A movement of ocean water characterised by regularity, either of a cylic nature, or more commonly as a continuous stream flowing along a definable path. Three general classes, by cause may be distinguished: ( a) currents related to sea water density gradients, comprising the various types of gradient current ; ( b) wind-driven currents , which are those directly produced by the stress exerted by the wind upon the ocean surface; ( c) currents produced by long-wave motions. The latter are principally tidal currents , but may include currents associated with internal waves , tsunamis , and seiches . The major ocean currents are of continuous, stream-flow character, and are of first-order importance in the maintenance of the earth's thermodynamic balance.