Eddies

Ocean currents are not continually getting faster and faster, and this is because an equilibrium has been reached whereby the rate at which energy is supplied is being balanced by the rate at which it is being dissipated. Ultimately, the ocean's kinetic energy is converted to heat, through frictional interaction with the sea-bed (this is particularly true of tidal currents in shallow shelf seas) or internal friction at the molecular level, i.e. molecular viscosity. There is a continual transfer of energy from the identifiable currents (i.e. the mean flow) to eventual dissipation as heat, via a succession of eddies of generally decreasing size. The frictional effect of these eddies is the eddy viscosity discussed in Section 3.1.1. Eventually, within small eddies a few centimetres across or less, kinetic energy is converted to heat energy by molecular viscosity.

This idea of a 'cascade' of energy flow, from large-scale features down to the molecular level, was neatly summarized by the dynamicist L.F. Richardson (1881-1953). in the following piece of dogerell:

'Big whirls have little whirls which feed on their velocity. Little whirls have lesser whirls and so on to viscosity."

(A paraphrase of Augustus de Morgan, who had himself paraphrased Jonathan Swift.)

Richardson was in fact referring to atmospheric motions, but could as well have been referring to flow in the ocean.

Eddies form because flowing water has a natural tendency to be turbulent and chaotic. Theoretically, it would be possible for an idealized uniform current to flow smoothly, as long as the current speed was below a certain critical value. In reality, wherever there are spatial variations in flow velocity (i.e. horizontal or vertical current shear), any small disturbances or perturbations in the flow will tend to grow, developing into wave-like patterns and/or eddies. Such effects are described as non-linear because they are not predictable simply by adding together flow velocities.

Figure 3.31 Examples of eddies in the oceans.

(a) Eddies in the Mediterranean off the coast of Libya. They were photographed from the Space Shuttle and (like those in Figure 1.1) show up because of variations in surface roughness. The picture shows an area about 75 km across; the white line is a ship's wake or bilge dump.

Figure 3.31 Examples of eddies in the oceans.

(a) Eddies in the Mediterranean off the coast of Libya. They were photographed from the Space Shuttle and (like those in Figure 1.1) show up because of variations in surface roughness. The picture shows an area about 75 km across; the white line is a ship's wake or bilge dump.

(b) Eddies in the North Atlantic Current to the south of Iceland. The eddy pattern is made visible by an extensive bloom of cocco-lithophores, phytoplankton with highly reflecting platelets. The distance from top to bottom of the image is about 350 km.

(c) The complex eddying currents around Tasmania, made visible by means of the Coastal Zone Color Scanner carried aboard the Nimbus-7 satellite. The colours are false and represent different concentrations of phytoplankton carried in different bodies of water, estimated on the basis of the amount of green chlorophyll pigment in surface water. Australia can be seen at the top of the image, which is about 1000 km across.

Although the formation of eddies generally results in energy being removed from the mean flow (i.e. from the identifiable current system), eddies may also interact with the mean flow and inject energy into it. They may also interact with one another, sometimes forming jets or plumes, but more often producing a complex pattern of eddies flowing into and around one another, forming more and more intricate swirls (as illustrated by the images in Figure 3.31 and Figure 1.1).

The pattern of large-scale current systems like the Gulf Stream or the Antarctic Circumpolar Current - i.e. the average current pattern (or mean flow) we attempt to represent geographically in maps like Figure 3.1 - is the oceanic equivalent of climate. Only relatively recently have oceanographers begun to get to grips with the study of the ocean's variability over short time-scales - i.e. with the ocean's 'weather'. In fact, the random fluctuations and eddies so characteristic of the ocean were formerly regarded merely as a nuisance, obscuring the mean flow.

Although eddies occur over a wide range of space- and time-scales, it became clear in the 1970s that variable flows with periods greater than the tidal and inertial periods are dominated by what are now called mesoscale eddies (the prefix 'meso-' means 'intermediate'). Mesoscale eddies are the oceanic analogues of weather systems in the atmosphere, but the differing densities of air and water mean that while cyclones (depressions) and anticyclones have length-scales of about 1000 km and periods of about a week, mesoscale eddies generally have length-scales of 50-200 km and periods of one to a few months. Mesoscale eddies travel at a few kilometres per day (compared with about 1000 km per day for atmospheric weather systems), and have rotatory currents with speeds of the order of 0.1 m s_l. In most mesoscale eddies, but not all. flow is in approximate geostrophic equilibrium.

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