I.

Introduction

Oceans and Oceanography, the ocean is a vast body of salt water that covers about three-quarters of the Earth's surface; oceanography is the scientific study of the physical, chemical, and biological processes that maintain its structure and motion. Marine science is also concerned with the boundaries of the ocean, with the atmosphere above, and with the seafloor below as well as at coastlines, and with ice.

II.

Ocean Basins

In the Southern hemisphere there is a circumpolar zone (the Southern Ocean) that connects the southern extremities of the S-shaped Atlantic, the vast triangular Pacific and the smaller Indian Ocean. There are several smaller semi-enclosed seas, of which the Arctic, the Baltic, and the Mediterranean are typical, that connect with larger oceans and modify their properties.

The average depth of the ocean ranges from around 4,000 m (13,100 ft) to 5,000 m (16,400 ft). Close to the land the seafloor is commonly found at shallow depths of less than 200m (655ft) which slope gently, sometimes rising to form offshore banks or islands. These shallow regions typically extend 100 to 200 km (60-120 mi) out from the land masses to form the continental shelves, regions of economic importance for fisheries, for the recovery of oil and gas, and for the disposal of waste. Seawards of the continental shelf, at the so-called shelf-break, the seafloor deepens rapidly, by about 3,500m (460 ft) to the continental rise, a gradually sloping zone of sediment extending about 600 km (360 mi) to the flat abyssal plains of the deep ocean floor.

The central axes of the main ocean basins are connected by the mid-ocean ridge system, extensive mountain chains with inner troughs that are intersected by fracture zones. The mid-ocean ridges (see Oceanic Ridge), fundamental to understanding the evolution of the ocean basins, are explained by plate tectonics. They are associated with earthquakes, volcanoes, and with hydrothermal vents that transfer hot chemically-rich fluid from the Earth's interior and are associated with unusual sulphide-dependent biological communities. From the mid-ocean ridges, molten rock wells up and spreads internally, adding new material to the Earth's rigid crustal plates. The plates are moving apart, at a few centimetres per year. In areas where the plates overlap, such as the rim of the Pacific Ocean, crust is subducted and returned to the mantle, forming trenches which reach depths exceeding 7 km (4.4mi). The deepest known depth, in the Mariana Trench east of the Philippines, is 11,033 m (36,198 ft).

It is useful to distinguish between the shallow continental shelves and the deep ocean, but it must be kept in mind that even the deepest trenches are shallow relative to the diameter of the Earth: the depth-to-width ratio is about 1 to 1,000. The ocean, like the atmosphere, consists of a shallow layer of fluid held on the rotating Earth by the force of gravity.

III.

Ocean Water

The ocean contains 97 per cent of the water on Earth, the atmosphere 0.001 per cent. The processes that interchange and transform water into vapour, liquid or solid are fundamental to weather and climate, and to life itself.

Water is one of the most common substances but it has unusual physical and chemical properties. It is one of very few naturally occurring liquids and it occurs in all three phases: water vapour, liquid water, and solid ice. It has a large specific heat and latent heat, so that large amounts of energy are needed to raise its temperature, to melt ice and to vaporize water. These properties to a large extent control the distribution of temperature on Earth, oceanic climates being more equable than those of continental regions. There are many other properties of water—high solvent power, high dielectric constant, high surface tension, among others—that ensure reactions essential to life proceed rapidly. Most of these properties are not much affected by the presence of the dissolved salts that distinguish the saline seawater from the much smaller amount of freshwater.

Seawater is a complicated solution that probably contains all the stable elements; present analytical techniques have identified about half of them but most are present in only small amounts—less than 1 part per million. The major constituents of a typical kilogram of seawater are 965 grams of water together with 19.353 grams of chloride, 10,760 of sodium, 2,712 of sulphate, and 1,294 grams of magnesium together with smaller amounts of calcium, potassium, bicarbonate, bromide, strontium, boron, and fluoride. Samples of seawater from almost everywhere in the open ocean have been found to contain these major constituents in very nearly constant proportions, so that seawater can be treated as a uniform mixture of them (the salinity) diluted with a varying amount of freshwater. Because of this near-constancy of composition, the salinity can be accurately estimated by measuring the electrical conductivity of a sample at a known temperature.

