Transmission of Energy In the Ocean
When sunlight (which contains all colors) strikes the ocean surface, the water molecules scatter the light rays. This scattering causes the water to appear blue because the blue light in the sun's rays is scattered more than light of other colors.
The appearance of the water is also affected by plankton. If there is enough plankton near the surface of the water, the combined effect of the water molecules and plankton will cause the water to appear green, gray, or even yellow. Coastal waters, which contain relatively large amounts of plankton, are usually green, while mid-ocean waters are blue. A species of alga that lives near the surface of the Red Sea occasionally gives a red color to the surface of that body of water.
Nearly all of the sunlight that strikes the ocean is absorbed within 600 feet (183 m) of the surface. A small amount of blue-green light penetrates considerably deeper—down to about 2,000 feet (610 m) if the water is sufficiently clear. Beyond the maximum depth of sunlight penetration, the only illumination is that produced by luminescent sea creatures.
The Persian Gulf, which has an average depth of only 200 feet (61 m), contains the world's warmest ocean water. Surface temperatures of 96° F. (36° C.) have been recorded there. Over most of the ocean, however, the surface temperature ranges from about 86° F. (30° C.) at the Equator to about 29° F. (-1.7° C.) near the poles. This general decrease from Equator to poles is greatly modified by ocean currents, however. Both the Arctic and Antarctic oceans contain icebergs and sea ice throughout the year.
The heat rays (infrared radiation) from the sun are absorbed by seawater much more rapidly than the light rays. Only a very small amount of solar heat radiation penetrates more than three feet (90 cm) below the surface. Usually, the water temperature decreases gradually with depth. At depths below 10,000 feet (3,048 m) the water temperature is only a few degrees above the freezing point, even in the tropics.
When water near the ocean's surface absorbs a large amount of heat from the sun, a thermocline may form between this surface water and the water below. A thermocline is a layer of water in which the temperature decreases with depth more rapidly than it does in the water above or below. A thermocline acts as a barrier to the mixing of the warmer water above with the cooler water below. In temperate areas, thermoclines usually occur only during summer; in tropical areas, thermoclines are usually permanent.
Water temperatures down to depths of about 900 feet (274 m) are usually obtained with a bathythermograph. This instrument provides a continuous, permanent record of water temperature in relation to depth while it is being lowered on a cable. The record is scratched on a smoked-glass slide by a temperature-sensitive stylus. At depths below 900 feet, temperatures are obtained with electrical and mercury-in-glass thermometers.
There are many sources of sound in the ocean, including marine animals, wave action, and ship engines and propellers. In general, sound waves travel through ocean water more readily than light waves do. For this reason sound is useful in detecting and locating objects underwater. In addition to its use in echo sounders, sound is used for navigation, locating schools of fish, and detecting submerged submarines. The methods (and equipment) that make it possible to use sound for these purposes are known as sonar.
The speed of sound in the ocean is approximately 4,990 feet (1,520 m) per second, about 4 1/2 times the average speed of sound in air. The exact speed depends on the temperature and pressure of the seawater, and therefore varies with depth.
The minimum speed of sound in the ocean occurs around a depth of 3,280 feet (1,000 m). The layer of water at this depth acts as a channel for sound waves, because sound waves leaving the layer are refracted (bent) back towards it. Loud sounds of low frequency produced in this channel have been detected by hydrophones (underwater microphones) halfway around the world.
