Chapter

The waters of the ocean are continually moving - in powerful currents like the Gulf Stream, in large gyres, in features visible from space (Figure 1.1), and in smaller swirls and eddies ranging in size down to a centimetre across or less.

Figure 1.1 These spiral eddies in the central Mediterranean Sea (made visible through the phenomenon of 'sun glint') were photographed from the Space Shuttle Challenger. Measuring some 12-15 km across, they are only one example of the wide range of gyral motions occurring in the oceans.

Figure 1.1 These spiral eddies in the central Mediterranean Sea (made visible through the phenomenon of 'sun glint') were photographed from the Space Shuttle Challenger. Measuring some 12-15 km across, they are only one example of the wide range of gyral motions occurring in the oceans.

VVhut drives all this motion'.'

The short answer is: energy from the Sun, and the rotation of the Earth.

The most obvious way in which the Sun drives the oceanic circulation is through the circulation of the atmosphere - that is, winds. Energy is transferred from winds to the upper layers of the ocean through frictional coupling between the ocean and the atmosphere at the sea-surface.

The Sun also drives ocean circulation by causing variations in the temperature and salinity of seawater which in turn control its density. Changes in temperature are caused by fluxes of heat across the air-sea boundary; changes in salinity are brought about by addition or removal of freshwater, mainly through evaporation and precipitation, but also, in polar regions, by the freezing and melting of ice. All of these processes are linked directly or indirectly to the effect of solar radiation.

If surface water becomes denser than the underlying water, the situation is unstable and the denser surface water sinks. Vertical, density-driven circulation that results from cooling and/or increase in salinity - i.e. changes in the content of heat and/or salt - is known as thermohaline circulation. The large-scale thermohaline circulation of the ocean will be discussed in Chapter 6.

I low din;-: ilit- muiiion of the Earth contribute to ocean circulation patterns',1

Except for a relatively thin layer close to the solid Earth, frictional coupling between moving water and the Earth is weak, and the same is true for air masses. In the extreme case of a projectile moving above the surface of the Earth, the frictional coupling is effectively zero. Consider, for instance, a missile fired northwards from a rocket launcher positioned on the Equator (Figure 1.2(a)). As it leaves the launcher, the missile is moving eastwards at the same velocity as the Earth's surface as well as moving northwards at its firing velocity. As the missile travels north, the Earth is turning eastwards beneath it. Initially, because it has the same eastward velocity as the surface of the Earth, the missile appears to travel in a straight line. However, the eastward velocity at the surface of the Earth is greatest at the Equator and decreases towards the poles, so as the missile travels progressively northwards, the eastward velocity of the Earth beneath becomes less and less. As a result, in relation to the Earth, the missile is moving not only northwards but also eastwards, at a progressively greater rate (Figure 1.2(b)). This apparent deflection of objects that are moving over the surface of the Earth without being frictionally bound to it - be they missiles, parcels of water or parcels of air - is explained in terms of an apparent force known as the Coriolis force.

Figure 1.2 (a) A missile launched from the Equator has not only its northward firing velocity but also the same eastward velocity as the surface of the Earth at the Equator. The resultant velocity of the missile is therefore a combination of these two, as shown by the double arrow.

(b) The path taken by the missile in relation to the surface of the Earth. In time interval 7",, the missile has moved eastwards to Mh and the Earth to fi,; in the time interval T2, the missile has moved to M2, and the Earth to G2. Note that the apparent deflection attributed to the Coriolis force (the difference between M, and G1 and M2 and G2) increases with increasing latitude. The other blue curves show possible paths for missiles or any other bodies moving over the surface of the Earth without being strongly bound to it by friction.

Was this article helpful?

0 0

Post a comment