The properties of freshwater depend on pressure and temperature; those of seawater are affected by salinity also. The density of seawater, for example, depends on temperature, pressure, and salinity in a complicated way: it decreases as the temperature increases but increases with the salinity and the pressure. The density is important because the ocean tends to move so that the densest water is at the bottom and the least dense at the top. Another important physical property of seawater is its high capacity for absorbing electromagnetic radiation, especially sunshine. Even in the clearest water almost all (99 per cent) of the incoming sunlight is absorbed in the top 100 m (330 ft), where it can be utilized in photosynthesis to transform inorganic carbon and nutrient elements into biological organisms such as plankton. At deeper depths the ocean is essentially dark, and its properties can be changed only by mixing processes.

Sound waves, however, can be transmitted through the ocean with relatively little loss: a depth charge detonated off Perth, Western Australia, can be detected off Bermuda in the North Atlantic. Man and marine animals therefore use sound to communicate underwater. Ocean depths are measured by echo sounding, in which the depth is calculated from the time taken by a pulse of sound to go to and into the seafloor and back. Sonar works similarly but the beam is transmitted at an angle to the vertical to detect or to image submarines, fish shoals or the shape and texture of the seafloor. GLORIA (the Geological Long Range Inclined Asdic), for example, uses a towed subsurface sonar to image the floor of the deep ocean for several kilometres each side of a surface research vessel.

IV.

Ocean Structure

The superficial appearance of the ocean is now well known from space observation. We live on what has been called “the blue planet” and from space see mostly blue sea, white clouds, and the relatively small amounts of land. We can distinguish ocean waves and swell and, by careful study of coastlines, the daily and half-daily movement of the whole ocean basins that constitute the tides. Such visual observations are confined to the surface; other important properties require observations from ships.

The distribution of sea surface temperature is the property best known, because it can be measured from space as well as by using simple methods and it is reported by merchant ships. In the open ocean it decreases from values of 30°C or more, near the equator to about -2°C near the ice in high latitudes. The salinity is more difficult to measure so is less well known; it is relatively low in high latitudes and has subtropical maxima at about 25°N and 25°S, with an equatorial minimum between. The distribution is linked to the difference between evaporation and precipitation, the low equatorial salinity being linked to high tropical rainfall (hence there are jungles and rainforests on land) and the twin maxima to the low rainfall of the subtropical anticyclones (with deserts on land). Both surface temperature and salinity are distributed approximately zonally, with contours running east to west. There are anomalies near coasts, associated with ocean currents and with a phenomenon known as upwelling. Upwelling regions are found near the eastern boundaries of the ocean where winds blowing along the coast can produce a mean offshore surface current. Deeper water (from perhaps 500 m/1,640 ft)) upwells to replace it, bringing lower temperatures. The upwelled water is often rich in nutrient salts, and as a result the upwelling areas are biologically highly productive and rich in fish and other forms of marine life.

Observations at depths below the surface are less numerous but scientists have a good knowledge of the mean distribution of temperature, salinity, and oxygen, with less complete information about other constituents. By far the best known is the temperature structure. The range is the same as that at the surface (-2°C to 30°C, just about the range of temperature in which people can live) but there is much more cold water than warm; the average temperature is 3.5°C. All the water warmer than 5°C is confined to a relatively shallow layer between latitudes 50°N and 50°S.

