Study of the oceans. Its subdivisions deal with each ocean's extent and depth, the water's evolution and composition, its physics and chemistry, the bottom topography, currents and wind, tidal ranges, biology, and the various aspects of human use. Computer simulations are widely used in oceanography to plot the possible movements of the waters, and many studies are carried out by remote sensing.
Oceanography involves the study of water movements – currents, waves, and tides – and the chemical and physical properties of the seawater. It deals with the origin and topography of the ocean floor – ocean trenches and ridges formed by plate tectonics, and continental shelves from the submerged portions of the continents.
History Interest in the ocean received great impetus from the laying of submarine telegraph cables in the 1850s. Before this, interest had been concentrated on the more practical aspects of navigation: the influence of currents on ships' courses and chartmaking. Many early navigators, among them Capt James Cook, made valuable scientific observations on their voyages. Using information from ships' navigation logs, Matthew Fontaine Maury was the first to appreciate the interaction between wind and ocean currents. His work was first published 1856.
The first major oceanographic expedition was that of HMS Challenger, which sailed round the world 1872–76; under the supervision of Charles Wyville Thompson and John Murray, 362 hydrographic stations were made, 360 million sq km/140 million sq mi of ocean floor were mapped and 4,417 new marine animal species were discovered. From 1925 to 1927 the German ship Meteor undertook an extensive study of the physical, chemical, and sedimentary structure of the South Atlantic. Modern international expeditions involve ships, aircraft, and scientists, often combining meteorological and oceanographic experiments.
Distribution and depth The oceans cover about 71% of the Earth's surface. The proportion of land to water is 2:3 in the northern hemisphere and 1:4.7 in the southern hemisphere. The average depth of the oceans is approximately 10.91 km/6.78 mi in the Challenger Deep, a valley in the floor of the Mariana Trench, east of the Philippines (for comparison, the peak of Mount Everest is 8.85 km/5.50 mi above sea level). Some 76% of the ocean basins have a depth of 3–6 km/1.8–3.7 mi and only 1% is deeper. The deepest parts of the ocean occur mainly in Pacific trenches. The deepest sounding in the Atlantic Ocean is almost 9 km/5.6 mi in the Milwaukee Deep, part of the Puerto Rico Trench. In some ocean basins the sea floor is relatively smooth, and stretches of the abyssal plain in the northwestern Atlantic have been found to be flat within 2 m/6.5 ft over distances of 100 km/62 mi. Comparing the average depth of about 4 km/2.5 mi with the horizontal dimensions, which are of the order of 5,000–15,000 km/3,100–9,320 mi, gives a ratio similar to that of the width and thickness of a single sheet of paper.
As the continents are approached, the abyssal plain rises through the continental slope to the continental shelf. The width of the continental shelf varies enormously, but its average width is 65 km/40 mi and its average depth about 130 m/425 ft. These are important areas for fishing and petroleum deposits and have a great influence on local tides. The mass of the oceans is calculated to be 1.43 × 1018 t, with a mean density of 1.045 gm/cc and a mean temperature of 3.9°C/39.02°F.
Temperature On average, temperature distribution in the oceans has three distinct layers. From the surface to a depth of usually less than 500 m/1,640 ft, the water is quite uniformly warm. The temperature decreases comparatively rapidly in a layer 500–1,000 m/1,640–3,280 ft thick to about 5°C/41°F. This region is called the main thermocline and beneath it lie the deep ocean waters, where temperature decreases slowly with depth. Towards higher latitudes the thermocline becomes less deep and in subpolar regions the water column is uniformly cold.
The temperature beneath the main thermocline is fairly uniform throughout the oceans, but the temperature above the thermocline depends on latitude and the predominant currents. The mean annual surface temperature in the tropics is about 30°C/86°F; towards the poles this may drop to −1.7°C/28.9°F, the freezing point of sea water.
