olelog

We seem to have different views on what a desert really is, although I suppose it is generally accepted that they are dry with extremely little vegetation and rather desolate. But a general conception that they are always hot and sandy is not true. Sand covers only about 20% of Earth's deserts, and the desert I am going to write about today is a cold winter desert. Actually it got a 14-cm deposit of snow due to consecutive snowing in December 2002. In January 2008 the whole desert was covered by snow and the temperature got as low as minus 32 degrees Celsius.

Apart from that Taklimakan Desert is however a true sand desert. It is reputed to be the world's second largest shifting sand desert with about 85% made up of shifting sand dunes. It is the largest desert in China covering an area of 327,000 km².



The Taklimakan Desert is hemmed in to the north by the snow-covered Tian Shan Mountain range and to the south by the rugged Kunlun Mountains. It is sitting in a depression in between called the Tarim Basin. Although the map above shows rivers crossing the basin they are relatively dry as precipitation in the Tarim Basin is extremely scanty due to the surrounding high mountains (“rain shadow”), and in some years it is nonexistent.

The NASA Earth Observatory image below shows the Taklimakan Desert as a vast region of sand desert.



Another more recent NASA Earth Observatory image (of 5 April 2012) shows dust storms in the Taklimakan Desert. Dust was thickest along the desert’s southern margin. Dust storms are common in the Taklimakan Desert. Marching sand dunes, some reaching a height of 200 m, cover most of the desert floor. The dunes are virtually devoid of vegetation, but plants survive along the desert perimeter, and experience distinct seasonal variations.



Deserts take up about one third (33%) of the Earth's land surface.

• http://earthobservatory.nasa.gov/IOTD/view.php?id=1925
• http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=77597&src=nha
• http://www.chinatraveldiscovery.com/china-facts/the-largest-deserts-in-china.htm
• http://www.eoearth.org/article/Taklimakan_desert

The Empty Quarter, or as it is called in Arabic, Rub' al Khali (الربع الخالي‎) is said to be the largest sand sea in the whole world. It encompasses most of the southern third of the Arabian Peninsula, including most of Saudi Arabia and areas of Oman, the United Arab Emirates, and Yemen, and covers some 650,000 km2. It holds about half as much sand as the Sahara Desert.



The desert is 1000 km long, and 500 km wide. Its surface elevation varies from 800 m in the southwest to around sea level in the northeast. The terrain is covered with sand dunes with heights of up to 250 m, interspersed with gravel and gypsum plains. As seen in above satellite image the sand is a reddish-orange color due to the presence of feldspar.

For thousands of years this territory has resisted settlement as one of the Earth's hottest, driest, and most unyielding environments - hence the name "quarter of emptiness". However oil of excellent quality has been found there and is exploited. The al-Ghawar oil field is one of the world's largest.

Dust from The Empty quarter regularly give rise to dust storms also to neighbour regions. The NASA satellite image below shows a dust storm that swept over the Arabian Peninsula in early February 2012. The image was captured on 2 February 2012. This dust storm follows a familiar pattern for this region, with especially thick dust occurring in the southwest. Toward the northeast, the dust thins enough to show the Persian Gulf, Qatar, and Bahrain.

• http://earthobservatory.nasa.gov/IOTD/view.php?id=9054
• http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=77070
• http://ngm.nationalgeographic.com/ngm/0502/feature1/index.html
• http://www.infoplease.com/ce6/world/A0842602.html#ixzz1lKqaYr1o

No, I am not talking COP15 today - everybody else is. Let me stick to anything else, like linear dunes. Linear dunes may be more than 160 kilometers long. Linear dunes may occur as isolated ridges, but they generally form sets of parallel ridges separated by miles of sand, gravel, or rocky interdune corridors.

scale = 5 km

Vast, windswept plains of sand dunes (ergs), occasionally interrupted by rocky outcrops, stretch across much of Algeria. Except for exceedingly rare oases, these seas of sand are usually as good as empty of life, including humans (well I have seen snakes out there, so there must be something to eat - now and then.

This natural-color image from the Landsat 5 satellite shows the emptiness of the Erg Iabès in western Algeria’s Adrar province (with Adrar as capital). Adrar, a Berber word meaning "mountain", is by the way the name of several areas in Northwest Africa. This rather small erg (compared to the country’s Grand Ergs) occupies the wide gap between the El Eglab Massif to the west and the Tademaït Plateau to the northeast.