Apart from near-surface seasonal and diurnal changes, the typical structure is of a layer of nearly isothermal water near the surface, separated by a layer of relatively rapid temperature change (the main thermocline) from a thick cold layer extending to the seafloor. North and south of latitude 50° the temperature varies little with depth. In middle latitudes the surface temperature increases and the depth of the main thermocline is a maximum, about 1 km (3,300 ft). At low latitudes the surface temperature is high and the thermocline shallow (about 100 m/330 ft) with a rapid change of temperature with depth. This structure is to some extent explicable in terms of the physical properties of seawater: in general the colder the water the heavier, so it is to be expected that the densest (cold) water will sink to fill the deep ocean basins. The coldest water is found at the surface in polar regions in winter, after its heat has radiated away into the long polar night: it sinks and makes the deep ocean cold, even under the tropics and the equator. Exactly how and where the sinking takes place is still being investigated. Salinity, as well as temperature, affects the density, especially at low polar temperatures. The main sinking regions appear to be of limited extent, confined to the Weddell Sea, in the Atlantic sector of the Southern Ocean, and to the Greenland-Iceland-Norwegian Sea in the North Atlantic. The salinity structure of the ocean is more complicated than its temperature structure. In general the densest water, with the lowest temperature, is found at the bottom. Salinity usually affects the density less so can be more variable with depth. The processes that affect the salinity (rainfall to dilute the water and evaporation to concentrate it) occur at the surface and form water masses with a particular combination of salinity and temperature. Once a water mass leaves the surface its temperature and salinity can be altered only by mixing other water masses. Most of the mixing processes treat heat and salt in the same way, so a water mass tends to retain its own particular temperature/salinity (T/S) characteristic.

Temperature and salinity are the most important tracers for indicating the source regions of water masses. They are called conservative tracers because there is no process, away from the surface, that puts in or takes out heat or salt, so in the deeper layers the temperature and salinity are conserved. A T/S diagram, showing how the salinity varies with temperature in a particular water column, provides a sort of fingerprint that allows water masses to be traced over thousands of kilometres of travel, being only gradually modified by slow mixing with other water masses. The detailed processes that bring about the mixing present a central problem in modern physical oceanography.

There are other tracers which, though not conservative, are valuable because they can provide an indication of time. Water at the sea surface is usually saturated (or even supersaturated) with atmospheric gases, including oxygen. When the water leaves the surface its oxygen content is gradually reduced because it is the life support of marine creatures and is used in the decomposition of detritus. The decreasing oxygen content thus provides some indication of the time since the water left the surface. In some regions, where the water is stagnant, all the oxygen is used up and hydrogen sulphide is present instead. The Black Sea is a classic example—it is said to be so called because sulphides blacken metal objects lowered into it.

Other tracers, called transient tracers, have distributions that change with time, often due to the influence of humankind. Tritium, for instance, is the heaviest isotope of hydrogen; its concentration in the ocean is almost entirely due to radioactive fallout from nuclear weapons testing after World War II. Its spread in the ocean has clarified some ocean-circulation rates and the magnitude of mixing. Tritium is radioactive, decaying with a half-life of 1,245 years to form a stable isotope, Helium³. Measurements of the tritium and Helium³ in the same sample provide an estimate of the time since the sampled water left the surface. Both the measurements and the interpretation are complicated but are producing valuable insight into deep ocean circulation. Other man-made tracers, such as freons, are also producing valuable results, and studies are being made of the possibility of deliberately injecting tracers, such as sulphur hexafluoride, to study transport and mixing.

V.

Ocean Currents

Ocean currents near the surface affect ships and most of the information about them comes from mariners' reports of their drift from their intended track. In spite of the different shapes of the Atlantic, Indian and Pacific oceans they have a broadly similar surface current pattern dominated by an ocean-wide clockwise circulation (or gyre), the currents being much stronger in the narrow region near the western boundary. The Gulf Stream of the North Atlantic and the Kuroshio in the Pacific are the best known currents in the ocean; the corresponding Somali Current in the Indian Ocean is complicated by seasonal variation of the monsoon. Near the equator in all oceans there are two westward-flowing Equatorial Currents; in the Pacific and Indian Oceans and in part of the Atlantic, they are separated by a eastward-flowing Equatorial Countercurrent. In the Southern Ocean there is no continuous continental barrier (although the narrow Drake Passage may have a related effect) and the main surface current flows round the Earth in the east-going Antarctic Circumpolar Current. The published charts of surface ocean currents give the average, climatological, mean—on a particular occasion the current may be quite different, especially with currents like the Gulf Stream which meanders and sheds ring-shaped eddies in a complicated way. The major surface currents vary with wind and weather, but can be regarded as semi-permanent.

