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2.2.1 Temperature distribution

The distribution of temperature at the surface of the oceans is zonal in nature, with isotherms (lines of constant temperature) oriented in an east-west pattern. The annual mean temperature distribution shown in Figure 2.1 illustrates this general zonal gradation. This pattern is due largely to the zonal distribution of the solar energy received at the sea surface. Specific exceptions to this pattern occur in regions of upwelling (where colder water from below is brought to the surface through the action of the winds), and in the vicinity of major (baroclinic) current systems such as the Gulf Stream (where the temperature field is distorted). The relatively low equatorial and tropical sea-surface temperatures in the eastern Pacific and Atlantic oceans are generally ascribed to the effects of upwelling. The more meridional trend of the isotherms off the northeast coast of the United States, for example, is evidence of the Gulf Stream current system. Examining only annual averages, however, can sometimes be misleading. The monsoon circulation in the Indian Ocean, for example, makes interpretation of an annual mean temperature field questionable.

The temperature field in the ocean exhibits a high degree of stratification with depth. Since the isotherms are nearly parallel to the horizontal plane, this type of structure is referred to as horizontal stratification. This is evidenced in Figure 2.2, which presents zonal (annual) averages of temperature in the Atlantic Ocean by 1o latitude belts. These zonal averages do not include the Mediterranean Sea, the Baltic Sea or the Hudson Bay.

Figure 2.1 Annual mean temperature (°C) at the sea surface. The distribution of surface temperatures shows a strong latitudinal dependence due largely to the zonal distribution of solar energy received at the sea surface (Levitus, 1982).

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