8.2: Wind-Driven Currents

On a windy day at the shore, we can easily see that winds cause ocean water motions such as waves. Winds also create currents that can transport large volumes of water across the oceans.

Generation of Currents

When winds blow across the ocean surface, energy is transferred from wind to surface water as a result of the friction between the wind and the surface. The energy transferred to the ocean surface sets the surface layer of water in motion, generating both waves and currents. The process of energy transfer from winds to waves and currents is complex and depends on many factors, including wind speed, surface tension of the water, and roughness of the surface (that is, whether waves are already present, their height, and whether they are breaking). Therefore, the percentage of wind energy that is converted into kinetic energy of ocean currents is highly variable. With a steady wind, the speed of the surface current is generally between 1% and 3% of the wind speed, so a wind of 60 km•h^–1 will generate a surface current of about 1 to 2 km•h^–1.

Surface water set in motion by the wind flows horizontally across the water below it. Because of internal friction between the surface water and the water below, wind energy is transferred downward. If we consider the water column to consist of a series of very thin horizontal layers, we can envision the moving surface layer transferring some of its kinetic energy to the next lower layer by friction, setting that layer in motion. The second layer, in turn, transfers some of its resulting kinetic energy to the layer below it, setting that layer in motion (Fig. 8-2a). However, only a fraction of the kinetic energy of each moving layer is transferred to the layer below it. Consequently, the speed of a wind-driven current decreases progressively with depth. Wind-driven currents are restricted to the upper 100–200 m of the oceans and generally to even shallower depths.

Figure 8-2. (a) Wind energy is converted to water movements, called currents, by friction between the moving air and the water surface. The resulting kinetic energy of the water at the surface is transferred vertically downward into the water column by friction between the water molecules. Friction occurs when the molecules of one layer are moved, as shown in the enlarged diagram of three adjacent molecules. As the upper-layer molecules are moved, the distance between them and the molecules immediately below changes. At point A, the distance between molecules is slightly decreased. With a decrease in distance the repulsive force between the molecules’ electron clouds increases faster than the attractive gravitational force between the molecules (partly because the electron clouds are closer to each other than the two centers of mass are). Thus, the molecule below is “pushed” forward and downward. Conversely, the distance between molecules at the point labeled B is slightly increased. Since the repulsive force between the electron clouds is reduced by more than the gravitational force is reduced, the molecule below is “pulled” forward and upward. This process transfers kinetic energy downward through successive layers of molecules. (b) Sloped sea surfaces can be created in the open ocean when winds transport the surface water layer either away from a divergence, as shown here, or toward a convergence that then becomes an elevated area of sea surface. (c) Sloped sea surfaces can also be created at a coastline when winds transport the surface water layer offshore or onshore. The sea surface slopes are greatly exaggerated in these diagrams.

Restoring Forces and Steering Forces

Once current motion has been started, it will continue for some time after the wind stops blowing because the water has momentum. It is like a bicycle that continues to roll forward after the rider stops pedaling. Just as a bicycle slows and eventually stops because of friction in the wheel bearings and between tires and road, ocean currents slow and would eventually stop if winds stopped blowing and did not restart. The energy of a current is dissipated by friction between water layers flowing over each other. However, the frictional force between moving layers of water is small. This is one reason why the currents created by winds can flow for long periods after the winds stop and can also flow into and through regions with little wind, such as the Doldrums (Figs. 7-10, 7-11).

There is a second reason for the continued flow of wind-created currents after the winds stop. Transport of surface water by wind-driven currents while winds are blowing can cause the sea surface to be sloped (Fig. 8-2b,c). Sea surface slopes created by wind-driven transport are extremely small, no more than a few centimeters of height difference across hundreds or thousands of kilometers of ocean surface. However, when the surface is sloped, a horizontal pressure gradient is formed (CC13), aligned in the same direction as the sea surface slope. The water flows from high pressure toward low pressure and tends to restore the ocean surface to a flat horizontal plane. 

Once set in motion, the speed and direction of any current are modified by friction and three other factors. First, any body in motion on the Earth, including water moving in currents, is subject to deflection by the Coriolis effect (CC12). Second, the presence of coasts can block current flow and cause the water to mound up or to be deflected. Third, current speed and direction are affected by the presence of horizontal pressure gradients.

The following sections examine how the interactions between climatic winds and the restoring and steering forces account for the location and characteristics of surface currents in the oceans (Fig. 8-3).

Figure 8-3. The wind-driven surface currents of the oceans and the locations of the major convergences and divergences are related. The surface currents form subtropical gyres both north and south of the equator in each ocean basin, and there are subtropical convergences within these gyres. Convergences are also found at higher latitudes between the subtropical gyres and the Antarctic Circumpolar Current, and between the subtropical gyres and the complicated high-latitude gyres in the Northern Hemisphere. Divergences lie on either side of the equator between the subtropical gyres and surrounding Antarctica.