The remaining stage of the hydrologic cycle (Section 6.1) consists of the circulation of water in the oceans. This has considerable impact on onshore climates (Note 11.A); the oceans affect the atmosphere's temperature (Note 3.G), moisture content (Note 4.D), stability (Section 7.6), rainfall (Section 10.4) and winds (Chapter 14). These effects are felt especially at the coast, and thus are important, for instance, to the 80 per cent of Australia's population who live within 30 km of the sea.
The amount of water in the oceans is huge. The average depth is 3,730 m (Note 1.A), which is over four times the mean elevation of the world's land above sea-level. It takes a 0.4 kg steel ball over an hour to drop 11 km to the bottom of the Mariana Trench in the north-west Pacific. Also, the area of the oceans is vast, covering 70.9 per cent of the Earth's surface (Note 1.A).
The predominance of ocean surface is a special feature of the southern hemisphere. Just over 80 per cent of the hemisphere is covered by sea and about a quarter of the rest is permanently covered by ice. The percentage of ocean is especially high at the latitudes of Australia, and around Antarctica (Figure 11.1).
The oceans resemble the atmosphere in consisting of a fluid, containing heat, being subject to convection (Note 11.B) and horizontal circulation, flowing under the influence of slope and pressure difference, and carrying contaminants. Much of the theory of atmospheric movement applies also to ocean flows.
The fluids also differ greatly. Water is substantially incompressible and over a thousand times more dense than air, so that even slow ocean currents contain much greater momentum—enough to affect the rotation of the Earth. The density of air ranges widely with altitude, whereas there are relatively slight variations of the density of the sea, due chiefly to differences in temperature or salt concentration. In addition, oceans are contained within large connected basins while the atmosphere is almost unbounded. Furthermore, the oceans hold much more heat; the total heat capacity of the whole atmosphere is equal to that of merely 3 m depth
of the oceans. Also, the Sun's radiation heats the sea at the top, creating a lighter upper layer and hence extreme stability, which helps preserve the layered structure of the oceans, discussed in Section 11.2. All the Sun's infra-red radiation is absorbed within a millimetre of the ocean's surface, while 90 per cent of the visible radiation is absorbed in the top 75 m. Overall, half the solar radiation is absorbed within 6 cm. This heating at the top contrasts with the situation in the atmosphere, which is heated by the ground below, creating instability and convection, already discussed in Chapter 7.
The oceans affect climates in several ways, on both a large and a small scale. They play a part in the transfer of latent heat (Figure 4.2), carry sensible heat to other latitudes (Note 5.F), and sea-surface temperatures affect convective rainfalls.
More immediately, the surface of the ocean influences the planetary boundary layer (PBL) of the atmosphere, the well-mixed layer against the surface (Figure 1.11), whose condition largely defines our climate. It is this lowest turbulent layer of a few hundred metres depth which exchanges heat and moisture, and whose wind contributes momentum to the ocean. Conversely, it is the top few dozen metres of the sea which are chiefly influenced by rainfall, air temperature and the flows of radiation at the surface. This sluggish layer is as well mixed as the PBL as a result of surface waves and downwards convection from any cooled surface. The two adjacent layers, of the ocean and the atmosphere, influence each other so greatly that we say they are 'tightly coupled', interacting substantially and automatically. They are what we shall mainly consider in this chapter.
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