Wind action on the surface does not simply blow the water in the same direction as the wind, except in very shallow depths. The earth's rotation causes a deflecting effect so that surface water is moved at an angle to the wind. This deflection, generally known as the Coriolis effect after the French engineer and mathematician who first derived an equation for it, influences any object moving on the earth's surface, and is due to the rotational movement of the earth beneath the moving body. In most cases the effect is so small compared with other forces
involved that it can be ignored, but in movements of the atmosphere and oceans the Coriolis effect has a magnitude comparable with the forces producing the motion, and must certainly be taken into account in understanding the course of ocean currents (Ingmanson and Wallace, 1995).
The Coriolis effect is equivalent to a force acting at 90° to the direction of movement, tending to produce a right-handed or clockwise deflection in the northern hemisphere and a left-handed or counterclockwise deflection in the southern hemisphere. It is proportional to the speed of movement and to the sine of latitude, being zero at the Equator. The Coriolis deflecting force, F, acting on a body of mass m moving with velocity V in latitude $ can be expressed
F = 2cV sin co being the angular rate of rotation of the earth.
Thus the Coriolis deflection acts to the right of wind direction north of the Equator and to the left south of the Equator. Theoretically, in deep water of uniform density it results in a deflection of 45° to wind direction at the surface. This deflection increases with depth. The speed of the wind-generated current decreases logarithmically with depth and becomes almost zero at the depth at which its direction is opposite to the surface movement. The deflecting effect is less in shallow or turbulent water. A sharp temperature gradient near the surface has an effect similar to a shallow bottom. The warmer surface layer tends to slide over the colder water below, following wind direction more closely than it would if the temperature were uniform throughout the water column.
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