Temperature, pressure, and salinity are the three most important properties of seawater, and they determine the other physical properties associated with seawater. This differs from pure water, where only pressure and temperature determine the physical properties. Wave motion and the presence of small suspended particles in seawater are also important variables that affect the properties of seawater. Wave motion causes a change in the processes of chemical diffusion, heat conduction, and transfer of momentum from one layer to another. The suspended particles increase the scattering of radiation, thereby absorbing more radiation than a similar layer (thickness) of pure water. The variables of wave motion and suspended particles, although important, cannot be measured.

In addition to temperature, pressure, and salinity, other common physical properties of seawater are water color, transparency, ice (which weve already covered in our discussion of the surface), and sound velocity. Some of the lesser known properties include specific heat, compressibility, osmotic pressure, eddy viscosity, electrical conductivity, radioactivity, and surface tension. Many of the lesser known properties can only be determined using complex mathematical calculation and formulation that incorporates data on one or more of the common physical properties, especially temperature, pressure, and/or salinity.

Learning Objective: Identify the oceans heat source, the total and diurnal range of ocean temperatures, the factors that control the distribution of heat in the oceans, and the oceans vertical-temperature profile.

The ocean, like the atmosphere, is heated by the Suns incoming radiation. In all latitudes the ice-free portions of oceans receive a surplus of radiation. Some of this heat is given up to the atmosphere, and some of it is retained. Because the sea retains a portion of this heat, the sea-surface temperature is normally higher than the air temperature. However this is true only when average conditions are considered. Whether the sea-surface is warmer or colder than the air above it at any particular moment in time is dependent upon the locality, the season of the year, the character of the atmospheric circulation and the character of the ocean currents. The temperature of the ocean ranges from about 2C to 30C. Ocean water that is nearly surrounded by land may have higher temperatures, but the open sea, where the water is free to move about, hardly ever heats above 30C. Here, the ocean currents distribute the heat and tend to equalize the temperature. Deep and bottom water temperatures are always low, varying between 4C and 1C.

The annual variation of sea-surface temper-ature in any region depends upon the variation of incoming radiation, the character of the ocean currents, and the character of the atmospheric circulation. The annual range of surface temperature is much greater over the oceans of the Northern Hemisphere than those of the Southern Hemisphere. This wider range of temperatures appears to be associated with the character of the prevailing winds, particularly the cold winds blowing from the continents. On the other hand, the annual range of ocean temperatures in the Southern Hemisphere is most definitely related to the range of incoming solar radiation, because of the absence of large land masses south of 45S. Here, the prevailing winds travel almost entirely over water. This brings about a far greater degree of consistency in the annual sea-surface temperature patterns and a much smaller annual temperature range compared to the Northern Hemisphere.

The temperatures near the equator experience a semiannual variation. This corresponds to the twice yearly passage of the Suns most direct rays across the equator.

Sea-surface temperatures change from day to night just like those of the atmosphere, but to a much lesser degree. The diurnal variation of sea-surface temperature in the open ocean is on the average only 0.2C to 0.3C. The greatest diurnal variation takes place in the tropics, with lesser variation at higher latitudes. The range of diurnal variation is dependent on the amount of cloudi-ness and the direction and speed of the wind.

The annual variation of temperature in sub-surface layers depends on several additional factorsnamely, the variation in the amount of heat that is directly absorbed at different depths, the effect of heat conduction, the variation in currents related to lateral displacement, and the effect of vertical motion. Diurnal temperature variations in subsurface layers are largely unknown. What we do know is that they are extremely small.

VERTICAL-TEMPERATURE STRUCTURE. The basic vertical temperature structure of the ocean in its simplest form is best described using the three-layered ocean model, which we will discuss following this section on seawater properties. Generally, there is little temperature change with depth through an upper or mixed layer, a sharp temperature decrease through a main thermocline layer, and a return to a gradual decrease in temperature through a deep water layer.

Learning Objective: Define salinity; recognize its effect on seawater density; state its ranges in the open ocean and the major factor that controls it. 

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing