Over the Equator, large hot air masses rise into the upper layers of the atmosphere. Pursuing a natural convection effect, these will seek to migrate towards the colder temperatures of the Earth's Poles. Due to its rotation and a resulting Coriolis force, these air masses are deviated clockwise in the Northern Hemisphere. This happens at around 30° of latitude over the Atlantic Ocean. When sinking down, they create a high pressure area (called Anticyclone) generally centered on the Azores Islands.
These winds originating from the Equator are largely driven by temperature differences on a planetary level. As this translates into atmospheric pressure variations, they can therefore be considered as global winds.
Unlike local winds that are influenced by the Earth's topography, the "Trade Winds" as they are referred to, result instead from the combined influences of the rotation of the Earth (Coriolis forces) and its longitudinal temperature differences.
The latter effect is relevant to the solar radiation on a spherical surface which -due to its incidence angle- cannot be as intense on the center of a sphere (Earth's Equator) versus its Poles. As the hot air masses generated over the Equator are deviated to the right by the rotation of the Earth, this results into a perpetual clockwise circular air movement over the surface of the North Atlantic Ocean.
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 (75 % of World reserves*) trapped at the bottom of Morocco’s Atlas Mountains, confirm that the 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 (vertebrates for the most part) whose remains, pushed by the Trade Winds induced currents have accumulated in this area.
The concentrations of phosphorus-rich organic debris covering thousands of square kilometers drifted away over the Atlantic Ocean and ended-up trapped at the bottom of North Africa's Atlas mountain ranges. These, highlighted by the yellow triangles shown on the map to the left (over Khouribga, Benguérir, Youssoufia and Boucrâa) accumulated during the Paleocene and Eocene epoch some 50 Millions years ago. Today, they represent by far the largest phosphate-rock deposits available worldwide, that are commercially mined.
Further to the South on the African coast and the junction between the Sahara desert with the Atlantic Ocean, the Trade Winds created a zone of global energy exchange characterized by a dry climate dominated by steady winds. In this area, the thermal winds generated daily over the Sahara's hot surfaces are superimposed upon the larger Atlantic Ocean Trade Wind system. This generates one of the most extensive and steadiest wind regime 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-led migrations and empire-building. It must be noted that the latter is in fact responsible for most of the world's modern political geography.
On a geomorphologic scale over the African continent, the Trade Winds have shaped the vast majority of the Saharan coastline into inert rocky plateaus, called "Hammadas".
Today, with funding made available by the North Atlantic Treaty Organization
under its scientific collaborative frameworks with neighboring countries, 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 an end-user driven regional academic research and industrial partnership in support of the Sahara Wind Project
the measurements confirmed the exceptional quality and scale of the aforementioned North-African Trade wind resource.
(*) Source: U.S. Geological Survey, Mineral Commodity Summaries, January 2013