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Ekman Transport is another phenomenon important to an understanding of geostrophic currents. Named for the Swedish oceanographer who first described it, and other principles, Ekman Transport is the net spiral motion down a water column created by Coriolis force and drag. Away from the surface and the direct effects of the wind, the current direction veers slightly toward the right or left with increasing depth down to about 100 m. A good way to envision this is to think of the water column as a stack of layers or playing cards, with each layer moving slightly to the right (or left in the southern hemisphere) of the layer above it (Figure 1.9). Due to friction, current speed decreases with increasing depth so that each successively deeper layer moves more slowly than the one above. In theory the resultant net flow over the upper 100 m is at 90° to wind direction, 90° to the right of wind direction in the northern hemisphere, and 90° to the left in the southern. However, the actual net flow of the Ekman Spiral is closer to 45° in both halves of the globe.

Figure 1.8 Coriolis effect. Apparent curved path of an object not coupled to the Earth's surface, moving in the northern hemisphere.

Source: Brown et al. (1989), figure 1.2a (p. 7). Reproduced with the permission of Pergamon Press.

Figure 1.9 Ekman transport. Net spiral pattern of wind‐driven motion down through a water column due to Coriolis effect and drag. The result is a net flow 90° to the right of the wind direction in the northern hemisphere. See color plate section for color representation of this figure.

Geostrophic currents are the result of a dynamic balance between the driving force of the wind, the turning effects of the Coriolis force, and pressure gradients caused by differences in sea‐surface height. Ekman Transport and wind stress act to create a slight hill of water, or topographic high, roughly in the middle of a gyre. Water in the high attempts to flow downhill but is offset by the Coriolis force so that the current in the gyre becomes parallel to the elevated sea surface, flowing clockwise in the northern hemisphere and counterclockwise in the southern.