Why is the climate in Australia mostly dry?

The basic feature of Australia's climate is that the proportion of arid areas is the largest, the annual precipitation is semicircular, and the whole continent is generally warm and hot. (1) The arid area is vast, with the highest proportion. The average annual precipitation of the whole continent is 470 mm, which is the least among all continents except Antarctica, much less than that in Africa, and only equivalent to three-fifths of the average annual precipitation of all continents in the world. The area with annual precipitation less than 250 mm (2.765 million square kilometers) accounts for 35.9% of the total area. The area of 250 ~ 500mm (2491.400km2) accounts for 32.4%, accounting for 68.3% of the total area of the mainland. The area above 1 1,000 mm (511,360 square kilometers) accounts for only 6.5%, which is far smaller than Africa and South America, and the smallest of the seven continents except Antarctica. (2) The regional distribution of annual precipitation in Australia presents a semicircle pattern that is open to the west, that is, it decreases from the north, east and south to the inland and western coastal areas. It is rainy in the north, and the annual precipitation is between1000 ~ 2000 mm. Keynes in the northeast is the most, with an average annual precipitation of 4206mm and a maximum of 5360 mm. It is the least near Lake El in the central plain, less than 100 mm or even less than 50 mm. This distribution pattern is unique in all continents. (3) The whole continent is generally warm and hot, especially in summer, and the inland areas are as hot as the Sahara desert. In the hottest month, the average temperature in three-tenths of inland central and western regions 1 month is above 30℃, while in central Tasmania it is 16℃, and the lowest in the southeastern alpine region, but it is also around 10℃. As for the average maximum temperature of 1 month, except for the northern part of the mainland, which is rarely adjusted by monsoon, it is above 40℃ in the north and south of the whole continent. The extreme maximum temperature is relatively high, such as 49.4℃ in Mabel, 53℃ in Koronka and 55℃ in Stout, central China (135 e, 23.5 S), which is the hottest place in Oceania. The whole continent is hot not only in summer, but also for a long time. The average daily temperature in inland areas of 20 ~ 23 south latitude continuously exceeds 32.2℃ 150 days, and the high temperature of 37.7℃ in some areas lasts for 64 days. Compared with Africa and South America at the same latitude, winter in Australia is also quite warm and hot. The average temperature in July in most parts of the mainland is above 10℃, and the 15℃ isotherm crosses the central part of the mainland, above 25℃ in the north, with the lowest temperature of 8 ~ 10℃ in the mountainous areas in the southeast, and only a few hills can reach below 0℃. Extreme minimum temperature, frost can be seen in inland areas at night, even falling below 0℃ (for example, Alice Spring once recorded -3.8℃), even in the southern coastal areas, frost rarely occurs. In a word, the climate characteristics of Australia are unique among the three tropical continents, which are different from those of South America and Africa, and are formed under the comprehensive influence of its latitude, continental contour, topography, ocean current, air pressure and wind force.

Australia lies between south latitude10 45 ′ ~ 39 08 ′ and south latitude 28 23 ′. It is the continent with the smallest latitude span, with small temperature difference between north and south and simple temperature distribution. The Tropic of Capricorn crosses the central part of the mainland, and 99% of the area belongs to the tropics and subtropics, which makes the areas with warm and hot temperatures all year round and desert with little rain particularly vast. The outline of the Australian mainland is relatively complete, which increases the distance from the inland to the ocean, reaching more than 1 500 kilometers at the maximum, which affects the penetration of ocean air into the inland. In addition, the whole contour line is long from east to west and narrow from north to south, which expands the area controlled by the southeast trade wind belt, and there is no adjustment of water surface such as vast rivers and lakes in the inland, which makes most areas more dry and hot. Topographically, the vast central and western regions are relatively flat, with little fluctuation, less cyclone rotation and stable weather, which is not easy to cause rainfall, thus narrowing the climate difference between the vast central and western regions. In addition, the mountains in the east are 4,500 kilometers long from north to south, and the height exceeds 1 1,000 meters. The eastern slope is steep and the western slope is gentle, which also oppresses the eastern coast, blocks the warm and humid airflow brought by the southeast wind of the eastern Pacific, makes the eastern slope rainy and the western slope dry, reduces the area of the rainy area in the east and expands the scope of the inland arid area in the west. The central plain runs through the north and south, which is conducive to the operation of the north-south airflow, making the central inland basin not a desert land that never rains. The ocean current around Australia is the North Australian Warm Current along the northern coast of the mainland and the East Australian Warm Current in the east, which affects the warming and humidification of the northern and eastern coasts of the mainland and becomes a rainy area. The southern coast is west wind drift, and the western coast is Western Australia's cold current, which affects the lack of rain on the southern coast of Australia and the formation of the western desert. Judging from the atmospheric circulation, the summer high pressure zone in the Australian continent moves southward, the northern part of the continent is the center of thermal low pressure, and the southern backflow high pressure zone is located in the southern part of the continent, so the northwest monsoon blows in the north, which is hot and rainy; Southeast wind blows in the south-central part of the mainland, which is hot and dry; Only the eastern slopes of the mainland face the wind and have more precipitation. In winter, the pressure belt moves northward and combines with the land cold high pressure. The high pressure center is located in the northern inland of the mainland. Most of the mainland is covered by high pressure, and the wind blows from the center of high pressure, so it is dry and rainy. Only the southwest and southeast corners of the mainland blow westerly winds from the sea, and there are many cyclones and rainy weather. The eastern hillside is still windward, with more precipitation. Tasmania is located in the south of 40 south latitude, with westerly winds all the year round, and belongs to a temperate maritime climate. Therefore, from the distribution of annual precipitation, it has become a rainy pattern in the north, east and south, and gradually decreases to the inland and west.