Why is the Desert so Dry?
Monday, July 27, 2009 8:12:54 AM
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:
go and enjoy it!