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physical oceanography


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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.

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Subjects: Maritime History.


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