Oceans’ Structure

The ocean has a complex structure, both at its surface and in the vertical dimension, descending to the ocean floor. This internal structure results in layering concerning temperature, salinity, density, and how the ocean responds to the passage of light and sound waves.

Temperature Layering

One highly significant aspect of ocean structure is the layering of water based on temperature and salinity differences. To understand the reasons for the thermal layering of the ocean, one must bear in mind that sunlight is the primary source of heating for the ocean. About 60 percent of this entering radiation is absorbed within the first meter of seawater, and about 80 percent is absorbed within the first ten meters. As a result, the warmest waters in the ocean are found at its surface.

Surface temperatures in the ocean are not the same everywhere. Ocean surface temperatures are closely related to latitude because more heat is received at the equator than at the poles. As a result, they are distributed in bands of equal temperature extending east and west, parallel to the equator. Temperatures are highest along the equator because of the near-vertical angles at which the sun’s rays are received. As latitude increases toward the poles, ocean temperatures gradually cool due to the decreasing angle of incidence of the incoming solar radiation.

Measurements of ocean surface temperature range from a high of 33 degrees Celsius in the Persian Gulf, a partly landlocked, shallow sea in a desert climate, to a low of -2 degrees Celsius near ice in polar regions. There, salt in the water lowers the water’s freezing point below the normal 0 degree Celsius level. Because the salinity of this cold water is so high, it sinks to the ocean floor and travels along it for substantial distances. Ocean surface temperatures may also vary with time of year, with warmer waters moving northward into the Northern Hemisphere in the summertime and southward into the Southern Hemisphere in the wintertime. These differences are most noticeable in midlatitude waters. In equatorial regions, water and air temperatures change little seasonally, and in polar regions, water tends to be cold all year because of ice.

Vertically downward from the equator toward the ocean floor, water temperatures become colder. This results from the fact that solar heating only affects surface waters and that cold water is denser than warm water. When waters at the surface of the ocean in the polar regions are chilled by extremely low winter temperatures, they become denser than the underlying waters and sink to the bottom. They then move slowly toward the equator along the sea floor, lowering the temperature of the entire ocean. As a result, deep ocean waters have much lower temperatures than might be expected by examination of the surface waters alone. Although the average ocean surface temperature is 17.5 degrees Celsius, the average temperature of the entire ocean is a frigid 3.5 degrees.

Oceanographers recognize the several ocean layers based on temperature stratification. First, there is an upper, wind-mixed layer consisting of warm surface water up to 500 meters thick. This layer may not be present in polar regions. Next is an intermediate layer, below the surface layer, where the temperature decreases rapidly with depth; this transitional layer can be 500 to 1,000 meters thick and is known as the main thermocline. Finally, a cold, deep layer extends to the ocean floor. In polar regions, this layer may reach the surface, and its water is relatively homogeneous, with temperature slowly decreasing with depth.

Because the upper surface layer is influenced by atmospheric conditions, such as weather and climate, it may contain weak thermoclines due to the daily heating and cooling cycle or seasonal variations. These are temporary, however, and may be destroyed by severe storm activity. Nevertheless, most ocean water lies below the main thermocline and is uniformly cold, the only exception being hot springs on the ocean floor that introduce water at temperatures of 300 degrees Celsius or higher. Plumes of warmer water emanating from these hot springs have been detected within the ocean.

Salinity

A second phenomenon responsible for layering within the ocean is the water’s salinity variation. For the ocean as a whole, the salinity is 35 parts per thousand. Considerable variation in the salinity of the surface waters from place to place results from processes that either add or subtract salt or water. For example, salinities of 40 parts per thousand or higher are found in nearly landlocked seas in desert climates, such as the Red Sea or the Persian Gulf, because high evaporation rates remove the water but leave the salt behind. High salinity values are also found at the open ocean's surface at the same latitudes where there are deserts on land (the so-called horse latitudes). There, salinities of 36 to 37 parts per thousand are common.

