The vertical structure of the ocean is determined by gravity, whereby the depth at which a water mass is going to lie is determined by the density of the water mass, which is turn related to its temperature and salinity. Temperatures are expressed in degrees Celsius (°C), and salinities in practical salinity units (psu). A salinity in psu is actually a dimensionless number, and refers to the conductivity ratio of the sea water sample to a standard KCl solution. A salinity of 35 (psu) does not exactly equal 35 grams of salt per liter of solution.
Ocean temperatures result predominantly from heat exchange with the atmosphere, together with mixing by winds. The temperatures of the uppermost layers of the ocean (above 100-150 m or so in practice) are fairly directly driven by atmospheric heating and cooling, and also exhibit seasonal variations. Below that depth, ocean temperatures exhibit a strong vertical gradient, the so-called thermocline. Friction of the wind on the ocean surface is also able to set the upper 1000 m or so of the ocean in motion. In this process, the wind also tends to slowly mix the heat from the upper layers with the underlying water masses, but this process is mostly effective above the thermocline.
Below the thermocline, temperatures in the deep ocean are fairly constant in space. This is due to the fact that temperature variations in the ocean take place predominantly through heat exchange with the atmosphere. As a result, water masses that are deep enough to be sheltered from atmospheric influence retain the temperature that they acquired when they were last in contact with the atmosphere for very long periods of time, exhibiting neither seasonal nor annual variations. Water masses located at depths of 700 to 1200 m (the depth range in which cold water is typically taken in for OTEC systems) were last in contact with the atmosphere in the Southern Ocean, where they acquired their temperature (and salinity) properties. In physical oceanography, these water masses are usually referred to as intermediate water masses. Along the equatorward edge of the subarctic oceans (mostly Southern Ocean, but also subarctic edges of the Atlantic and Pacific Oceans), they are pushed equatorward and tend to slowly disappear under the warmer (and thereby less dense) water masses in a process called subduction, which in turns shelters them from atmospheric influences.
In a similar manner to temperature, ocean salinities result from freshwater exchange with the atmosphere through evaporation and precipitation. Once sheltered from the atmosphere by overlying water masses, the salinity of deep water masses changes only very slowly.
Temperatures between 700 and 1200 m typically range between 4 and 6°C throughout most of the world ocean. Only in the arctic and subarctic regions are temperatures lower (map on the left). The north Atlantic Ocean and to some lesser extent the northern Indian Ocean stand out in Figure 3.2 with warmer temperatures at depths. This results from somewhat different dynamics in the semi-enclosed Mediterranean Sea, which is isolated from the Atlantic Ocean by a relatively shallow (350 m deep) sill in the Strait of Gibraltar. This shallow sill prevents deep waters from the Atlantic from entering the Mediterranean Sea, so that waters around 1000 m in the Mediterranean are formed locally from winter cooling and do not cool down much more than 10°C or so. However, waters tend to flow out of the Mediterranean Sea and cascade down the sill into the Atlantic Ocean (as a result of their salinities being much larger than those of typical Atlantic water masses) so that warmer waters from the Mediterranean are brought into the Atlantic at relatively unusual depths. A similar sill isolates deep water masses of the Red Sea from deep water masses in the Indian Ocean but leads to warmer waters from cascading down into the Indian Ocean.
Since oceanic temperatures around 1000 m deep are fairly homogeneous around 4-6°C, a temperature difference of at least 20°C between the cold and warm sources for an OTEC system requires waters with a temperature of 24°C or more. An examination of observed water temperatures at 30 m indicates that such temperatures can only be reached in the tropical regions (Figure 3.3, map on the right).