Except in the tropics the amount of heat the ocean receives at a given latitude varies with the seasons. In late spring and summer the surface temperature increases and heat is mixed downwards by turbulence, to form a mixed surface layer bounded underneath by a seasonal thermocline. This mixed layer is rarely thicker than 100 m/330 ft, and the seasonal thermocline has been shown to consist of many layers several metres deep at a uniform temperature, separated by thinner regions where temperature changes rapidly with depth. During the winter the surface temperature of the layer slowly decreases and the seasonal thermocline is eroded. The annual variation in surface temperature is at most about 10°C/50°F and often less.
The capacity of tropical waters to store heat has a great influence on the global circulation of the atmosphere, because this is driven by large-scale convection in the tropical atmosphere.
Composition and salinity Salinity is usually expressed as the amount of dissolved salts contained in 1,000 parts of water, with an average value of about 35 parts per thousand (ppT). Areas of particularly heavy precipitation, such as the tropics, and those with slight evaporation or a great inflow of fresh water, have a low salinity. In the Baltic Sea, for instance, the salinity is always below 29 ppT.
Regions of the trade winds and permanent anticyclonic conditions show high salinity, but the enclosed seas (such as the Mediterranean and the Red Sea) have the highest. The most striking contrasts of salinity are only a surface feature, and are greatly reduced in deeper waters. In the case of the Dead Sea, river water has been pouring down for thousands of years into a comparatively small lake where evaporation has been consistently high, and, as a result, the very high salinity of 200 ppT has been reached.
Pressure The pressure at any depth is due to the weight of the overlying water (and atmosphere). For every 10 m/33 ft of depth the pressure increases by about one standard atmosphere (atm), which is the pressure at sea level due to the weight of the atmosphere. Thus, at a depth of 4,000 m/13,125 ft the pressure is 400 atm, and is 1,000 atm or more at the bottom of the deepest trenches. Even at such enormous pressures marine life has been found. One of the effects of living at these great pressures is revealed when animals are brought up quickly in a trawl only to break into pieces on account of the sudden reduction in pressure. In the laboratory small unicellular creatures have been subjected to pressures as high as 600 atm without suffering any apparent harm.
Density The density of a sample of sea water depends first on its temperature, then on its salinity, and lastly on its pressure. Increasing salinity and pressure cause an increase in density, and higher temperatures reduce density. The range of density values found in sea water is remarkably small, varying from about 1.025 at the surface to about 1.046 at 4,000 m/13,125 ft.
As density is so closely linked to temperature, the density distribution in the oceans tends to mirror the temperature distribution. Surface density is minimal in the tropics with a value of 1.022 and increases towards higher latitudes with values of about 1.026 at 60°N or S. Vertical sections show the same three-layer structure as temperature, with a less dense, mixed upper layer lying on the thermocline which surmounts the bottom layer. In subpolar regions the density may be quite uniform throughout the water column. Small though these density differences may be, they have a profound effect on vertical motions in the ocean and large-scale circulation.
Currents Ocean currents can be divided into two groups. Wind-driven currents are primarily horizontal and occur in the upper few hundred metres of the ocean. Thermohaline currents are caused by changes in the density of sea water due to changes in temperature and salinity. They are mainly vertical currents affecting the deep oceans. Trade winds blowing from the east in low latitudes and the westerly winds of mid-latitudes along with the Coriolis effect produce a great clockwise gyre in the oceans of the northern hemisphere and an anticlockwise gyre in the southern hemisphere. The consequent return currents such as the Gulf Stream from the Equator towards the poles are relatively narrow and strong and occur on the western boundaries of the oceans.
There are also westerly counter-currents along the Equator, cold northerly currents from the Arctic, and a strong circumpolar current around Antarctica, which is driven by the ‘roaring forties’ (westerly winds). The currents in the northern Indian Ocean are more complicated and change direction with the monsoon. Some parts of the centres of the main oceanic gyres have very little current, for example the Sargasso Sea, but other areas have recently been shown to contain large eddies that are similar to, but smaller than, depressions and anticyclones in the atmosphere. The source of these eddies is uncertain but one possibility is large meanders that break off from strong western boundary currents such as the Gulf Stream. These eddies are the subject of recent research, as are the deep ocean currents, which have recently been shown to be faster than was thought.
