes and tropical storms during December 1997-Jaunary 1998 contributed to record October-April rainfall totals at Darwin (2499 mm). At Katherine, located southeast of Darwin, a series of major rainfall episodes during late December (Fig. 62) was followed by an additional 914 mm of rain in January. For comparison, the annual average rainfall total at this station is 1000 mm. This excessive rain culminated with a major flooding episode in late January, triggered by more than 375 mm of rain during 25-26 January.

Elsewhere during the season, rainfall was below normal along most of the east coast, as well as across sections of the west and northwest. The most significant dryness was observed along the coastal regions of Queensland and New South Wales, where deficits at many locations exceeded 400 mm. However, these deficits soon disappeared as above-normal rainfall covered eastern Australia (defined by the boxed region in Fig. 61b) from April through November (Fig. 61d), including the climatologically drier months of June-September. This above-normal rainfall was consistent with the development and intensification of La Niņa conditions. For the April_November period as a whole, rainfall surpluses averaged 50-200 mm over most of the region, with the largest anomalies (exceeding +400 mm) observed in eastern New South Wales. Above-normal rainfall (50-200 mm surpluses) also covered much of central and western Australia during this period. In association with this excess precipitation, the areas most affected by flooding included extreme southern Queensland, much of the northern half of New South Wales, and northwestern Australia.

In contrast, extreme southeastern Australia again experienced below-normal rainfall throughout the year, as severe rainfall deficiencies extended beyond two years. The most severely affected regions were located immediately east and southeast of Melbourne, which experienced their driest two-year period in the historical record.

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