Ekman Current and Transport

The turbulence in the surface mixed layer is associated with a vertical momentum flux, the Reynolds stress, which has a vertical gradient. An equilibrium can develop in the surface mixed layer when the vertical gradient of the Reynolds stress and the Coriolis force acting on the water parcels moving in the surface mixed layer balance each other. 

The upper ocean wind-driven current was described realistically for the first time by Walfried Ekman s landmark theory of 1905. The velocity distribution in the near surface layer of the ocean cannot be determined without additional information about the variation of the Reynolds stress vector with depth. Ekman (1905) assumed the Reynolds stress vector to be equal to the vertical shear of the mean current vector times a constant vertical eddy viscosity. The resulting current profile below the sea surface is the well known Ekman spiral with current speed decreasing exponentially with depth and current direction turning clockwise linear with depth from 45° right-handed to the wind stress vector at the sea surface. 

Many attempts have been made to verify Ekman s theory with observations. Clockwise turning mean current spirals that decay smoothly with depth have been observed by numerous investigators (Davis et al., 1981 Weller, 1981 Price et al., 1986, 1987 Weller et al., 1991 Rudnick and Weller, 1993 Wijffels et al., 1994 Chereskin, 1995 Lee and Eriksen, 1996 Weller and Plueddemann, 1996). Several ofthem observe a spiral thatis much flatter than an Ekman spiral in that the observed current rotates less with depth than predicted by Ekman s theory. 

An alternative perspective is the assumption that the Reynolds stress is an external function of time and depth described as a volume force independent of the mean current (Pollard, 1970). Such a conceptual model is valid if the turbulence generating the Reynolds stress in the surface mixed layer is caused by physical mechanisms that are independent of the mean current. Pollard (1977) suggested that Langmuir circulation might by a very effective mechanism to distribute Reynolds stress downward in the surface mixed layer from the sea surface where all the momentum is received from the wind stress. 

Although the details of the vertical structure of the wind-driven current in the surface mixed layer depend on the vertical distribution of the Reynolds stress in the surface layer, the vertical integrated wind-driven current, the Ekman transport, depends only on the wind stress at the sea surface. 

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