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This article comes from the official account of Wechat: Earth knowledge Bureau (ID:diqiuzhishiju), with the original title: "Super drought, domination from the south!" "author: Xiao Zhe, drawing: Baba Dad, proofreading: Chao Qian, Editor: tuna

In the 2nd century AD, the Greek geographer Ptolemy concluded in his Geography that there must be a huge "unknown southern continent" between the equator and the South Pole, balancing with the continents of the Northern Hemisphere. In medieval manuscripts, the southern tip of the African continent is connected to the "southern continent", and the Indian Ocean is an "inland sea".

Ptolemy's map of the world, the imaginative part of the world view.

At that time, people thought that the monsoon was blown by the wind god.

(photo: Wikipedia) ▼

In the hundreds of years after the great voyage began, Dias, who crossed the Cape of good Hope, did not encounter the predicted "southern continent" here. Before the discovery of the Antarctic continent, it was only in the South Pacific that Europe discovered a wild "unknown continent" that was much smaller than predicted. The British seemed to be comforting themselves and called it Australia, meaning "southern continent".

It was not until 1770 that Captain James Cook listed Australia as British territory. Later, due to the outbreak of the American War of Independence, Britain changed the criminals who had been exiled to America to Australia.

What are the natural conditions of this "southern continent", which is not high in existence and even unpopular?

Australian topography ▼

The arid continent of Australia, located between 10 and 39 degrees south, is the smallest continent in the world, about 80 percent the size of China, and the southernmost island of Tasmania can extend south to 44 degrees south.

From the point of view of the satellite, the land surface of Australia is red, which is very similar to the surface of Mars but different from other continents on Earth.

In fact, some red areas are basically uninhabited by ▼.

The key to creating such a red land lies in Australia's own climatic conditions.

In areas where the climate is generally cool, rock weathering is mainly a physical process caused by frozen water, temperature change, life activities and so on. However, the areas of the Australian continent with annual precipitation of less than 250 mm account for 359% of the total area, and the average annual precipitation is the lowest of all continents except Antarctica. In such a hot and dry climate, chemical weathering caused by rust erosion of a large number of iron-bearing rocks is more common.

In addition, after Australia separated from the Antarctic continent about 5500 million years ago, it moved to lower latitudes towards Beijing drifters, which was less affected by Quaternary glacial activities. This allowed weathered iron oxide to accumulate for millions of years, eventually forming a thick, vast, barren red desert.

If Australia hadn't broken up with Antarctica and floated to where it is now,

Then the climate may be very different (photo: Wikipedia) ▼

And the hot climate of Australia is obviously closely related to its latitude--

The solar altitude angle in the equatorial region is higher, the solar radiation received in a year is more, the temperature increases rapidly, the atmosphere is heated up, and a low pressure zone is formed on the ground. On the other hand, the rising warm air diverges to the north and south sides at high altitude and sinks to form a subtropical high pressure belt in the northern and southern hemispheres around 30 degrees south and north latitudes. the surface temperature is high and the pressure is high, which makes it difficult for the upper air water vapor to condense into precipitation, muggy and little rain.

The Australian continent corresponds to the control range of the downdraft in the subtropical high pressure belt of the southern hemisphere. there is a high pressure system across the continent from west to east, which is shown as an anticyclone rotating counterclockwise. A single anticyclone can be up to 4000 kilometers in diameter and can remain quasi-stationary for several days, leaving most parts of the continent in sunny and cloudy weather and forming vast arid desert zones.

Anticyclones over the subtropical high

(figure: ABC Weather) ▼

Especially from June to September, as the solar direct point moves northward near the Tropic of Cancer, the southern hemisphere enters winter, and the global planetary wind belt shifts northward as a whole, making the subtropical high belt located between 30 °and 35 °south latitude, resulting in drier and less rain in most parts of Australia in winter.

From the average annual precipitation line of Australia, we can see its general dry and wet zone distribution ▼.

