8.5: Open-Ocean Surface Currents

Surface currents of the open oceans (Fig. 8-3) are wind-driven currents initiated by Ekman transport and maintained as geostrophic currents. Although they typically extend to depths of several hundreds of meters, they may extend as deep as 2000 m in some limited cases. The most obvious features of ocean surface currents are the gyres in subtropical latitudes of each ocean. Separate subtropical gyres are present north and south of the equator in each ocean except in the Indian Ocean, where the Northern Hemisphere gyre would occur at a location occupied by landmasses. Subtropical gyres in the Northern Hemisphere flow clockwise; those in the Southern Hemisphere flow counterclockwise.

At latitudes above the subtropics, ocean surface currents are more complicated. The North Atlantic and North Pacific oceans have high-latitude gyres similar to, but less well formed than, the subtropical gyres. The high-latitude gyres rotate in the opposite direction (counterclockwise) from the northern subtropical gyres. In the Southern Hemisphere, high-latitude gyres are absent because the surface current that flows clockwise from west to east is not interrupted by a landmass, so it flows completely around Antarctica.

Geostrophic Currents on a Water-Covered Earth

If there were no continents, geostrophic surface layer currents would be relatively simple. The climatic winds and convection cells that would be present on a rotating, water-covered Earth are shown in Figure 7-10. Figure 8-8a shows these idealized climatic winds, and the directions in which the wind-driven layer of ocean water would be moved by Ekman transport due to the winds. In the trade wind and polar easterly zones, Ekman transport is partly toward the pole and partly toward the west. In the westerly wind zones, Ekman transport is toward the equator and to the east. Surface water layer divergences and upwelling of subsurface water occur at the equator and at the atmospheric upwelling regions between the polar and Ferrel cells in each hemisphere. Surface water convergences occur at the atmospheric downwelling regions between the Hadley and Ferrel cells in each hemisphere. Thus, Ekman transport would produce a depression of the sea surface at the equator and between the polar and Ferrel cells in each hemisphere, and an elevation of the sea surface between the Hadley and Ferrel cells in each hemisphere (Fig. 8-8b).

Figure 8-8. Ocean surface layer currents are driven by climatic winds and modified by the presence of landmasses. (a) The climatic winds on an idealized water-covered Earth would cause Ekman transport of surface waters in the directions shown. (b) Those water movements would lead to the formation of convergence and divergence, and geostrophic currents would flow on the pressure gradients created by the Ekman transport of the surface layer. These geostrophic currents would flow around the Earth from east to west, or vice versa, between the latitudes of the convergences and divergences. (c) However, landmasses block the flow of geostrophic currents and divert them to the north or south, resulting in the formation of geostrophic gyres.

The sloping sea surfaces created by climatic winds establish horizontal pressure gradients within the water column and thus establish geostrophic currents. If an equilibrium were reached, the geostrophic currents would flow around the Earth from east to west or from west to east across the pressure gradients (Fig. 8-8b). Currents would flow to the west in the trade wind zones and polar regions and to the east in the westerly wind zones. Geostrophic currents on the real Earth cannot flow in this way because they are interrupted by continents in all but the westerly wind zone around Antarctica. The Antarctic Circumpolar Current does, in fact, flow from west to east around the Earth in the Southern Hemisphere westerly wind zone (Fig. 8-3), just as the no-continent model suggests (Fig. 8-8b).

Geostrophic Gyres

Around each of the three major oceans, continents provide a western boundaries (the Atlantic coast of North and South America, Pacific coast of Asia and Australia, and Indian Ocean coast of Africa) and eastern boundaries (the Atlantic coast of Europe and Africa, Pacific coast of North and South America, and Indian Ocean coast of Southeast Asia and Australia). Within each ocean, geostrophic east-to-west or west-to-east currents are established at the same latitudes and in the same locations as those in the simple ocean-covered Earth model (Figs. 8-3, 8-8b). Where these currents meet a continent, they are blocked and diverted to the north or south.