At the equator, however, much lower salinity values are encountered despite the high temperatures and nearly vertical rays of the sun. The reason is that the equatorial zone lies in the so-called doldrums, a region of heavy rainfall. The ocean’s surface waters are, therefore, diluted, which keeps the salinity relatively low. Similarly, low salinities are also found in coastal areas, where rivers bring in large quantities of freshwater, and in higher latitudes, where rainfall is abundant because of numerous storms.

Despite the variation in salinity in the ocean’s surface water, the deep waters are well mixed, with nearly uniform salinities ranging from 34.6 to 34.9 parts per thousand. Consequently, in some parts of the ocean, surface layers of low-salinity water overlie the uniformly saline deep waters. In other parts of the ocean, surface layers of high-salinity water overlie the uniformly saline deep layer. Between these layers are zones of rapidly changing salinity known as haloclines. An important exception to this picture is a few deep pools of dense brine, such as those found at the bottom of the Red Sea, where salinities of 270 parts per thousand have been recorded.

Haloclines are very common in coastal areas. Off the mouth of the Amazon River, for example, a plume of low-salinity river water extends out to sea as far as 320 kilometers, separated from the normally saline water below by a prominent halocline. In many tidal rivers and estuaries, a layer of heavier seawater will extend many kilometers inland beneath the freshwater discharge as a conspicuous saltwater wedge.

Density Stratification

A prominent density stratification within the ocean results from the variation in ocean temperatures and salinity just described. As noted, two factors make water heavier—increased salinity, which adds more dissolved mineral matter, and decreased temperature, which results in the water molecules being more closely packed together. Therefore, the least-dense surface waters are found in the equatorial and tropical regions, where ocean temperatures are at their highest. The density of surface ocean waters increases toward the higher latitudes because of the falling temperatures. Low-density surface water is found only in areas where large quantities of freshwater are introduced by river runoff, high precipitation, or melting ice.

Vertical density changes are even more pronounced. As water temperatures decrease with depth, water densities increase accordingly. This increase in density, however, is not uniform throughout the ocean. At the poles, the surface waters are almost as cold as the coldest bottom waters, so there is only a slight increase in density as the ocean floor is approached. By contrast, the warm surface waters in the equatorial and tropical regions are underlain by markedly colder water. As a result, a warm upper layer of low-density water is underlain by an intermediate layer in which the density increases rapidly with depth. (This middle layer is known as the pycnocline.) Below it is a deep zone of nearly uniform high-density water.

Convective overturning takes place when this normal density stratification is upset. In a stable density-stratified system, the less dense surface water floats on top of the heavier, deeper water. Occasionally, however, unstable conditions will arise in which heavier water forms above lighter water. Then, convective overturning takes place as the mass of heavier water sinks to its appropriate place in the density-stratified water column. This overturning may occur gradually or quite abruptly. In lakes and ponds, it occurs annually in regions where winter temperatures are cold enough. In the ocean, convective overturning is primarily associated with the polar regions, where extremely low winter temperatures result in the sinking of vast quantities of cold water. In addition, convective overturning has been observed in the Mediterranean during the wintertime, when chilled surface waters sink to replenish deeper water.

Light Penetration and Sound Waves

Oceanographers also recognize stratification in the ocean based on the depths to which light penetrates, and they divide the ocean into two zones. The upper zone, which is known as the photic zone, consists of the near-surface waters that have sufficient sunlight for photosynthetic growth. Below this zone is the aphotic zone, where there is insufficient light for photosynthetic growth. The lower limit of the photic zone is generally taken as the depth at which only 1 percent of the surface intensity of sunlight still penetrates. In the extremely clear waters of the open ocean, this depth may be 200 meters or more.