Deep ocean currents mostly run in the reverse direction to surface currents, in that dense water sinks off Newfoundland and Tierra del Fuego and then drifts towards the Equator in deep western boundary currents. In the Arctic the sinking occurs because of the cooling of Arctic water in winter, whereas in the Antarctic it is because of an increase in salinity, and hence density, due to surface sea water freezing. The return vertical flow of this thermohaline circulation occurs as a very slow rise in dense deep water towards the surface over most of the ocean. This rise is a result of winds blowing over the water which increases the depth of the wind-mixed layer and the thermocline bringing up denser water from below.
Ocean circulation and currents play a dominant role in the climate of oceanic land margins. The ocean is warmer than the land in winter and cooler in summer, so that the climate of coastal regions is equable in that annual temperature variations are smaller than in the centre of large land masses, where the climate is extreme. Warm and cold ocean currents can produce climatic contrasts in places at similar latitudes, for example Western Europe and eastern Canada.
Light The colour of the sea is a reflection in its surface of the colour of the sky above. The penetration of light into the ocean is of crucial importance to marine plants, which occur as plankton. Thus they are able to survive only in the top 100 m/330 ft, or less if the water is polluted. The amount of light to be found at any depth of the ocean depends on the altitude of the Sun, on the weather and surface conditions, and on the turbidity of the water (see turbidity current). Sunlight can penetrate the sea's surface only when the Sun is vertically overhead and the sea is calm. Light that does not pass through the surface penetrates no deeper than about 150 m/500 ft. Not all the colours of the spectrum are absorbed equally: red rays are quickly absorbed, but blue and violet light penetrate much further. In clear waters, such as the Sargasso Sea, violet light may be present at 150 m/500 ft, though at a very low strength.
An excellent example of the absorption of light by sea water is the Blue Grotto on the island of Capri, Italy, within which everything is enveloped in pure blue light. This phenomenon occurs because light entering the cave must first pass through the surface water that almost fills the entrance.
Sound The speed at which sound travels underwater depends on density, which in turn depends on temperature, salinity, and pressure. In water at 0°C/32°F with salinity of 35 ppT and pressure of 1 atm the speed of sound is 1,445 m/4,740 ft s−1. The speed of sound increases by about 4 m/13 ft s−1 for an increase in temperature of 1°C/33.8°F, by 1.5 m/4.9 ft s−1 for an increase of 1 ppT in salinity and by 1.7 m/5.6 ft s−1 for an increase in pressure of 10 atm. This dependence on density means that the distribution of sound–speed displays the three vertical regions shown by temperature and density.
Below the mixed upper layer the velocity of sound decreases with depth in the region of greatest temperature change (thermocline). From about 1,000–1,500 m/3,280–4,920 ft the pressure dependence is the most important factor and the velocity of propagation increases with depth.
The variation in the speed of sound leads to the phenomenon of refraction, whereby the direction of propagation of the sound waves is altered. Thus sound waves that start in the vicinity of the minimum speed of sound at 1,000 m/3,280 ft or more tend to remain at this depth as energy spreading out in the vertical is refracted back to the original level. This gives rise to a sound channel called the SOFAR channel. Even quite small sound sources, for example, from whales, may be heard in the SOFAR channel at distances of many thousands of kilometres.
A similar sound channel, called the surface sound duct, may be formed if conditions make the speed of sound increase with depth just below the surface. Positions of maxima in the speed-of-sound profile produce shadow zones as sound energy in this region is refracted away from this depth.
Underwater sound is used to measure the ocean depth, by echo sounding, and for detection of underwater bodies such as submarines and shoals of fish (see sonar). The limit on the accuracy of depth determined by an echo sounder is due to lack of knowledge of the speed of sound at each particular location, and smaller-scale variations in the mixed upper layer can cause great difficulty in the interpretation of sonar returns.
See also tide.
Ocean Surface Topography from Space
Woods Hole Oceanographic Institution Home Page
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