EKMAN DRIFT IN HOMOGENEOUS WATER

Measurements made with satellite-tracked buoys drogued in different la yers between the sea surface and 30-m depth under homogeneous winter c onditions in the North Sea allow analysis of the Ekman currents under a large variety of wind conditions. The experiment lasted from Novembe r 20, 1991, until February 29, 1992. The first 4 weeks of this period, during which the buoys stayed close together, are used to determine t he Ekman stresses. The total current field is a superposition of barot ropic currents due to sea level variations and Ekman currents. The cla ssical Ekman theory is not able to describe properly the observed defl ection of the currents to the right of the wind direction and their de cay with depth. This deflection is 10-degrees near the sea surface and increases to approximately 50-degrees in 25-m depth. The relation bet ween wind stress and the stress field in the interior of the water is given by a tensor, which describes the rotation and the variation of t he stress with increasing depth. The concept of eddy viscosity is appl icable, if a viscosity tensor is used to relate stress and vertical sh ear. The viscosity tensor is a function of the vertical coordinate onl y and is independent from the wind stress. It shows maximum values in 15- to 20-m depth and may be due to Langmuir circulation cells. Furthe r studies are needed to determine the physics of this tensor. Its magn itude in the interior of the mixed layer exceeds 1000 cgs units. Conse quently, Ekman currents are weak and may not be the dominant currents within the mixed layer.