Coupled with the fact that the overall topography of Australia is high on both sides and low in the middle, the vast plateau mountains in the west and the high watershed in the east make the central plain even more short of water, and the drought in a large area of the whole continent makes Australia's overall population only 25.89 million.

The whole of Australia is not completely occupied by deserts. There is a lot of green in the coastal areas of the north, east and southwest of Australia, where many prosperous cities are located, and these are the gifts of the oceans that surround the continent.

Gifts from the north when the rising air from the equator reaches the subtropical high pressure belt and sinks, the accumulated air near the ground returns to the equator and forms the northeast trade wind and the southeast trade wind in the northern and southern hemispheres respectively under the action of the earth's rotation. The two trade winds converge and rise in the equatorial region, where they form an equatorial convergence zone.

In practice, the equatorial convergence zone swings along the equatorial line with the seasons.

(photo: Wikipedia) ▼

From December of each year to February of the following year, summer enters in the southern hemisphere, the subtropical high belt moves southward to between 35 °and 40 °south latitude, the equatorial convergence zone advances southward, and the northeast trade wind in the northern hemisphere changes from the equator to the northwest wind.

In addition, due to the difference in thermal properties between land and sea, the surface temperature of the mainland increases rapidly to form a thermal low pressure, and the air flow convergence finally forms a strong monsoon from the northern ocean, which brings abundant precipitation to the northern part of the Australian continent.

But in addition to the influence of the monsoon, this small piece of land surrounded by the ocean is also more deeply and directly affected by the ocean.

The warm Southern Equatorial current flows southeast from the Coral Sea, between the Great Barrier Reef and Chesterfield Reef, and forms the East Australian current off the southeast coast of Queensland, Australia, accumulating Fraser Island, the largest sand island in the world, under the combined action of waves and winds.

After that, the ocean current strengthens along the east coast of the Australian mainland in southeastern Queensland and northern New South Wales and goes south to the Tasman Sea.

Due to the westward strengthening effect caused by the earth's rotation, the East Australia warm current can transport 30 million cubic meters of sea water per second and flows in a strip area 100km wide and 500m deep, making it the strongest ocean current around Australia. Ocean currents are strongest in summer in the southern hemisphere and weaken in winter.

In the movie finding Nemo, it is this East Australian stream that helps Marin go to Sydney to find his son Nemo. In reality, many tropical fish in the north, such as tuna, also travel south by ocean currents. Although it is a warm current, the East Australian current carries few nutrients, but it has a profound impact on the marine biodiversity of the Coral Sea and the Tasman Sea.

Both tuna and southern bluefin tuna are usually found in East Australia.

Atlantic bluefin tuna (northern bluefin tuna)

The same genus and different species, the north is more handsome (upper north and lower south) ▼

The East Australian current carries a large amount of warm water southward, coupled with the uplift blocking effect of the Great Watershed in eastern Australia, intercepting abundant moisture from the warm current on the east coast of Australia.

The pleasant climate makes it the most populous area in Australia and the most frequented area for tourists. Sydney, Melbourne and Brisbane are all located in Australia's top three cities.

At the same time, on the southwest coast on the other side of the continent, another warm and humid climate brought by another southward current created Perth, Australia's fourth largest city.

On the west coast of the global continent at the same latitude, the Benguela and Peruvian cold currents on the eastern side of the African continent and the eastern side of the South American continent have created the unusually dry Namib Desert and the Atacama Desert. Where does this warm and humid current come from?

The distribution of deserts in the world, watch ▼ on horizontal screen.

It also starts with the waters to the north of Australia. Under the prevailing trade winds in the tropics, the warm equatorial current on the surface of the Pacific Ocean flows all the way to the west, preventing it from accumulating in the Indonesian archipelago, forming the Western Pacific warm Pool, which is the largest and hottest warm water in the world. Some of these currents can enter the Indian Ocean through the sea lanes between the Indonesian archipelago, known as the Indonesian crosscurrent.

This is the only place in the world where warm equatorial water can be extracted from an ocean.

