Ekman spiral

The corkscrew flow, also Ekman Spiral (after Vagn Walfrid Ekman ) is a vertical velocity profile that occurs because wherever the wind blows ( away from the Earth's equator ) is continuous over a vast expanse of water. In sum - over the depth range of the Ekman spiral - water is transported transverse to the wind direction. This is the understanding of the wind-driven circulation material, see Detection and significance of the Ekman - transport.

Description

The wind moves the water through friction with it. Whose motion is deflected by the Coriolis force, on the northern hemisphere to the right. Each deeper water layer is entrained by each overlying and braked by the underlying. The different directions and magnitudes of the friction forces balance precisely the Coriolis force on the considered layer. From this condition it follows that the deeper water layers move slower and the movement towards increasingly deviates from the wind direction. Specifically, the velocity vectors form a logarithmic spiral with Therein is the angle of rotation in radians. After half a revolution of the magnitude of the velocity has obtained on a fraction.

The depth dimension of the coil results from the viscosity and the density of the water and of the vertical component of the rotational speed. In the so-called Ekman depth - in middle latitudes at about fifty meters - the water flows opposite to the wind direction. This is considered as the limit of the influence of wind friction.

The surface of flow is turned through 45 ° relative to the wind direction and reached about three percent of the wind speed.

Just as there are on the surface, an Ekman spiral, there is also an Ekman spiral in the air above the water surface or on the ground or in the water above the seabed, if this water flows. It has a reverse rotation. Thus, the resulting speed turns with greater depth to the left ( in the northern hemisphere ). In shallow water, the two spirals overlap. If the water is shallow enough, the resulting water surface flow in the direction of the wind.