Long, linear dunes such as the ones pictured here align in the direction of the prevailing winds, and they usually form under the influence of strong winds. However, linear dunes are not the only types of dunes that are found in ergs. For example, weaker winds blowing over a linear dune in the non-prevailing wind direction may create star dunes. Barchan dunes are shaped like a crescent, with a more gradual slope on the windward side, and a steeper slope on the downwind side.

• http://earthobservatory.nasa.gov/IOTD/view.php?id=41476&src=eorss-iotd

Let me concentrate myself on two deserts - the driest and the oldest in the world - the Atacama Desert in South America and the Namib Desert in Southern Africa. Unlike for instance the Gobi desert, that is so far away from the sea that the water can’t make it that far and so it falls before arriving to the desert, they are both situated next to a large ocean.

To understand why these two deserts are where they are we need a bit of elementary oceanography. I suppose the coriolis effect is more or less well known. It makes ocean currents curve to the right (clockwise) in the northern hemisphere and to the left (counter-clockwise) in the southern hemisphere. This way it forms large circular ocean currents known as gyres. This map shows the 5 largest ocean gyres in the world.

Please notice in particular the South Atlantic Subtropical Gyre and the South Pacific Subtropical Gyre. Currents flowing from equator towards the poles will be warm currents (like the Gulf Stream). Currents flowing towards the equator will be cold. Due to the ocean gyres a cold current is flowing northwards along the west coast of Chile - the cold Humboldt current (also known as Peru Current). Likewise a cold current is flowing northwards along the west coast of Namibia - the cold Benguela current.

And this leads us to what is possibly the driest place on on earth (actually another outstanding candidate for this honour is a low spot in the Lut Desert of eastern Iran). The Atacama Desert is a virtually rainless plateau in South America, extending nearly 1000 km between the Andes mountains and the Pacific Ocean. Parts of Chile's Atacama Desert haven't seen a drop of rain since record-keeping began. Here a place called Arica gets just 0.76 millimetres of rain per year. At that rate, it would take a century to fill a coffee cup (not even a tee cup for my dear British readers). The precipitation (moisture equivalent to rain) in Atacama averages less than 1 centimetre per year from fog. Measurable rainfall (more than a millimetre of rain) occurs every five to 20 years and heavy rains fall only two to four times a century. No vegetation grows here. It is what is termed ‘absolute desert’.

The desert is to a great extent created by the cold Humboldt current. A great mass of ice cold water surges out of the Antarctic Ocean and flows north along the South American continental shelf. The shallowing land forces the cold deep waters up to the sea surface where the waters may encounter warm winds that blow land-ward. The warm air cools as it moves across the cold current and the air becomes too cold to hold much moisture. No rain clouds, therefore, can reach the coast and the land dries into a hostile area for life. In the winter, fog rises from the upwelling cold currents, blankets the desert, and gives moisture to the land. The mountain ranges also play a role. The Atacama is blocked from moisture on both sides by the Andes mountains to the east and by coastal mountains to the west. The trade winds blow westward on the east side of the Andes, but the desert lies in the rain shadow of the Andes.

I would indeed like to compare the situation with the Namib Desert and the cold Benguela current in Southern Africa. The Namib Desert is considered to be the oldest desert in the world, having endured arid or semi-arid conditions for at least 55 million years. Its aridity is caused by the descent of dry air cooled by the cold Benguela current along the coast. It has less than 10 mm of rain annually and is almost completely barren. The cold waters of the north-flowing Benguela current move from the western coast of South Africa and Namibia towards north and Northwest up to the line where it joins the southern equatorial current which is a warm current. Its waters are cold because there are very deep waters that were brought upward due to the rotation of Earth from west to east. This upward movement of deep waters are sometimes increased by southern Trade winds which blow west from the Kalahari Desert towards the ocean. The cold current creates the desert conditions of the shore of Namibia, and the persistent fogs of the Skeleton Coast.

That these two deserts are both located at the same southern latitude (trade wind zone) on the west coast of a large continent is no coincidence. The Tropic of Capricorn passes through both deserts.

Deserts can, by the way, be classified by their geographical location and dominant weather pattern as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. The Atacama Desert covers three of these categories: trade wind, mid-latitude, and coastal.

This is a special post written for the the sixth Carnival of the Arid coming up soon at Coyote Crossing. I’ll give you the URL here in due time...

... and here it comes:

• http://faultline.org/index.php/site/item/carnival_of_the_arid_6/

go and enjoy it!