Stratification in the ocean based on the behavior of sound waves has also been observed. Because sound waves travel nearly five times faster under water than in the air, their transmission in the ocean has been extensively studied, beginning with the development of sonar, the echo sounder. So-called scattering layers have been recognized as regions that reflect sound, usually because of the presence of living organisms that migrate vertically, as layers within the water column, depending on light intensity. The sofar (sound fixing and ranging) channels are density layers within the ocean where sound waves can become trapped and can travel for thousands of kilometers with extremely small energy losses. These channels have the potential to be used for long-distance communications. Shadow zones are also caused by density layers within the ocean. These layers trap the sound waves and prevent them from reaching the surface. One advantage of shadow zones is that submarines can travel in them undetected.

Study of the Ocean

The measurement of water temperatures at the ocean’s surface is quite simple. A thermometer placed in a bucket of water scooped out of the ocean at the bow of a boat will suffice, provided the necessary precautions have been taken to prevent temperature changes caused by conduction and evaporation. For oceanwide or global studies, satellites provide near-simultaneous readings of ocean surface temperatures with an accuracy of 1 degree Celsius. These satellites utilize infrared and other sensors, which measure the amount of heat radiation emitted by the ocean’s surface to within 0.2 degrees.

Measuring the temperature of the deeper subsurface waters posed a problem, however, because a standard thermometer lowered over the side of a ship will “forget” a deep reading on its way back to the surface. As a result, the so-called reversing thermometer was developed in 1874. This thermometer has an S-bend in its glass tube. When the thermometer is inverted at the desired depth, the mercury column breaks at the S-bend, thus recording the temperature at that depth. Modern electronic instruments record subsurface water temperatures continuously. These devices can be dropped from a plane or ship or moored to the ocean floor.

Measuring the salinity of seawater is more challenging than one might think. An obvious way to determine water salinity would be to determine the amount of dried salts remaining after a weighed sample of seawater has been evaporated. Still, in actual practice, that is a messy and time-consuming procedure, hardly suitable for use on a rolling ship. Various other techniques have been used over the years based on such water characteristics as buoyancy, density, or chloride content. By far, the most popular relies on seawater’s electrical conductivity. In this method, an electrical current passes through the seawater sample. The higher the water's salt content, the lower the electrical resistance of the solution and the faster this current is observed to pass. Using this method, oceanographers have determined the salinity of seawater samples to the nearest 0.003 part per thousand. This is an important advantage because the salinity differences between deep seawater masses are minute.

The densities of surface water samples can easily be measured by determining the water sample’s buoyancy or weight. The real difficulty comes with determining a subsurface water sample’s density. If this water sample is brought to the surface, its temperature, and, therefore, its density, will change. Although sophisticated techniques are available for determining density at depth, in actual practice, the density is not measured at all. Instead, it is computed from the sample’s known temperature, salinity, and depth. The density of the water is almost wholly dependent on these three factors.

Various methods are available for measuring the depth of light penetration in seawater. A crude estimate can be made using the Secchi disk, first introduced in 1865. This circular, white disk is slowly lowered into the water, and the depth at which the disk disappears is noted visually. More sophisticated measurements can be made using photoelectric meters. Another reliable indicator of the maximum depth of light penetration in the sea is the lowest level at which photosynthetic growth can occur. For sound studies within the ocean, various methods are used to create the initial sound, including using explosives in seismic profiling. The returning echo is detected using a receiver known as a hydrophone.

Principal Terms

convective overturn: the renewal of the bottom waters caused by the sinking of surface waters that have become denser, usually because of changes in temperature or salinity

doldrums: the equatorial zone where winds are calm and variable, and there is heavy thunderstorm rainfall

halocline: a zone within a body of water characterized by a rapid rate of change in salinity

horse latitudes: the belts of latitude approximately 30 degrees north and 30 degrees south of the equator, where the winds are very light and the weather is hot and dry

pycnocline: a zone within a body of water, characterized by a rapid rate of change in density

salinity: the quantity of dissolved salts in seawater, usually expressed as parts per thousand

saltwater wedge: a wedge-shaped intrusion of seawater from the ocean into the bottom of a river; the thin end of the wedge points upstream

thermocline: a zone within a body of water, characterized by a rapid change in temperature

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