Australia: The Land Where Time Began
The Ekman Response - Ekman Transport
The Ekman Response is a phenomenon of the near-surface layer which is a response to wind forcing, forcing flow in the surface layer of the ocean off to the right in the Northern Hemisphere and to the left in the Southern Hemisphere as a result of the Coriolis force. In the surface layer turbulence acts like friction, each thin layer in the surface layer pushing the one below a bit more to the right, in the Northern Hemisphere, with a slightly lower velocity than the layer above it. The result of this is an "Ekman Spiral", decreasing velocity with increasing depth, the entire spiral occurring within the top 50 m of the ocean. To calculate the total transport in the Ekman layer all the velocities are added together which gives the total transport in the Ekman layer, the net effect is the Ekman transport moves at exactly right angles to the wind direction in the Northern Hemisphere and to the left in the Southern Hemisphere. The Ekman velocities are small and don't generate turbulence. Therefore they do not have a direct effect on the development of mixed layers, and they are affected by turbulence in the upper layer but not by the stratification of the mixed layer. For conveying the effect of the wind to the ocean the Ekman response is crucial, and for the development of large-scale, long timescale circulation in the ocean.
The Ekman transport is "exactly" perpendicular to the wind direction and in the Northern Hemisphere is to the right of the wind direction and to the left in Southern Hemisphere. In respect of application of Ekman layers to general circulation, only the Ekman transport matters. Therefore, both the eddy viscosity and thickness of the Ekman layer are unimportant.
Ekman bottom layer
Turbulence near the bottom of the ocean, and therefore mixing, can be generated by currents or current shear that results from interaction with the ocean floor. When the water is shallow, as in coastal waters, complete mixing of the water column occurs if the depth is shallow enough and the currents rapid enough (Simpson, 1998; Brink, 2005) and a more extended discussion on the web site for the text book1 (Section S7.4.2). A bottom Ekman layer can form at longer timescales on the shelf in which there are balanced frictional and Coriolis forces that are balanced (Ekman, 1905) and Section 7.5.3), with the layer also being affected by the slope of the seafloor.
Viscous (frictional processes) that extend several 10s of metres down into the ocean, can communicate wind stress to the surface layer of the ocean. The response is strongly affected by Coriolis acceleration in this frictional layer, that is wind driven, is the Ekman layer. In an Ekman layer the physical processes include only frictional (viscosity of eddies) and Coriolis acceleration. At the surface of the ocean the velocity in the Ekman layer is at its strongest, decaying exponentially downwards and disappearing at a depth of about 50 m. It coexists with, though is not the same as, the mixed layer depth or euphotic zone depth.
The 2 characteristics of an Ekman layer that are most unusual, when compared with frictional flow that does not rotate, are the horizontal velocity vector and the net transport integrated through the Ekman layer is exactly to the right of the wind flow in the Northern Hemisphere and to the left in the Southern Hemisphere.
If there is a flow along the bottom, bottom Ekman Layers can form that are 50-100 m thick. If the water is shallow enough the top and bottom Ekman layers can overlap with the tendency of the top layer to turn right, in the Northern Hemisphere, will overlap with the left-turning bottom layer. If at the top surface there is a wind stress that would produce an Ekman layer of depth DE in deep water, then a water depth h, then the approximate angle, α, is as listed in Table S7.2. As the depth increases the net flow is progressively more in the direction of the wind.
Ekman transport convergence & wind stress curl
According to the authors1 when wind stress varies with position so that Ekman transport varies with position, then within the Ekman layer there can be a divergence or convergence. Downwelling of water out of the Ekman layer results from convergence and upwelling into the Ekman layer results in divergence. It is this mechanism that connects wind powered frictional forcing of the surface layer to the interior, geostrophic ocean circulation.
In the Northern Hemisphere upwelling into the Ekman layer results from positive wind stress curl and downwelling from the Ekman layer results from negative wind stress curl. Downwelling is Ekman pumping and upwelling from the Ekman layer is sometimes called Ekman suction.
The trade winds, or westward wind stress, results in Equatorial upwelling due to Ekman transport, and these winds cause northward Ekman transport in the Northern Hemisphere and southward Ekman transport in the Southern Hemisphere. The result is upwelling along the Equator, though there is only small wind stress curl, more or less because of the Coriolis parameter dependence.
Ekman convergence and divergence can also occur along the coastline, the divergence being due to the boundary conditions at the coast and not to wind stress curl. In this situation Ekman transport is perpendicular to the coast if the wind blows along the coast, which requires that to feed the Ekman layer there must be either upwelling or downwelling at the coast. This is 1 mechanism by which coastal upwelling is induced and the subtropical western boundary current systems are caused to form.
Ekman response and wind forcing - observations
There are major consequences for the wind-driven ocean circulation that result from the Ekman theory that make it important to confirm and refine Ekman's theory by observations of the ocean. It is difficult to make observations of the Ekman response because of the time-dependence of the wind. Observations of the California Current has produced an Ekman-like response that is easily visible as the direction of the wind was relatively steady (Chereskin, 1995).
For large parts of the Pacific Ocean an Ekman response to the wind is apparently in the average 15 m velocity from surface drifters, with velocities to the right of the wind stress in the Northern Hemisphere and to the left in the Southern Hemisphere.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|