There are few subsurface currents of a semi-permanent kind. Perhaps the most interesting are the Equatorial Undercurrents found in the Atlantic and the Pacific Oceans, and sporadically in the Indian Ocean, flowing from the west at speeds of over 1 metre per second (3 ft per second) at a depth of about 100m (330 ft) on the equator. Other semi-permanent subsurface currents are found when dense water is formed in a basin with a shallow sill: the dense water overflows the sill as a current into the ocean basin outside. Typical examples are the flow of heavy deep water from the Mediterranean Sea into the Atlantic Ocean at Gibraltar, and from the Red Sea into the Indian Ocean at the Strait of Bab el Mandeb. Dense water also flows into the Atlantic Ocean over various sills in the ridge connecting Greenland, Iceland, and Scotland.

Otherwise our knowledge of subsurface currents is difficult to summarize because they are so variable. Cold water originating in the northern North Atlantic or in the Weddell Sea occupies all the deep ocean basins so there must be deep flow towards the equator, but the path taken is not well established. In the North Atlantic there is thought to be a deep vertical-meridional cell with deep water flowing southwards at cold temperatures. There is no source of deep water in the Pacific Ocean and the relatively sluggish circulation mainly takes place above 800 m (2,640 ft), warm water flowing north in the Kuroshio and returning in the central and eastern Pacific at lower temperatures. The Indian Ocean too has no deep water formation. Some water has been observed to flow polewards as subsurface western boundary currents, notably as a countercurrent under the Gulf Stream at depths below 2,000 m (6,550 ft). Elsewhere in the ocean the mean current is obscured by the variability introduced by mesoscale ocean eddies. These resemble meteorological depressions and anticyclones but are smaller (typically 100 m/330 ft) across and with currents of order 10 cm per second (4 in per second). Such circulations typically have a lifetime of about 100 days and the variable currents associated with them obscure the much smaller mean currents. Although the mean speed of deep ocean currents is small, they transport large quantities of heat and freshwater so are important to the maintenance of climate

VI.

Air-Sea Interaction

Apart from tides, all atmospheric and oceanic motion is powered by the Sun. There are two basic questions: What happens to the sunshine? And what happens to the water? Most of the solar energy falls on the tropics whereas the outgoing long-wave radiation is more uniformly distributed with latitude. The excess of heat in low latitudes is transferred polewards by motions in the atmosphere and the ocean. The atmosphere can be thought of as a gigantic, inefficient, heat engine absorbing heat in the hot equatorial belt and losing it nearer the poles. In lower latitudes the air rises; forming equatorial rainbelts and travelling polewards before sinking in the subtropical anticyclones and returning equatorwards as the trade winds. Polewards of 30°N and 30°S winds are basically westerly, but with travelling depressions and anticyclones that bring unsettled weather to middle latitudes. Both the low-latitude meridional cell and the smaller scale disturbances transfer heat from tropic to pole. They also determine the general circulation of the atmosphere, the winds of the world.

It is these wind patterns that force the mean-surface currents of the ocean, which are thought to be mainly wind-driven. The deeper currents are driven by density differences, to produce the thermocline circulation that is brought about by the sinking of surface water that is sufficiently cold and saline to be dense enough to sink to great depths and to fill the deep ocean basins. The mechanisms are obscure and it may be that the wind-driven and the density-driven circulations interact. Computer models of the ocean, and of the coupled atmosphere and ocean, are being used to study the motions involved. It is very important to gain a deeper understanding of the present climate so as to get more confidence in climate prediction and the scale and intensity of any global warming.

A major international programme, the World Ocean Circulation Experiment (WOCE), part of the World Climate Research Programme (WCRP), gathers resources from about 30 countries and, between 1990 and 1998, made unprecedented surface and satellite observations; it is expected to produce much increased knowledge of the structure and circulation of the oceans. The Global Ocean Observing System (GOOS), established in 1992, part of the Global Climate Observing System (GCOS), provides observations to monitor changes in ocean circulation, as well as data concerning the climate and the biological, chemical, and physical composition of the world’s oceans.