An area flowing to another ocean (figure: Wang Jianning, 2017) ▼

Between the tropical Indian Ocean and the tropical Atlantic, and between the tropical Atlantic and the Pacific, the equatorial warm current is blocked by the continent, thus creating some distinctive features of the southern Indian Ocean.

The warm water accumulated in the western Pacific warm pool flows through Indonesia to the Indian Ocean, and some of it flows south along the coast of Western Australia, reaching Cape Leyne and then southeast, along the Greater Australia Bay. and serve as the Zine current to the west coast of Tasmania.

On the lighthouse of Cape Luin, you can see the Indian Ocean and the Southern Ocean.

In 1897, William Saville Kent conducted a survey of marine animals and plants in the Arohus Islands and found that the winter water here is much warmer than the coastal waters near the coast, suggesting that there may be warm tropical waters offshore.

It was not until the advent of satellite technology in the 1970s that the existence of the southward current was finally confirmed, and it was not until the 1980s that Creswell and Golding named the current the Luin current.

The Loyne current brings warm water from the tropical Indian Ocean to Western Australia.

(figure: Catia M. Domingues, 2007) ▼

The Luin current has created a mild climate in southwest Australia, which has an annual rainfall of about 1250 millimeters.

After summer entered in the southern hemisphere from November to March of the following year, the equatorial convergence zone is now located in the northern part of the Australian continent, forcing a strong southerly wind to the west coast and greatly weakening the southward current. In winter and autumn, as southerly winds weaken, so does the Luin current, bringing rain to southwestern Australia.

The monsoon and two ocean currents from the north bring precious water vapor and a pleasant climate to Australia, while from Australia to the south, there are more powerful forces that dominate the continent.

Dominated from the south in the southern hemisphere, the subtropical high belt almost circles from east to west. Because of the characteristics of its anticyclone, it is also known as the subtropical ridge, and the south side of the ridge corresponds to a strong prevailing westerly belt. The Antarctic circumpolar current, also known as westerly drift, is driven by strong westerly winds.

The Antarctic circumpolar current is like a boundary, keeping the warm current away from the Antarctic continent ▼.

The westerly winds here are so strong that the latitudes of the Southern Ocean are called "Roaring Forties", "Furious Fifties" and "Screaming sixties" by navigators.

The Antarctic circumpolar current is the only current that completely surrounds the earth because there is no land in the high latitudes of the southern hemisphere, and the biggest limitation it encounters is the Drake Strait, the widest strait in the world with a width of about 1000 kilometers.

Although the velocity of the Antarctic circumpolar current is not fast, the maximum surface velocity is about 0.75m / s, but its average transport volume is about 130 million cubic meters, which is larger than that of any other surface current.

After entering the southern Indian Ocean, the strong Antarctic circumpolar current moves northward to form the Western Australian current under the influence of geostrophic deflection, which occupies almost the whole southeast Indian Ocean and merges into the South Equatorial warm current at about 20 °south latitude.

The Antarctic circumpolar current also participates in the global ocean current cycle through water exchange.

Has a strong influence on the global climate, ▼.

On the other hand, the length of the land coastline on the west side of the Australian continent is shorter and protrudes westward, which makes the ocean current flow distance is very short, which is much shorter than the Benguela cold current and the Peruvian cold current at the same latitude, so the water temperature of the Western Australian current is 4 ℃ to 6 ℃ higher than both. This also makes it the weakest of the major cold snaps in the world.

It is also difficult for it to reach the Australian coast because of the existence of the southward Luin current and the weak cold current in Western Australia. So in fact, many areas around the Australian mainland are surrounded by warm currents, and it is difficult for cold currents to deeply affect Australia's climate.

Due to Australia's unique geographical location

There has always been a lot of research on the factors affecting the climate of the Australian continent, ▼.

Continuing southward through the surrounding westerly belt, it is a high-latitude region centered on the Antarctic continent, where the pressure and most of the changes in the mid-latitude of 40-50 degrees take the form of a "warp".