The westward-flowing currents within the trade wind zones of the Northern and Southern Hemispheres are called the North Equatorial Current and the South Equatorial Current, respectively (Fig. 8-3). These Equatorial currents carry large volumes of water from east to west until they meet the western boundary continent. Surface water “piles up” there, and the Equatorial currents are deflected away from the equator. The currents then flow toward higher latitudes as western boundary currents until they enter the westerly wind zone, where they join the west-to-east geostrophic flow. When the currents reach the eastern boundary land mass, they are deflected primarily toward the equator. After turning toward the equator, they flow as eastern boundary currents until they enter the trade wind zone, rejoin the Equatorial Current, and complete their circuit, or gyre (Figs. 8-3, 8-8c).

As western boundary currents flow toward the pole, they are subject to the Coriolis effect. Therefore, if there were no opposing horizontal pressure gradient, they would be deflected to the east. Similarly, as eastern boundary currents flowed toward the equator, they would be deflected to the west (Fig. 8-9a). In each case, the deflection would be toward the center of the gyre. In addition, Ekman transport of surface water in the trade wind and westerly wind zones and the Coriolis deflection of the gyre currents, including the boundary currents, all tend to move water toward the center of the gyre. As a result, an equilibrium situation is created in which there is a mounded sea surface at the center of the gyre (Fig. 8-9b). Currents that make up the gyre flow around the mound in geostrophic balance between the Coriolis effect and the horizontal pressure gradient that surrounds this central location.

Figure 8-9. Geostrophic currents of the subtropical gyres. (a) Wind-driven Ekman transport due to the trade winds and westerlies, and the Coriolis deflection of the boundary currents, both act to elevate the ocean surface at the center of each subtropical gyre. (b) This elevated sea surface creates a horizontal pressure gradient that tends to move water outward. As water flows on this pressure gradient, it is deflected by the Coriolis effect and flows geostrophically around the mound. At equilibrium, the sea surface elevation does not vary because the tendency of water to be moved to the center of the mound is balanced by the pressure gradient, which generates geostrophic currents. Thus, the wind energy of the trade winds and westerlies is transferred into the geostrophic currents that flow continuously and form the subtropical gyres.

Hence, gyres are geostrophic currents that flow on mounded surfaces. The center of each subtropical gyre is called a “subtropical convergence” (Fig. 8-3). Subtropical gyres are essentially the same as the idealized gyres shown in Figure 8-9. However, they are modified in shape and location by the configuration of each ocean’s coastlines (Fig. 8-3).

Geostrophic ocean gyres act like giant flywheels, which use angular momentum to store rotational energy. They spin at an almost constant speed that represents the average wind energy input to the gyre. If winds cease or are abnormally light, the geostrophic current gyre, or flywheel, slows very gradually because turbulence and friction between the gyre currents and the adjacent water masses or seafloor dissipate the energy very slowly. When winds resume, Ekman transport builds the height of the mound, increasing the sea surface slope and speed of the gyre. This process occurs very slowly because massive volumes of water must be moved to elevate the sea surface even a millimeter or two over a mound that is hundreds or thousands of kilometers in diameter. Winds that drive the ocean gyres are variable over periods of days, but very long periods of calm or unusually high winds are rare. Because gyre current speeds change very slowly, they are relatively stable and generally do not reflect normal wind speed variations.

Westward Intensification of Boundary Currents

Western boundary currents include the Gulf Stream and the Brazil, Kuroshio, East Australian, and Agulhas Currents (Fig. 8-3). Eastern boundary currents include the California, Peru, Canary, and Benguela Currents. Western boundary currents are narrower, faster, deeper, and warmer than eastern boundary currents (Table 8-1). They are generally so deep that they are constrained against the edge of the continental shelf and do not extend across the continental shelf to the shore.

