At the Equator, large hot air masses rise to the upper layers of the atmosphere and tend to move towards the Poles where the temperatures are colder. In the Northern Hemisphere, at around 30° latitude because of the rotation of the Earth (the Coriolis force), these air masses are deviated clockwise and begin sinking down creating a high pressure area (called Anticyclone) over the North Atlantic Ocean.
Hence, the winds generated over the Equator revolve clockwise around the North Atlantic Ocean's high pressure area centered over the Azores Islands. Largely driven by temperature differences on a planetary level which translates into atmospheric pressure variations, these winds are therefore considered global winds.
The "Trade Winds" as they are referred to are therefore not influenced by the earth's topography. They result instead from the combined influences of the rotation of the earth (Coriolis forces) and its temperature differences from Equator to Poles.
The latter are due to the effects of solar radiation incidence angles on a spherical surface which are not as intense at sphere's center (earth's Equator) versus its extremes (Poles). The hot air masses generated at the Equator are subsequently deviated to the right by the rotation of the earth resulting into a clockwise movement over the North Atlantic.
It may be relevant to mention that these fundamentals cannot change, regardless of any climate change considerations, so long that the earth is round and rotates while exposed to direct solar radiation.
The circular movements of the Trade Winds over the Atlantic are shaping oceanic surface currents by wind stress in the exact same pathways. Geological evidence provided by the existence of the world’s largest sedimentary phosphate deposits (42 % of World reserve base*) trapped in front of Morocco’s Atlas Mountains, confirm that Trade Winds have been active for millions of years.
Indeed, high marine biological productivity associated with upwelling ocean currents along continental margins brought phosphorus-rich cold waters from deeper ocean levels nearer to the surface. This nourished and stimulated growth of plants and animals whose remains accumulated on the sea bottom.
The concentrations of phosphorus-rich organic debris covering thousands of square kilometers drifted away over the Atlantic and have been trapped at the bottom of North Africa's Atlas mountain range. These, highlighted by the yellow triangles shown on the map to the left (over Khouribga, Benguérir and Youssoufia respectively) have accumulated over millions of years. They represent by far, the largest phosphate-rock deposits available worldwide, that are commercially mined today.
Further to the South, on the African coast and the junction between the Sahara desert with the Atlantic Ocean, the Trade Winds are creating a zone of global energy exchange caracterized by a dry climate dominated by steady winds. Indeed, thermal winds generated daily over the Sahara's hot surfaces are actually superimposed upon the larger trade wind system coming from the Atlantic. This generates one of the most extensive and steadiest wind systems available on earth.
The North to North-East wind directions resulting from the merging of these global and local effects are typical for a Trade Wind region. The Trade Winds are actually the main factor responsible for the Sahara's extreme dryness by moving away the clouds from the ocean.
The Trade Winds have been known for centuries. During the Age of Sail, the pattern of these prevailing winds determined transatlantic sailing routes impacting thereby the history of European empire-building. The latter is in fact responsible for most of the world's modern political geography.
The Trade Winds shaped the vast majority of the Saharan coastline into inert rocky plateaus, called "Hammadas".
With funding made available by the North Atlantic Treaty Organization
, complete data on wind characteristics -in particular wind speeds at various heights- have been consistently monitored on desert sites throughout Morocco and Mauritania for several years. Established through regional academic and industrial partnerships in support of the Sahara Wind Project
the measurements confirmed the exceptional quality and scale of the aforementioned North-African Trade wind resource.
(*) Source: United States Geological Survey - Geology and Nonfuel Mineral Deposits of Africa and the Middle East - 2009