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Prepared By: Introduction The sea is a mysterious wilderness, full of secrets. Ever since the evolution of oceans, man was trying to probe into its mysteries, but in many ways they are still unknown and mysterious. Although man conquered the entire land, the problem of conquering the sea is much more formidable and complex. Oceanography is a science dealing with the study of oceans . It is not a single scientific discipline, but rather a coordinated application of many other sciences to the oceans. Broadly speaking, Oceanography deals with the physical, chemical, biological, geological and meteorological aspects of this water planet. Advances in sciences and technology enabled the marine scientists and sea -faring explorers to have a better understanding of the oceans and to throw light on many of its unsolved problems. Further, development of an array of mechanical and electronic equipments paved the way for the collection of samples and data from any depth in the sea with considerable degree of precision. Recently remote sensing technique, aerial photography and satellite imaging are widely in use for the various oceanographic investigations. Pressure resistant underwater observation cabins such as 'Bathyscaphe' can go down even to 11,000 meters in the sea and make direct observations of the deep sea. Automatically operating underwater cameras can provide extensive information about the nature of sea bottom , bottom fauna and speed of deep water movements without the risk involved in diving. The present article is an attempt to explain briefly some of the very important physical oceanographic parameters such as water temperature, salinity, dissolved oxygen, illumination and currents considering their significance to the life in the sea. Water temperature Since water has a peculiar property of having high specific heat which is about three thousand times that of air, seawater temperature remains more or less steady over vast areas of the sea unlike in terrestrial environment where the temperature fluctuates hourly. Because of its tremendous capacity to store heat and also release it , the sea is able to control the climate of large areas of land. Sun is the main source of heat available on the earth. About 50% of the sun's energy that reaches the sea surface is in the infra -red rays. Near the surface itself these rays get absorbed. Hence only a thin surface layer gets heated directly. In low latitudes, heat absorption at surface produces a warm, light surface layer overlying the cold , denser, deep layers. The temperature gradually decreases from surface downward to certain depth and then sharply to some depth. This layer of sharp temperature gradient is called thermocline . The sea loses heat in many ways. If the sea surface is warmer than the overlying air, heat will be lost. Also through surface evaporation and consequent cooling, a large amount of heat is lost especially in the middle and lower latitudes. A substantial amount of heat is also lost during the formation of ice. So generally speaking, seawater receives heat in lower latitudes and loses heat in the higher latitudes. The temperature distribution in the sea takes place in a number of ways. As cold water is usually heavy, polar waters sink and flow along the bottom towards the equator. To take the place of this sunken water there exists a counter flow of tropical warm water which flows pole-ward along the surface. In addition to this pattern of water flow, there occurs a vertical water circulation which is characteristic of temperate waters . During winter, surface water is cooled and becomes heavy and consequently sinks. Sub-surface water which is warmer and lighter, ascends. But this kind of vertical water movement is usually restricted to shallow waters. The temperature stability of the sea is the result of the continuous movement of the water. The highest sea temperatures are found at the surface in low latitudes. Generally, the temperature remains fairly constant through out the year in high and lower latitudes. In middle latitudes, surface temperature varies with season according to the climatic changes. Salinity Ever since the formation of the oceans, all kinds of salts were being washed away from land to sea. Sea water is a chemical mixture in which all the elements listed in the Periodic table are present in solution in varying proportions. The average salinity of seawater is considered as 35 ppt. The Standard seawater with salinity of 35 ppt has a composition : Sodium chloride 77.76%, Magnesium chloride 10.88%, Potassium sulphate 2.46%, Calcium carbonate 0.34% and Magnesium bromide 0.22%. In addition to these, the seawater contains many trace elements as well which are in very low quantity. In the open sea, there is very little variation in salinity except very near the surface. In the surface waters of tropics and in those regions affected by the trade winds, excessive evaporation may increase salinity. In partially enclosed areas like the Red Sea, shallow 'sabka' areas and lagoons of Arabian Gulf, evaporation may be so great that hyper-saline conditions may prevail. On the other hand, in polar waters , melting of ice in summer may bring down salinity to a large extent. The large volume of fresh water brought in by large rivers like the Amazon, the Congo and the Ganges may dilute seawater locally. But considering the vastness of the sea, such local variations are negligible. Moreover, the continuous movement of the water will promptly correct such local and seasonal imbalances. Hence the salinity will remain steady for long periods over vast areas. One remarkable aspect regarding sea water is that although the total salinity might undergo change, the relative proportion of the major constituents remains unchanged. Therefore there is a relative constancy of composition of sea water. Dissolved oxygen The Oxygen content of seawater varies approximately between 1-6 ml/l. High