In high latitudes, heat passes from the sea to the atmosphere. Surface cooling of the water produces convectional mixing, and there is, therefore, little difference in temperature between the surface and the deep layers. Through the whole depth of water the temperature range is usually within the limits of —1.8 to 1.80C. There is often an irregular temperature gradient within the top 1000 m because the surface is diluted by fresh water from precipitation or melting ice. This forms a low-density layer of colder water above slightly warmer, but denser, water of higher salinity entering from middle latitudes (Figure 4.1a). Below 1000 m the temperature is almost uniform to the bottom, decreasing only slightly with depth.
At low latitudes, heat absorption at the sea surface produces a warm, light surface layer overlying the cold, denser, deep layers. Here the temperature gradient does not descend steadily but shows a distinct step, or thermocline, usually between about 100 and 500 m (Figure 4.1b), where temperature falls quite sharply with depth. This zone is termed a discontinuity layer. Above it, surface mixing maintains a fairly even warm temperature, a stratum referred to as the thermosphere. Below the thermocline is the psychrosphere where the water is cold, and there is only a slight further decrease of temperature towards the bottom. To a considerable extent the thermocline acts as a boundary between a warm-water population above and a cold-water population below.
In middle latitudes, the surface water becomes warm during the summer months and this leads to the formation of temporary, seasonal thermoclines near the surface, commonly around 15-40 m depth (Figure 4.1c). In winter, when the surface water cools, these temporary thermoclines disappear and convectional mixing may then extend to a depth of several hundred metres. Below the level to which convectional movements mix the water, there is usually a permanent but relatively slight thermocline between about 500 and 1500 m.
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