This phenomenon was first discovered and named "Antarctic Oscillation" by Chinese atmospheric scientist Gong Daoyi in 1998. Because of the obvious zonal characteristics of this mode, most of the changes of air pressure are parallel to the latitude, which is also known as the annular mode of the southern hemisphere.

There are two models of Antarctic Oscillation (▼) in summer and winter in the Southern Hemisphere.

The positive and negative phase change of this mode causes the north-south movement of the westerly belt and the ridge of high pressure, which has a great impact on the climate of Australia:

In winter, the northern side of the westerly belt is located in southern Australia. After the cold front forms from the southern sea, it can cross the Australian coast along the westerly belt, bringing normal winter rainfall.

When the annular model in the southern hemisphere is in a strong positive phase, the westerly belt is abnormally southward, and the rainfall system moves southward, making South Australia drier. The retreat of the strong westerly belt allows warm and humid air along the east coast to move south, bringing more precipitation to the area.

But when the annular mode is in a strong negative phase, the abnormal northerly westerly winds make the south colder and bring more snow to the Australian Alps. Unusually northerly westerly winds will blow across the continent, the west will receive more water vapor from the ocean, and precipitation will increase, but by the time it reaches the east coast, the water vapor will have been exhausted, causing drought in the east.

In summer, the subtropical ridge moves southward and the westerly belt basically moves to the sea. When the annular mode in the southern hemisphere is in the negative phase, the ridge of high pressure is abnormally northward, which makes the eastern part of Australia drier, and the northern and central parts of Australia are prone to extreme high temperatures. The abnormal south of the high pressure ridge caused by the positive phase will make the east coast wetter.

The climatic conditions of the Australian continent are not only affected by these north-south systems, but on the whole, it is under the joint shaping of the atmosphere and the ocean that the Australian continent is what it is today.

The real situation is obviously more complicated than the above theory. The wildfires caused by extremely high temperatures in early 2020 have brought Australia to the attention of the world, and more uncertainties brought about by climate change in the future will also make the "southern continent" face more challenges.

Reference website and literature:

1. Https://www.redmap.org.au/article/ocean-currents-in-australia/

2. Https://www.griffith.edu.au/data/assets/pdf_file/0017/213524/East-Australian-Current.pdf

3. Https://phdessay.com/the-leeuwin-current/

4. Https://www.gostudy.com.au/australia/climate-australia/

5. Https://www.abs.gov.au/ausstats/abs@.nsf/Lookup/by%20Subject/1301.0~2012~Main%20Features~Australia%27s%20climate~143

6. Https://oceanservice.noaa.gov/facts/roaring-forties.html

7. Https://www.imosoceanreport.org.au/time-series/environment/current-variability/

8. Https://www.geographynotes.com/wp-content/uploads/2017/03/clip_image002-79.jpg

9. Https://www.nesdis.noaa.gov/news/why-does-australia-look-red-space

10. Http://au.mofcom.gov.cn/article/ddgk/zwqihou/202103/20210303045529.shtml

11.https:// www.bom.gov.au/climate/sam

12.Alan P.Trujillo.Harold V. Thurman, translated by Zhang Ronghua et al. Introduction to Oceanography Chapter 7: ocean Circulation

13.Church, J.A., Cresswell, G.R. Et al. Poleward Flows Along Eastern Ocean Boundaries: The Leeuwin Current. Coastal and Estuarine Studies (1989), vol 34. Springer, New York, NY.

4.Ming Feng, Gary Meyers, et al. Annual and interannual variations of the Leeuwin Current at 32 °S. Journal of Geophysical Research. (2003)

15.Kuhnt, W., Holbourn, A., Xu, J. Et al. Southern Hemisphere control on Australian monsoon variability during the late deglaciation and Holocene. Nat Communication 6, 5916 (2015).

16. Gong Daoyi, Wang Shaowu. Antarctic Oscillation [J]. Science Bulletin, 1998 (03): 296301.

17. Li Xiaofeng. The concept of annular module [J]. Advances in Geoscience, 2015. 30 (03): 367-384.

* the content of this article is provided to the author and does not represent the position of the Earth knowledge Bureau

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