Table 8-1. Boundary Currents 

	Western Boundary Currents	Eastern Boundary Currents
Northern Hemisphere examples	Gulf Stream,  Kuroshio Current	California Current,  Canary Current
Southern Hemisphere examples	Agulhas Current,  Brazil Current	Peru Current,  Benguela Current
Width	Narrow (≤100 km)	Broad (≈1000 km)
Depth	Deep (to 2 km)	Shallow (≤500 m)
Speed	Fast (>100 km•day^–1)	Slow (<50 km•day^–1)
Volume transport	Large (50 ×106 m^3•s^–1)	Small (10–15 ×106 m^3•s^–1)
Boundaries with coastal currents	Sharply defined	Diffuse
Upwelling	Almost none	Frequent
Nutrients	Depleted	Enhanced by upwelling
Fishery	Usually poor	Usually good
Water temperature	Warm	Cool

 

The reasons for the westward intensification of boundary currents are quite complex. However, the major reason is related to the Earth’s rotation. Western boundary currents are intensified because the strength of the Coriolis effect varies with latitude.

We can look at westward intensification in a simplified way. The Coriolis effect, which increases from the equator to the poles, is weaker in the latitudes of the trade wind band than in the westerly wind band. Therefore, water moving eastward is deflected more quickly toward the south in the westerly wind zone than water moving westward is deflected toward the north in the trade wind zone (Fig. 8-10). Consequently, in the westerly wind zone, geostrophic flow tends to transport surface water toward the center of the gyre over the entire width of the ocean. In contrast, surface water transported westward in the trade wind zone tends to flow with less deflection (that is, in a circle of larger radius; CC12) across the ocean (Fig. 8-10), is constrained at the equator by the Ekman transport of the trade winds in the other hemisphere, and tends to pile up on the west side of the ocean.

Figure 8-10. The westward-moving geostrophic currents of the subtropical gyres flow near the equator, where the Coriolis deflection is weak, whereas the eastward-moving currents flow where the Coriolis deflection is much greater. Hence, the westward-moving water near the equator flows across the ocean more directly, piling up water on the west side. The greater Coriolis deflection further from the equator tends to turn parts of the eastward-moving gyre currents toward the equator even before it reaches the eastern boundary continent, “spreading” the southerly part of the gyre currents that returns water from the westerly region toward the equator.

As a result, the mound at the center of the subtropical gyre is offset toward the west side of the ocean (Fig. 8-11). The western boundary current is laterally compressed against the continent, and the sea surface slope is steeper toward the western boundary than toward the eastern boundary. Steeper sea surface slopes cause faster geostrophic currents. Therefore, western boundary currents are faster than eastern boundary currents. The westward offset of the subtropical gyre mound causes the pycnocline to be depressed (deeper) on the west side of the ocean in relation to the east side (Fig. 8-11). Hence, western boundary currents are narrower and deeper than eastern boundary currents.

Figure 8-11. In the Northern Hemisphere, the sea surface elevation in the center of the subtropical gyres is displaced to the west. Thus, the horizontal pressure gradients are stronger and the gyre currents are faster on the west side of the ocean than on the east side. In addition, because of this displacement, the mixed layer is thicker on the west side and the deep layer does not extend onto the continental shelf.

This simplified explanation serves reasonably well. However, for a more detailed understanding of westward intensification, it is necessary to examine a difficult concept called “vorticity” (see Online Box 8B1).

The characteristic differences between western and eastern boundary currents have major consequences for processes that sustain fisheries off the adjacent coasts (Chap. 12). The differences are also an important factor in the transport of heat poleward (Chap. 7). Warm western boundary currents are fast-flowing and deep. They have little time to cool and only a small surface area from which to lose heat as they move into higher latitudes. Therefore, heat energy carried away from the equator by western boundary currents is released to the atmosphere primarily when the water enters the westerly wind zone, where the gyre currents flow eastward. Heat transported by western boundary currents moderates the climates of regions into which they flow. For example, the Gulf Stream transports heat north and then east across the Atlantic Ocean, moderating western Europe’s climate.