physical oceanography

physical oceanography is the study of the physical processes taking place within the oceans and their interactions with the atmosphere. The high heat capacity of water relative to air means the oceans play a major role in the climate by redistributing heat around the globe. If the oceans did not exist, the poles would be much colder and the tropics much hotter. The mechanisms resulting in flows of water and the mixing of waters of different origins are of fundamental importance in understanding ocean processes. The rotation of the earth, and its influence on the atmosphere in generating winds, provide the basic processes whereby currents develop in the oceans. In the absence of both continents and winds, a pattern of rotating cells (or gyres) of currents would develop. This basic pattern is strongly modified by the land barriers and the general shapes of the continental boundaries. The density of sea water is determined mainly by its temperature, salinity, and the hydrostatic pressure. Seawater temperatures are generally warmer at the surface and cooler at depth. The seasonal thermocline, which at temperate latitudes forms in spring and disintegrates in autumn, is important biologically. Water below it is usually richer in the nutrients needed for the growth of the marine plant phytoplankton than the water above it. The nutrient-rich waters from under the thermocline are only brought to the surface during upwelling and in winter, when the surface waters are cooled and storms mix the surface waters down to depths of several hundred metres.

At the surface, water temperatures fluctuate as a result of solar radiation, heat exchanges with atmosphere, and evaporation (the latent heat of evaporation means the surface skin of the ocean is cooled when water evaporates from the surface). When seawater is cooled it becomes denser. Its density also increases if its salinity is increased as a result of evaporation or the formation of ice. Its density decreases (i.e. it becomes lighter) if it is warmed, or else diluted, with rain, the melting of ice, or the outflows from rivers. The outflow of the River Amazon can be traced several hundreds of kilometres from its delta, and the saltiness of the eastern Mediterranean has become higher since the building of the High Aswan Dam has reduced the outflow of the River Nile.

Thus, at latitudes where rainfall is low and evaporation is high, the surface water becomes heavier and sinks into the ocean's interior. Once a mass of water has left the surface, its properties of temperature and salinity are conserved, and are only changed by mixing with other types of water. So the water column in the ocean tends to be stratified into layers, and these increase in density with depth.

Under exceptional circumstances, the water densities become uniform from the surface to the bottom, so that water at the surface can then sink freely all the way to the bottom. This occurs regularly in the Weddell Sea and until recently in the Greenland Sea, but climate change has turned off this source of deep water, and it is now feared that this will lead to a change in the Gulf Stream. This sinking of very cold—and hence oxygen-rich—water drives the so-called thermohaline circulation of the whole ocean. This results in the total turnover of the oceans every 1,500 years which supplies the oxygen to the bottom waters of all the oceans that is needed by most of the animals living there.

One possible effect of the cessation of bottom water formation in the Greenland Sea is to reduce the flow of the Gulf Stream, which would have a substantial effect in cooling the climate of northern Europe. In the Atlantic where the water at the bottom has most recently been formed—and is described as being young—the deep water is rich in oxygen. In the Pacific and Indian Oceans, the bottom waters are old and contain far less oxygen, but are richer in nutrients (nitrates, phosphates, and carbon dioxide), released by the decomposition (regeneration) of material that has sedimented from the surface.

Bottom water formation is one of the important processes whereby carbon dioxide is being removed from the atmosphere and stored in the deep ocean. If, as predicted, climate change slows the large-scale (thermohaline) circulation, then the rate of build-up of carbon dioxide in the atmosphere will increase.

The oceans are transferring energy absorbed from solar radiation in the tropics to the higher latitudes. This is well illustrated by the contrast in the climates on the two sides of the Atlantic. On the eastern side the Gulf Stream, and its extension the North Atlantic Drift, carries warm water far to the north into the Barents Sea. So the climate of western Europe is mild, whereas down the east coast of Canada the climate is kept cool by the Greenland Current, which carries cold water and icebergs southwards. Massive computer models are now being built to predict the responses of the oceans to climate change and how they may generate even greater climate changes.

Traditionally the method used by physical oceanographers was to collect water samples and measure their properties. However, with the development of technologies that allow these properties to be measured in situ, the approach has been, more and more, to use instruments to collect the data. Such instruments are either lowered on cables or attached to moorings or to drifting buoys, or most recently mounted either on underwater vehicles or on towed bodies that undulate up and down as they are towed.

The use of satellites for remote sensing and precision navigation has revolutionized physical oceanography. Even so, collecting enough data with sufficient precision to follow and quantify the influence of important small-scale features such as eddies will require the total scientific budget for the whole world. As the power of computers has grown, so more and more effort is being devoted to constructing mathematical models to simulate the ocean. These can then be used to predict what is happening, and continually to check the model's output against real observations. Just as the accuracy of forecasts produced by marine meteorology has improved dramatically since the 1980s, a remarkable improvement has taken place in the information being produced by physical oceanography.

Bibliography

Open University, Ocean Circulation (1989).
Open University, Wave, Tide and Shallow Water Processes (1989).

http://podaac-www.jpl.nasa.gov/www.whoi.edu/science/PO/dept/www.es.flinders.edu.au/~mattom/IntroOc/new start.htmlThere is an excellent on-line textbook at http://oceanworld.tamu.edu/home/course_book.htm

M. V. Angel