Oceanography

EKMAN SPIRAL

Surface winds and ocean currents are intimately related. But how winds drive oceans currents is not so obvious. The process begins when winds blow across the water and drag on the surface. This sets a thin layer in motion which, in turn, drags on the one beneath, setting it too in motion. This process continues downward, transferring momentum.Such transfers of momentum between layers are inefficient, and energy is lost in the process. As a result, Current speed decreases with increasing depth below the surface. In an infinite ocean on a nonrotating earth, the waters would always move in the same direction as the wind that set it in motion.

Since the earth rotates, movements of surface waters are deflected to the right of the wind in the northern hemisphere is was noted by Fridtijof Nansen while studying the drift of Arctic ice. He found that the ice moved 20° to 40° to the right of the wind. To explain such effects, we assume a simple, uniform oceans with no boundaries. In such oceans, each layer sets in motion the layer beneath, so that the deeper layer moves more slowly than the one above. These movements can be represented by arrows (vectors) whose orientation shows current direction and whose length indicated current speed. The change in current direction and speed with increasing depth form a spiral when viewed from above. This is called the Ekman spiral. A spiral for the Southern Hemisphere exhibits the opposite sense of deflection, but current speeds still decrease with increasing depth.

Water column stability controls the depths to which wind effects penetrate below the sea surface. A strong pycnocline inhibits transfer of momentum from the surface to waters below the pycnocline. The limit for wind effects is usually taken to be the depth at which the subsurface current is exactly opposite to the surface current. Under strong winds, wind-drift currents may be as deep as 100 meters below the surface. Surface currents move at about 2% of the speed of the wind that caused them. For instance, a wind blowing at 10 meters per second causes a surface current of about 20 centimeters per second.