- Key People:
- V. Walfrid Ekman
- Related Topics:
- ocean current
Ekman layer
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What is the Ekman layer?
The Ekman layer is a zone in the ocean affected by wind-driven surface waters, named after Swedish oceanographer V. Walfrid Ekman. It extends approximately 100 meters deep.
Wind stress on the surface sets the water in motion. Within the Ekman layer, the wind stress is offset by the Coriolis force and frictional forces. Surface water moves at a 45° angle to the wind (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). As depth increases, the current speed decreases, and the direction rotates away from the wind, forming a spiral.
Ekman transport causes zones of convergence and divergence of surface water. Convergence forces surface water downward (downwelling), while divergence draws water from below (upwelling). Upwelling cools surface water and brings nutrient-rich water to the surface, creating biologically productive regions. The ocean current pattern produced by the wind-induced Ekman transport is called the Sverdrup transport.
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Where is the Ekman layer found in the ocean?
The Ekman layer is a vertical region in the ocean influenced by wind-driven surface waters, extending to approximately 100 meters in depth. Within this layer, wind stress is balanced by the Coriolis force and frictional forces.
Ekman transport, a phenomenon widely observed in the oceans, occurs when the average water particle within the Ekman layer moves at an angle of 90° to the wind. This movement is to the right of the wind direction in the Northern Hemisphere and to its left in the Southern Hemisphere.
Ekman transport results in convergence and divergence zones of surface water. Convergence forces surface water downward (downwelling), while divergence draws water from below into the surface Ekman layer (upwelling). The primary upwelling regions are along the eastern boundary of subtropical ocean waters, such as the coasts of Peru and northwestern Africa.
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Who discovered the Ekman layer and why is it named after them?
The Ekman layer is named after Swedish oceanographer V. Walfrid Ekman, who first deduced its existence in 1902. Ekman developed a theoretical model to explain observations of wind drift in the Arctic, leading to his discovery of this vertical region of the ocean that is affected by wind-driven surface waters. The Ekman spiral and Ekman transport are also named after him.
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How does wind affect the movement of water in the Ekman layer?
Wind stress, which is proportional to the square of wind speed and direction, sets the surface water in motion in the Ekman layer. Within this layer, the wind stress is balanced by the Coriolis force and frictional forces.
The surface water is directed at a 45° angle to the wind, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As depth increases, current speed decreases, and the direction rotates away from the wind, forming a spiral. The average water particle moves at a 90° angle to the wind, a phenomenon known as Ekman transport.
Ekman transport varies with the wind, creating convergence and divergence zones. Convergence forces downwelling, while divergence causes upwelling, affecting nutrient distribution and marine life.
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What is the Ekman spiral and what causes it?
The Ekman spiral is the theoretical effect of the Coriolis effect on current direction, assuming a steady wind over an ocean of infinite depth and uniform viscosity.
Wind-driven stress on the ocean surface causes the Ekman spiral. Surface water moves at a 45° angle to the wind (right in the Northern Hemisphere and left in the Southern Hemisphere). As depth increases within the Ekman layer, current speed reduces, and the direction rotates away from the wind, forming a spiral. At the layer's base, water motion opposes the wind direction.
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How does the Ekman layer influence ocean currents and upwelling?
The Ekman layer, named after Swedish oceanographer V. Walfrid Ekman, is a region in the ocean affected by wind-driven surface waters, extending to about 100 meters in depth. Wind stress sets the surface water in motion, balanced by the Coriolis force and frictional forces within the Ekman layer. The surface water is directed at a 45° angle to the wind (right in the Northern Hemisphere and left in the Southern Hemisphere). As depth increases, current speed reduces, and the direction rotates away from the wind, forming a spiral.
Ekman transport causes convergence and divergence zones due to wind variations. Convergence forces surface water downward (downwelling), while divergence draws water from below (upwelling). Upwelling occurs along the eastern boundary of subtropical ocean waters, such as Peru and northwestern Africa, cooling surface water and bringing nutrient-rich subsurface water into the sunlit layer, creating biologically productive regions.
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Ekman layer, a vertical region of the ocean affected by the movement of wind-driven surface waters. This layer, named for the Swedish oceanographer V. Walfrid Ekman, extends to a depth of about 100 metres (about 300 feet). Ekman deduced the layerâs existence in 1902 from the results obtained from a theoretical model he constructed to help explain observations of wind drift in the Arctic.
Wind-driven stress on the ocean surface is proportional to the square of the wind speed and the direction of the wind; it is this force that sets the surface water in motion. Within the oceanic Ekman layer, the wind stress is balanced by the Coriolis force and frictional forces. The surface water is directed at an angle of 45° to the wind, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. With increasing depth in the boundary layer, the current speed is reduced, and the direction rotates farther away from the wind direction following a spiral form, becoming antiparallel to the surface flow at the base of the layer where the speed is 1/23 of the surface speed. This so-called Ekman spiral may be the exception rather than the rule, as the specific conditions are not often met, though deflection of a wind-driven surface current at somewhat smaller than 45° is observed when the wind field blows with a steady force and direction for the better part of a day. The average water particle within the Ekman layer moves at an angle of 90° to the wind; this movement is to the right of the wind direction in the Northern Hemisphere and to its left in the Southern Hemisphere. This phenomenon is called Ekman transport, and its effects are widely observed in the oceans.
Since the wind varies from place to place, so does the Ekman transport, forming convergence and divergence zones of surface water. A region of convergence forces surface water downward in a process called downwelling, while a region of divergence draws water from below into the surface Ekman layer in a process known as upwelling. Upwelling and downwelling also occur where the wind blows parallel to a coastline. The principal upwelling regions of the world are along the eastern boundary of the subtropical ocean waters, as, for example, the coastal region of Peru and northwestern Africa. Upwelling in these regions cools the surface water and brings nutrient-rich subsurface water into the sunlit layer of the ocean, resulting in a biologically productive region. Upwelling and high productivity also are found along divergence zones at the Equator and around Antarctica. The primary downwelling regions are in the subtropical ocean watersâe.g., the Sargasso Sea in the North Atlantic. Such areas are devoid of nutrients and are poor in marine life.
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The vertical movements of ocean waters into or out of the base of the Ekman layer amount to less than one metre per day, but they are important since they extend the wind-driven effects into deeper waters. Within an upwelling region, the water column below the Ekman layer is drawn upward. This process, with conservation of angular momentum on the rotating Earth, induces the water column to drift toward the poles. Conversely, downwelling forces water into the water column below the Ekman layer, inducing drift toward the Equator. An additional consequence of upwelling and downwelling for stratified waters is to create a baroclinic field of mass (that is, a volume whose horizontal pressure gradient varies with depth due to horizontal variations in temperature and salinity). Surface water is less dense than deeper water. Ekman convergences have the effect of accumulating less dense surface water. This water floats above the surrounding water, forming a hill in sea level and driving an anticyclonic current characterized by geostrophic motion that extends well below the Ekman layer. Divergences do the opposite; they remove the less dense surface water, replacing it with denser, deeper water. This induces a depression in sea level with a cyclonic geostrophic current.
The ocean current pattern produced by the wind-induced Ekman transport is called the Sverdrup transport, after the Norwegian oceanographer H.U. Sverdrup, who formulated the basic theory in 1947. Several years later (1950), the American geophysicist and oceanographer Walter H. Munk and others expanded Sverdrupâs work, explaining many of the major features of the wind-driven general circulation by using the mean climatological wind stress distribution at the sea surface as a driving force.