Summary: Australasia's climate is strongly influenced by the surrounding oceans. Key climatic features include tropical cyclones and monsoons in northern Australia; migratory mid-latitude storm systems in the south, including New Zealand; and the ENSO phenomenon, which causes floods and prolonged droughts, especially in eastern Australia. Unfortunately, climate models at present cannot provide reliable predictions for these features under climate change.
The region's climate is strongly influenced by its oceanic context. Northern Australia, lying just south of the Western Pacific oceanic "warm pool," experiences tropical conditions, with a summer monsoon (Sturman and Tapper, 1996). Tropical cyclones (averaging six per year) are a major concern for northern coastal regions. Cyclones can track southward as far as New Zealand, bringing very high rainfalls (Sinclair, 1993a, b). Western and central Australia experience generally clear, dry conditions owing to large-scale subsidence. In the winter half-year, eastward-moving anticyclones cross the continent, and northern areas are influenced by mild, dry, southeast tradewinds. In the summer half-year, the anticyclones cross at higher latitudes-bringing fine weather to southern Australia and to New Zealand.
South of about 35�S there are increasingly strong westerly winds, within which large-scale atmospheric waves develop. The resulting migratory cyclonic and anticyclonic systems are key features of the weather of New Zealand, Tasmania, and the southern coasts of Australia, especially during winter (Maunder, 1971; Sturman and Tapper, 1996). The eastward progression of these weather systems brings cycles of warm northerly conditions followed by depressions, fronts, and sometimes severe outbreaks of cold air from the Southern Ocean.
Australia's generally low relief (apart from Tasmania and the Great Dividing Range parallel to the east coast) does not significantly affect the weather regimes. Because of the size of the continent, however, the rain-bearing weather systems progressively dry out as they penetrate inland, resulting in a very arid central desert region (Sturman and Tapper, 1996). Australia is the driest populated continent; two-thirds of the land is classified as arid or semi-arid (<400 mm annual rainfall). About 87% of all rainfall is lost to evaporation from the land surface and vegetation, compared to 60% for the United States and Europe. Less than 5% of Australia has an average annual runoff greater than 250 mm; 26% of the land surface contributes more than 88% of total runoff, and half the land surface has no direct discharge to the ocean.
An important feature of the region's climate is the ENSO phenomenon, which causes high year-to-year variability, especially of rainfall in northern and eastern Australia. The ENSO phenomenon involves the interaction of the tropical Pacific Ocean with the global atmosphere and shifts in the major climatic patterns of the globe every 2-5 years or so (McBride and Nicholls, 1983; Rasmusson and Wallace, 1983; Bureau of Meteorology, 1989; McCreary and Anderson, 1991; Trenberth, 1991). During "El Ni�o" years-which are marked by unusually warm sea surface temperatures in the tropical eastern Pacific Ocean-Australia experiences reduced rainfall and extensive and often prolonged droughts, with severe impacts on rural communities and agricultural production. Major wildfires also can occur. At the opposite phase of the oscillation, during "La Ni�a" years, flooding is common. Australia has been described as a land of drought and flooding rains.
Owing to its generally low rainfall and low runoff and the large seasonal and ENSO-related variations, Australia's hydrological features are significantly different from almost all other regions and continents (Finlayson and McMahon, 1988; McMahon and Finlayson, 1991; Fleming, 1995; Thomas and Bates, 1997). The variability of Australian rainfall and runoff is among the greatest in the world: two to four times those of northWestern Europe and North America for the same climatic zones. Compared with other nations, the frequency and duration of drought is extreme, though these are interspersed with sequences of above-average rainfall. About 25% of the variance in annual rainfall is attributable to the ENSO phenomenon. Most of Australia's major rivers have ceased to flow at least once during the past 100 to 200 years, and groundwater levels show remarkable variations through time.
In contrast, New Zealand has a maritime climate; few places are more than 100 km from the coast. Rainfall is relatively well-distributed by region and season, and the influence of the ENSO phenomenon on year-to-year variations is less than in Australia. Mountains rising to 2000-3000 m transverse to the prevailing westerly winds result in very wet conditions in western areas and drier "rainshadow" conditions in eastern areas. In the west of the South Island, annual rainfall can be as much as 12,000 mm, with high runoff, steep torrents, extensive permanent snow fields at high altitudes, and fast-moving glaciers. Barely a few tens of kilometers to the east, annual rainfall may be as low as 500 mm. The eastern plains experience summer drought, which sometimes extends to spring and autumn.
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