Oceanic Circulation. A Description of Currents in the Ocean

The water in the world ocean is always in motion. A variety of motions exists, each differing in the size and shape of its path and in the length of time that motion persists. Water circulates by following paths as large in scale as hemispheric, as in the North Pacific Ocean basin, and as small as the paths of eddies in the surf zone. Table 8-1 demonstrates the extent and diversity of the scales of circulation in the world ocean. The currents observed at any location in the ocean are a complex mixture of motions of several scales.

Table 8-1. Circulation in the World Ocean

If currents are observed over a substantial interval of time, the pattern of water motion changes depending upon the temporal scale associated with each current component. Analyses of ocean currents show components that are either steady, periodic, episodic, or random. The relationship between temporal scales and size scales of ocean currents is shown also in Table 8-1, which shows that certain generalizations regarding currents can be made Steady flow is associated with oceanic gyral circulation.

Seasonal changes in coastal currents are periodic; changes in current speed and direction every 6 to 12 hours are related to tidal circulation; and wave action produces current changes 5 to 25 sec apart. Periodic changes on the order of 15 hours in current direction only is related to inertial flow (i.e., flow that is no longer driven by forces that initiated the motion). Episodic changes in currents usually are related to storms or other atmospheric phenomena. Such episodes last from a few hours to several days. Random changes in current speed and direction are largely a result of turbulence in the sea.

Turbulence. Turbulence is of extreme importance in the world ocean because it is the mechanism by which mixing is accomplished. Thermal energy and chemical substances, such as oxygen, carbon dioxide, and plant nutrients, are distributed through the ocean largely by turbulent mixing. Plankton are moved through the surface layers by turbulence and are kept bathed in nutrients by that effect. The transport of sediments in the nearshore zone is initiated and maintained by turbulence induced by breaking waves.

Turbulence is chaotic motion in a fluid and is perceived as an assortment of eddies that change shape and direction of travel in no apparent pattern. It is seen commonly in smoke, clouds, and in stirred liquids, especially liquids into which another colored liquid is poured. In the ocean, turbulence is observed in current measurements as nonsystematic deviations from the average flow.

Fig. 8-1 shows a typical current record of steady flow containing turbulent noise. Such a record is analyzed by assuming that the random turbulent deviations from the average flow represent the sum of an unlimited number of harmonic oscillations, each characterized by a discrete frequency. The energy (1/2 mv²) associated with each frequency is extracted and plotted against its frequency to produce a spectrum of turbulent energy (Fig. 8-1B).

Figure 8-1. A record (A) of current measured at a site showing a steady flow containing turbulence. A turbulence spectrum derived from such measurements is shown in (B). (After B. Kinsman, Journal of Geophysical Research, Vol. 66, No. 6, 1961, copyright by the American Geophysical Union)

The spectrum portrays the relative abundances of the turbulent components in the flow. If we assume that each frequency component of motion is rotational (turbulent motion follows curved paths), we can regard the frequencies as representing the sizes of the turbulent eddies in the flow. Low frequencies represent large eddies and high frequencies represent small eddies. We refer to the scale of turbulence in terms of the diameter of a circular eddy corresponding to the spectral frequency of turbulence.

Large eddies, called energy-bearing eddies, are formed as long as a localized force is applied to water. These eddies then give rise to smaller eddies that move under their own momentum. These eddies, called inertial eddies, in turn form smaller and smaller eddies until they become so small that water viscosity converts the energy of their motion into heat. The formation of energy-dissipating eddies is the terminal stage in turbulent mixing.

The size of the largest eddy that forms in turbulent flow is determined by the rate that energy is supplied to the flow (by, say, the wind) and the proximity to flow boundaries. In the open ocean, intermediate-scale eddy motions can be quite large, on the order of several hundred kilometers. Near a coast the maximum eddy size is smaller but still appreciable (Fig. 8-2). Eddies the size of large embayments have been observed.

Figure 8-2. A satellite (LANDSAT-1) photo image of a portion of the coast of southern California. Large waves suspended fine sediments in coastal water, and tidal action carried the material into an offshore coastal current. Large eddies are seen where the current passes an irregularity in the coastline. The current has carried the sediment well over 30 km offshore. (Photo from NASA imagery)

Eddies in the inertial and energy-dissipating range are usually present near the surface of the world ocean. Only in the depths of ocean (away from the bottom) is the flow virtually free of turbulence of the inertial and energy- dissipating scales.

Surface Currents. Now let us turn our attention to the large scale circulation of the world ocean (we will consider coastal currents in the chapter on inshore oceanography). This circulation is characterized by many steady currents (Fig. 8-3). Short-term fluctuations do exist in the direction and magnitude of the major ocean currents, but, over a period of time, a consistent pattern prevails.

The most conspicuous features in Fig. 8-3 are the large gyres (currents moving in a circle) found in tropical and subtropical regions of each ocean basin. In the northern hemisphere, such gyres move in a clockwise direction, whereas the gyres of the southern oceans rotate counterclockwise. The currents on the western side of the subtropical gyres gain intensity as they flow to higher latitudes.

This westward intensification is best developed in the Gulf Stream and in the Kuroshio of the northern Atlantic and Pacific ocean basins, respectively. These currents attain speeds of approximately 250 cm per sec. The currents at the western boundaries of the southern oceans also appear to be intensified, but to a lesser degree than in the northern hemisphere.

Between the major gyres in the equatorial regions of the ocean basins, countercurrents flow in directions opposite to the adjacent currents. Countercurrents do not follow closed paths, as do gyres. They tend to flow along straight paths at low latitudes. The Pacific Equatorial Countercurrent is well developed over the width of the Pacific basin and attains speeds of 50 cm per sec The Atlantic Equatorial Countercurrent is generally restricted to the eastern Atlantic Ocean basin. The countercurrent in the Indian Ocean basin is not always present, because it is influenced strongly by the monsoons.

The pattern of movement of the colder water of the Arctic regions is more complex than in the lower latitudes. In the northern hemisphere, water movements appear to be influenced strongly by restrictive continental boundaries. The North Pacific Ocean basin contains its own circulation pattern. It is virtually closed off from the Arctic Ocean basin, and there is relatively little exchange to the north. In contrast, the North Atlantic Ocean basin is open to the Arctic Ocean basin, so great volumes of surface water are exchanged between the two basins.

In the southern hemisphere, the largest surface flow in the world, the West Wind Drift, flows from west to east around Antarctica. Here, virtually nothing obstructs the surface currents in the endless expanse of water that surrounds the Antarctic continent.

Deep Currents. Water movements are not limited to the surface of the world ocean. Although surface currents have greater speed and are more distinctly noticeable, they represent less than 10 percent of the water filling the ocean basins. Deep currents, on the other hand, are very difficult to measure because of their remoteness from the sea surface and their low velocity. Nevertheless, motion does exist at all depths.

Figure 8-4. Inferred direction of flow of bottom water in the world ocean. Area of origin of bottom water. (After Lonsdale, Johnson, Mantyla, and Taft and Jones, Wiist, and Wyrtki)

A variety of methods is used to discern the general nature of deep circulation; the geographical distribution of physical and chemical properties, such as oxygen and temperature, drifting devices, current meters, and bottom photographs, all provide evidence of bottom currents. The evidence produces a somewhat incomplete picture of the deep circulation in the world ocean. The chart in Fig. 8-4 shows distinct areas where deep water forms and then flows toward lower latitudes.