in the South Atlantic, driven westward by the trade winds, is the North Equatorial Current. The circuit is completed by the powerful Gulf Stream on the western limb and the Canary Current on the eastern one.
Figure 1.7 Coriolis effect. Differences in velocity of Earth's surface as a function of latitude. (a) Equatorial view. (b) Polar view.
Ekman Transport
Ekman Transport is another phenomenon important to an understanding of geostrophic currents. Named for the Swedish oceanographer who first described it, and other principles, Ekman Transport is the net spiral motion down a water column created by Coriolis force and drag. Away from the surface and the direct effects of the wind, the current direction veers slightly toward the right or left with increasing depth down to about 100 m. A good way to envision this is to think of the water column as a stack of layers or playing cards, with each layer moving slightly to the right (or left in the southern hemisphere) of the layer above it (Figure 1.9). Due to friction, current speed decreases with increasing depth so that each successively deeper layer moves more slowly than the one above. In theory the resultant net flow over the upper 100 m is at 90° to wind direction, 90° to the right of wind direction in the northern hemisphere, and 90° to the left in the southern. However, the actual net flow of the Ekman Spiral is closer to 45° in both halves of the globe.
Figure 1.8 Coriolis effect. Apparent curved path of an object not coupled to the Earth's surface, moving in the northern hemisphere.
Source: Brown et al. (1989), figure 1.2a (p. 7). Reproduced with the permission of Pergamon Press.
Figure 1.9 Ekman transport. Net spiral pattern of wind‐driven motion down through a water column due to Coriolis effect and drag. The result is a net flow 90° to the right of the wind direction in the northern hemisphere. See color plate section for color representation of this figure.
Geostrophic currents are the result of a dynamic balance between the driving force of the wind, the turning effects of the Coriolis force, and pressure gradients caused by differences in sea‐surface height. Ekman Transport and wind stress act to create a slight hill of water, or topographic high, roughly in the middle of a gyre. Water in the high attempts to flow downhill but is offset by the Coriolis force so that the current in the gyre becomes parallel to the elevated sea surface, flowing clockwise in the northern hemisphere and counterclockwise in the southern.
Ocean Gyres and Geostrophic Flow
Six great circuits are found in the world ocean, four in the southern hemisphere (South Atlantic, South Pacific, Indian, and Antarctic Circumpolar) and two in the northern (North Atlantic, North Pacific). The gyres correspond fairly well to the biogeographic distribution of oceanic species.
With the exception of the Antarctic Circumpolar Current (ACC), which is not technically a gyre since it flows uninterrupted around the Antarctic continent, all gyres flow around the continental margins (Figure 1.5). The circuits are bounded on the east and west by eastern and western boundary currents and on the north and south by transverse currents. The Gulf Stream in the North Atlantic is an excellent example of a western boundary current and the Canary Current, along west Africa, of an eastern boundary current. Partly because of the convergence of the westerly winds, western boundary currents are far stronger and more clearly defined than their counterparts in the east. Additional contributors to the greater strength of western boundary currents are the fact that the “hill of water” at the heart of the geostrophic circulation is displaced to the west by the eastward rotation of the Earth, creating a steeper pressure gradient and a stronger poleward flow of water. The phenomenon is termed “westward intensification.” Table 1.2 gives a comparison of the characteristics of eastern and western boundary currents.
Upwelling
Wind‐driven movement of water can also induce vertical circulation, particularly in coastal regions. On the west coasts of land masses in the northern hemisphere, winds out of the north or northwest cause along‐shore water movement, which is moved offshore (clockwise) by Ekman transport. In the southern hemisphere, winds from the south will result in similar coastal upwelling. The water moved offshore is replaced by cooler, nutrient‐laden water welling up from below (Figure 1.10), resulting in an ideal situation for increased productivity. Upwelling regions are the most fertile oceanic areas of the world, often supporting large fisheries for small coastal pelagic fishes such as sardines and anchovies. As we will see later, areas of high ocean productivity are often associated with zones of low oxygen at mid‐depths, resulting from the biological degradation of sinking particulates in a stratified water column. Upwelling can also occur offshore, which it consistently does at the equator where the north and south equatorial currents meet and at the Antarctic divergence. Downwelling, the opposite situation, occurs when winds and Ekman transport cause surface water to converge along a coast.
Deep‐Ocean Circulation
Oceanic waters are stratified by density, which is mainly a function of salinity and water temperature. The colder or saltier a parcel of water is, the greater its density. Temperature–salinity plots, or T‐S diagrams, help characterize ocean layering and water‐column characteristics. Figure 1.11, a T‐S curve for an oceanographic station in the Atlantic, is an example. Note that the lines of equal density, or isopycnals, each comprise a variety of temperatures and salinities; the same density can result from many different temperature–salinity combinations.
Vertical structure in the ocean can be divided into three density zones: an upper mixed layer, a layer of changing density, and the deep layer.
The upper mixed layer is a region of fairly uniform density because of the action of wind mixing, waves, and currents. Depending on place and season, it can vary from being very shallow (<30 m) to depths of greater than 200 m and is the only region of the ocean that interacts with the atmosphere. The upper mixed layer contains about 2% of the volume of the ocean.
Beneath the upper mixed layer is the pycnocline, where density increases rapidly with depth and the increasing density acts as a barrier between the upper and deep layers. If declining temperatures are mainly responsible for the increasing density, the pycnocline is also a thermocline. If salinity is the major cause, it is a halocline. The pycnocline extends from the bottom of the mixed layer to the cold and stable deep zone. In most regions of the world ocean, the pycnocline ends at about 1000 m of depth and contains about 18% of the world ocean volume.
