Answer to Question 1
The transition zone between the surface zone and the deep ocean is on the order of a kilometer in thickness and is characterized by a rapid increase in density with increasing water depth. The very sharp increase in density is called the pycnocline; the transition zone is referred to as the pycnocline zone. In some regions this density gradient is dominated by salinity changes, and salinity rises rapidly with increasing depth. In this case, the salinity gradient is specifically referred to as the halocline. In most other regions, temperature changes dominate the density gradient, and temperature drops rapidly with increasing depth. There the transition is called the thermocline. In any of these cases, a steep density gradient forms that makes this stratification very stable.
Answer to Question 2
Bottom water constitutes the densest water produced in the oceans. Near the poles, the surface waters are cooled below the normal freezing point (1.9C in some areas) by contact with the cold overlying atmosphere. (The freezing point is lower than that of pure water because of the presence of salt.) When that water freezes, it forms a layer of sea ice several meters thick that floats on the surface of the polar oceans. When the ocean surface freezes, most of the sea salt is excluded, because the salt does not fit into the crystal structure of the ice. As a result, the water just beneath the sea ice becomes saltier, and an underlayer of very cold, highly saline water forms. The combination of low temperatures and high salinities results in very dense water that sinks and flows down the slope of the basin and spreads toward the equator as the bottom layer of water in the deep-ocean basins.
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