Why world desert are located in the western margins of continents in subtropics?

Question

Why world desert are located in the western margins of continents in subtropics?

Answer 1

Interesting, the 'Chapter', in Physical Geography!.

Answer 2

Earth's rotation sets in as churning of wide basins of Oceans (Pacific, Atlantic, Indian), called 'gyres'. Gyres are clockwise in northern hemisphere and counter-clockwise in southern hemisphere, due to earth's Coriolis effect. Also the differential temparature regimes of parts of ocean and salinity variations caused by major river discharges and evaporation differences causes density variations of sea water leading to subsidence and upwelling of very huge water masses that move as streams (Gulf stream is an example). The unhindered flow of Antarctic circumpolar current has no counter part in northern hemisphere.

As a result 17 major surface currents were identified of which there are cool currents - Humbolt (Peru), Benguela (Angola), West Australian in southern hemisphere, Canaries, Californian currents in the northern hemisphere. These are the result of sudden upwelling of cool water masses from deep within the oceans. These cool currents are the follow up of the demise of warm currents like Gulf stream in Atlantic and Kurosheo in Pacific. It is noteworthy that these currents wash only the western margins of continents due to direction of gyres, that causes them. When these cool currents upwell at around latitudes of 30deg, the adjacent continental masses - Canries: West africa, Humbolt: Atacama (Chile/Peru), Benguela: SW Africa (Namib coast), West Australian: West Australia, experience a phenomenon. With these land areas getting heated up while the sea water is very cool, land winds blow dry (high pressure) from land to the sea surface (low pressure). If sea winds blow onto land they carry water vapour in form of clouds that gets precipitated as rain or snow. But the reverse, 'land => sea' wind is devoid of rain. This turns the continental areas in these latitudes from 30deg progressively down to 25deg into desert. Sahara (Africa), Atacama(S.America), Namib-Kaalahari (S'Africa) are formed like this. In Atacama the average annual rainfall is 4mm and no rains for four years is normal.

There is an interesting side effect. These cool upwelling currents bring a rich load of nutritious food from th Antarctic and Arctic waters, that feeds of huge shoals of fish, when the warm surface winds blow the cool water. In Atacama, there are large colonies of sea birds (no human would prefer to stay in that desolation) nesting on land feeding on this endless supply of fishes. Their droppings get petrified due to lack of rain, that is exported as farm manure 'Guano'. And Peru emerged as a leading fishing country because of the abundant fishes in its coastal waters.