Moisture and Rainfall
With the exception of the vast arid interior of Australia, much of Oceania is warm and humid nearly all the time. New Zealand and the high islands of the Pacific receive copious rainfall; before human settlement, they supported dense forest vegetation. Much of that forest is gone after 1000 years of human impact (see Figure 11.5B; see also Figure 11.8D). Travelers approaching New Zealand, either by air or by sea, often notice a distinctive long white cloud that stretches above the north island. Seven hundred years ago, early Maori settlers (members of the Polynesian group) also noticed this phenomenon, and they named that place Aotearoa, “land of the long white cloud,” a name that is now applied to all of New Zealand.
The legendary Roaring Forties (named for the 40th parallel south) are powerful air and ocean currents that speed around the far Southern Hemisphere virtually unimpeded by landmasses. These westerly winds (blowing west to east), which are responsible for Aotearoa’s distinctive bodies of moist air, deposit a drenching 130 inches (330 centimeters) of rain per year in the New Zealand highlands and more than 30 inches (76 centimeters) per year on the coastal lowlands (see Figure 11.5B). At the southern tip of New Zealand’s North Island, the wind averages more than 40 miles per hour (64 kilometers per hour) about 118 days per year. Farmers in the area stake their cabbages to the ground so the plants will not blow away.
By contrast, two-thirds of Australia is overwhelmingly dry (see Figure 11.5A). The dominant winds affecting Australia are the north and south easterlies (blowing east to west) that converge east of the continent. The Great Dividing Range blocks the movement of moist, westward-moving air so that rain does not reach the interior (an orographic pattern; see Figure 1.28). As a result, a large portion of Australia receives less than 20 inches (50 centimeters) of rain per year, and humans have found rather limited uses for this interior territory. But the eastern (windward) slopes of the highlands receive more abundant moisture. This relatively moist eastern rim of Australia was favored as a habitat by both the indigenous people and the Europeans who displaced them after 1800. During the southern summer, the fringes of the monsoon that passes over Southeast Asia and Eurasia bring moisture across Australia’s northern coast. There, annual rainfall varies from 20 to 80 inches (50 to 200 centimeters).
Overall, Australia is so arid that it has only one major river system, which is in the temperate southeast where most Australians live. There, the Darling and Murray rivers drain one-seventh of the continent, flowing west and south into the Indian Ocean near Adelaide. One measure of Australia’s overall dryness is that the entire average annual flow of the Murray-Darling river system is equal to just one day’s average flow of the Amazon in Brazil.
In the island Pacific, mountainous high islands also exhibit orographic rainfall patterns, with a wet windward side and a dry leeward side (see Figure 11.5C). Rainfall amounts on the low-lying islands vary considerably across the region. Some of the islands lie directly in the path of trade winds, which deliver between 60 and 120 inches of rain per year, on average. These islands support a remarkable variety of plants and animals. Other low-lying islands, particularly those on the equator, receive considerably less rainfall and are dominated by grasslands that support little animal life.