VII.

Using the Ocean

The economic uses of the ocean depend on such basic things in its large area and volume, together with the physical and chemical properties of seawater. Its combination of high density and low viscosity make it suitable for propelling ships; its complex chemical composition makes it capable of supporting a complicated food web starting with photosynthesis and including the proteinaceous fishes that humans find palatable and nutritious. Its opacity to sunshine makes it dark which, together with its vast volume, encourages the concealment in it of anything from sewage (see Sewage Disposal) to nuclear submarines. Its high specific and latent heats make it the regulator of the Earth's climate and the primary control on human existence. The ocean has been used since long before recorded history: nowadays there are many more people with more powerful machinery, tools, and sources of energy. Improved understanding is needed if its capacity is not to be over-exploited.

The ocean has traditionally been used as a support for ships, as a source of food, and as a sink for waste: it is increasingly recognized as a vital component in the regulation of climate. Valuable chemicals can be extracted from seawater and the recovery of minerals, including hydrocarbons, from the seafloor is a major industry which is gradually extending its operations into deeper water. Military activity such as anti-submarine warfare, on the other hand, is declining with the end of the Cold War, its deep ocean research and development being partly transferred to coastal waters. Surface ships are more concerned with waves than currents, and increasing use is being made of wave forecasts based on computer models using wind speeds from meteorological forecasts. The results are compared with ships' observations and especially with wave height observations from satellite altimeters, which also measure observations of surface wind speed. Other instruments (scatterometers) measure both wind speed and direction. Wave forecasts are also valuable to fishing vessels, as are special sonar fish-finding acoustic systems. Fisheries oceanography, however, is a very difficult subject. The varying abundance of the stocks is difficult to predict. Managing the industry so as not to exceed what are thought to be sustainable yields presents difficult intergovernmental problems, both of obtaining and of enforcing the necessary treaties. There is little hope that fish will supply more than a small fraction of the world's protein needs. There is such a large volume of ocean that dumping unwanted material in it is attractive to industries and to cities that wish to avoid paying the extra cost of dumping on land, or of processing or recycling their waste products. Most people have first-hand experience of marine water pollution but there are few good estimates of what is dumped where. More than three-quarters of marine pollution comes from sources on land and a third of it is airborne, including some pollutants from vehicle emissions. Only about 12 per cent comes from ships and boats, as a result of operational discharges, accidents, or general rubbish.

For many years now the value of offshore petroleum and gas production has exceeded that of the world fish catch. High-yielding reserves are still being found, although at gradually greater depths and in regions where environmental conditions are much harder for offshore structures to withstand and for their supporting service industries to operate. The exploitation of material on the floor of the ocean is mainly limited to the extraction of sand and gravel, from relatively shallow depths. There has been little progress in the proposed extraction of metals from the manganese nodules found in large amounts on the floor of the deep ocean, or of the metal-rich sediments known to exist in holes in the rift valley of the Red Sea, or those associated with the hydrothermal vents of the Atlantic and Pacific Oceans. Some chemicals, like bromine, continue to be extracted from seawater and there is growing interest in pharmaceutical products obtained from marine biota. The water itself represents a valuable resource for making fresh water in many parts of the world where flash distribution or reverse osmosis is economic, although the high latent heat of water imposes a high energy cost.

That the ocean acts as a regulator of climate is increasingly recognized but, in spite of the expansion of and progress in marine science this century, scientists are still very ignorant about the properties, populations, and processes of the ocean. Advanced computer models of the coupled atmosphere and ocean are being developed but need more and better information about ocean processes. Not until they reach a more advanced state can we hope to predict, with confidence, the changes in climate that may be being brought about by increasing carbon dioxide, methane, and other radiatively active gases in the atmosphere.

The ocean and the atmosphere are expected to last, in more or less their present form, for hundreds of millions of years. In the next few generations the population of the world will exceed ten billion, most in what are now developing countries; by then our survival will depend on a better understanding of the interaction between our finite biological and physical resources.