Oceans and Oceanography

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.