A convenient method of visualizing the sea is to divide it into layers in much the same way that we do the atmosphere. Using bathythermograph information (temperature versus depth profiles), the oceans display a basic three-layered structure: the mixed layer, main thermocline, and deep water layer. The latitudinal distribution of these layers is shown in figure 1-2-1, while the typical

Figure 1-2-1.North-south distribution of a simple three-layered ocean (North Atlantic) in winter.

thermal structure is shown in figure 1-2-2. Both figures are representative of winter.

The mixed layer is the upper layer of the three-layered ocean model. It is a layer of fairly constant warm temperatures which, in middle latitudes, extends from the surface to a maximum depth of about 450 meters, or 1,500 feet. This layer gets its name from the mixing processes that bring about its fairly constant warm temperatures. The two mixing processes are classified as mechanical and convective.

MECHANICAL MIXING. This mixing process is caused by wave action, surface storms, etc. The wave action stirs up the water. Warmer surface water is driven downward, where it mixes with colder subsurface water. Eventually, a layer of water with a fairly constant temperature is produced. This process is more important in summer than in winter, because surface waters are much warmer and less dense than sub-surface waters, thereby producing a stable water column. The mechanical mixing process is more rapid and irregular than the convective mixing process.

CONVECTIVE MIXING. This process occurs as a result of changes in water stability. When surface waters become more dense than subsurface waters, an unstable condition exists. Such a condition can occur when there is an increase in surface salinity owing to evaporation or the formation of ice, or by a decrease in the surface water temperature. A temperature decrease of .01C or a salinity increase of 0.01 , is sufficient to initiate the convective mixing process. In the former case, for example, a cold polar or arctic air mass moving over warm water cools the surface water before it can cool the subsurface water. As the surface waters cool and become colder than the subsurface waters, they become more dense and sink. As the colder surface water sinks, the warmer and less dense subsurface water rises to the surface to replace it. This process continues until the water is thoroughly mixed, the density difference eliminated, and the water column stabilized. Even though winds and the resultant wave action are generally stronger during winter, convective mixing, caused by the colder winter air temperatures, produces a deeper mixed layer than can be attained by mechanical mixing. It is for this reason that convective mixing is considered the more important of the two, and the pre-dominant process of winter.

The convection process is strongest in northern waters where vertical temperature and salinity gradients are not extreme and surface waters undergo a high degree of cooling. Convective mixing attributed to salinity changes is most noticeable in the Mediterranean and Red seas, where evaporation far exceeds precipitation. We have looked at both processes individually; however, the two processes can and often do take place simultaneously. When this occurs, the mixed layer normally attains a greater depth than would be attained by either process individually.

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