Spatial and temporal variation in precipitation at the start of the rainy season in tropical Australia
S. T. Garnett A C and G. Williamson A BA School for Environmental Research, Institute for Advanced Studies, Charles Darwin University, NT 0909, Australia.
B School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tas. 7001, Australia.
C Corresponding author. Email: stephen.garnett@cdu.edu.au
The Rangeland Journal 32(2) 215-226 https://doi.org/10.1071/RJ09083
Submitted: 20 December 2009 Accepted: 18 May 2010 Published: 30 June 2010
Abstract
The patterns of rainfall early in the rainy season vary substantially across northern Australia, even in sites with the same annual average. This has biophysical and economic implications in terms of land and infrastructure management, resource availability and capacity, and access. Daily patterns in long-term rainfall records in Australia north of 23°S subject to regular monsoonal rainfall were compared with threshold levels for dryland and wetland seed germination, initiation of the growing season, patterns of gaps between early storms and the heaviness of the first falls, correlations between thresholds, spatial variation in correlation with the Southern Oscillation Index (SOI) and temporal trends in mean threshold dates. The earliest rains sufficient to cause seed germination or generate fresh fodder occur in the north-west of the Northern Territory with the average date being later to the south, east and west. Initial falls of the rainy season are heaviest, however, on Cape York Peninsula so that the time between first falls and saturation is shortest in the east. The probability of extended gaps between rainfall events increased from north to south. When the SOI is taken into account, no change in timing could be detected at the few sites with records of sufficient duration. However, because of changes in SOI frequency, rains are tending to start earlier in the drier parts of the north and north-west and later in the east. This may be because anthropogenic climate change is resulting in fewer classical El Niño Southern Oscillation events and more frequent El Niño Modoki climate anomalies.
Additional keywords: climate change, El Niño Modoki, North Australia, rainy season.
Acknowledgements
We are particularly grateful to Sam Cleland of the Bureau of Meteorology for his encouragement and advice on the preparation of this manuscript, to Garry Cook of CSIRO for early discussion of the issues and for providing insight into the literature, and Ken Day of the Queensland Climate Change Centre of Excellence for comments as a reviewer, particularly for drawing our attention to the potential significance of the Pacific Decadal Oscillation.
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