8.11: Inertial Currents

Once established by the wind, currents will continue to flow even after the wind stops, until the momentum built up by the wind is dissipated by friction. If there is no other wind or pressure gradient force on the water after the wind stops, the water will flow in a curved path under the influence of the Coriolis effect. If the current is limited to a narrow range of latitude within which the Coriolis effect does not change significantly in magnitude (CC12), the water flows in circles. The circular currents are called “inertial currents.” Although inertial currents flow even after wind has stopped, they are ultimately wind-driven currents because winds are responsible for starting the water in motion and thus provide the momentum needed to drive the currents.

Inertial currents occur frequently throughout the oceans, but the water rarely moves in perfect circular patterns. In Figure 8-19, for example, a circular inertial current is superimposed on a mean drift to the north-northwest. The inertial current and mean wind-driven current were generated by winds blowing at different times, different velocities, and probably different locations. Studies of ocean currents are always complicated by the simultaneous presence of wind-driven and inertial currents, which are caused by different wind events but combine to form the current actually measured at any given place and time. In fact, measured currents are even more complex because geostrophic currents, tidal currents (Chap. 10), and wave-induced longshore currents (Chap. 9) are also combined with inertial and other wind-driven currents.

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