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Figure 3.16 Annual variation of temperature at different depths in soil at Kaliningrad, European Russia (above) and in the water of the Bay of Biscay (at approximately 47° N, I2°W) (below), illustrating the relatively deep penetration of solar energy into the oceans as distinct from that into land surfaces. The bottom figure shows the temperature deviations from the annual mean for each depth.
Sources: Geiger (1965) and Sverdrup (1945).
raise the temperature of an approximately 30 m thick air layer by 10°C. In this way, the oceans act as a very effective reservoir for much of the world's heat. Similarly, evaporation of sea water causes large heat expenditure because a great amount of energy is needed to evaporate even a small quantity of water (see Chapter 3C).
The thermal role of the ocean is an important and complex one (see Chapter 7D). The ocean comprises three thermal layers:
1 A seasonal boundary, or upper mixed layer, lying above the thermocline. This is less than 100 m deep in the tropics but is hundreds of metres deep in the subpolar seas. It is subject to annual thermal mixing from the surface (see Figure 3.15).
2 A warm water sphere or lower mixed layer. This underlies layer 1 and slowly exchanges heat with it down to many hundreds of metres.
3 The deep ocean. This contains some 80 per cent of the total oceanic water volume and exchanges heat with layer 1 in the polar seas.
This vertical thermal circulation allows global heat to be conserved in the oceans, thus damping down the global effects of climatic change produced by thermal forcing (see Chapter 13B). The time for heat energy to diffuse within the upper mixed layer is two to seven months, within the lower mixed layer seven years, and within the deep ocean upwards of 300 years. The comparative figure for the outer thermal layer of the solid earth is only eleven days.
These differences between land and sea help to produce what is termed continentality. Continentality implies, first, that a land surface heats and cools much more quickly than that of an ocean. Over the land, the lag between maximum (minimum) periods of radiation and the maximum (minimum) surface temperature is only one month, but over the ocean and at coastal stations the lag is up to two months. Second, the annual and diurnal ranges of temperature are greater in continental than in coastal locations. Figure 3.17 illustrates the annual variation of temperature at Toronto, Canada and Valentia, western Ireland, while diurnal temperature ranges experienced in continental and maritime areas are described below (see pp. 55-6). The third effect of continentality results from the global distribution of the landmasses. The smaller ocean area of the northern hemisphere causes the boreal summer to be warmer but its are winters colder on average than the austral equivalents of the southern hemisphere (summer, 22.4°C versus 17.1°C; winter, 8.1°C versus 9.7°C). Heat storage in the oceans causes them to be warmer in winter and cooler in summer than land in the same latitude, although ocean currents give rise to some local departures from this rule. The distribution of temperature anomalies for the latitude in January and July (Figure 3.18) illustrates the significance of continentality and the influence of the warm currents in the North Atlantic and the North Pacific in winter.
Sea-surface temperatures can now be estimated by the use of infra-red satellite imagery (see C, this
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