values occur at the surface, where dissolved oxygen tends to equilibrate with atmospheric oxygen. Also rapid photosynthesis by the marine algae might sometimes produce super-saturation. But the receipt of oxygen from both these sources varies depending on salinity, temperature, solar illumination and nature of sea surface. The amount of oxygen in the atmosphere is far in excess of that in saturated seawater. Hence wherever there is contact between the two, oxygen is bound to diffuse in to water till the saturation point is reached. This rate of diffusion is hundred times greater if the sea surface is agitated. Although seawater receives oxygen only at or very near the surface, life exists everywhere. Respiration and bacterial decomposition of organic matter that take place in deeper layers indicate the availability of oxygen. This is brought about by the continuous mixing of waters, particularly by the deepwater currents. A current originating in higher latitudes will hold more oxygen than one originating in lower latitudes as a result of the prevailing thermo-haline conditions in these two regions. Oxygen is lost from the sea water in different manner. Part of the oxygen will be consumed by the primary producers themselves and also the animals during respiration. Bacterial decomposition of organic substances also reduces the oxygen content in the sea. In spite of the vertical and horizontal mixing of the waters, there is one layer in the sea where oxygen deficiency is very acute. Oxygen decreases from surface vertically downward to this layer which is known as the Oxygen minimum layer. It is present in almost all the oceans at a depth somewhere between 150 and 1000 meter and is very prominent in low latitudes. A combination of factors is responsible for the creation of the oxygen-minimum layer. This layer often coincides with the thermocline. Illumination The sea is divisible into three zones depending on the presence or absence of solar illumination. They are the euphotic (well illuminated), dyphotic (faintly illuminated) and aphotic (not illuminated at all) zones. Illumination depends on the sun's position, latitude, season and cloud cover and hence the light penetration in the sea is related generally to these factors. Much of the incident light is reflected from the sea surface. Relatively more light is reflected from a ruffled surface than a calm one. Also there is a gradual decrease of light intensity with depth. 50% of the sun's radiation consists of infra red rays which are invisible. This is almost fully absorbed in the first one meter of water. Ultra violet rays are also quickly absorbed. So it is only the visible part of the spectrum from violet to red rays that matters in the sea. It has been reported that up to 60% of the incoming radiation is absorbed in the top one meter while 80% in the first 10 meters. In coastal waters, about 99.5% is absorbed in the first ten meters. This is due to rapid absorption of light rays by photosynthetic organisms and scattering of light by water molecules and suspended particles. As the light travels through water, its composition changes. The differential absorption of the solar spectrum partly accounts for the colour of the sea surface. The blue component penetrates more and the red least. The blue colour of the oceanic waters is due to the scattering of the blue rays upwards while yellow and red rays are largely absorbed.. The green colour of the coastal waters is due to the presence of large quantities of suspended matter reflecting back light of longer wavelengths and relatively greater absorption of blue light. Currents The sea is habitually restless. Owing to the influence of the rotation of the earth (Coriolis force) and partially because of the presence of the land masses obstructing the path of the currents, the currents are deflected towards the right in the northern hemisphere and to the left in the southern hemisphere. Hence the general pattern of oceanic circulation for the Northern Hemisphere is in clockwise direction and that for the Southern Hemisphere is in anticlockwise direction. In addition to this, there are wind induced currents which occur whenever wind blows consistently from the same direction for a significant period of time. Also tidal currents caused by the gravitational pull of the sun and the moon are considered to be the strongest of all water currents generally observed at sea. Importance Of Oceanographic Studies Oceanographic conditions govern the world weather. Sea surface data collected from a network of stations through remote sensors along with climatic data are often used for weather forecasting. Oceanographic conditions influence the distribution and migration of fishes and other marine organisms. in the sea. Oceanography plays an important role in defense as well. In the sea where sudden variations occur in the vertical temperature and salinity structure, the density gradients are also large enough to deflect back the sound waves transmitted from sonar. In case the density difference is enormously large, sound waves passing these layers undergo total deflection and hence a submarine or any other vessel hiding in this region is hard to be detected. Hence certain specific oceanographic data of the territorial waters are often treated as confidential from defense point of view. Long term oceanographic data on the speed and direction of waves and currents along with the bathymetry of the territorial waters is highly essential to take remedial measures on coastal erosion causing havoc to life and coastal structures in many countries. Record of near -shore oceanographic data is a pre-requisite for planning recreational facilities along the beaches and coasts and to demarcate safe areas for diving and water sports. A proper understanding about the coastal circulation pattern and bottom topography is a basic requirement for selecting suitable dumping sites for the treated effluents from factories and industrial establishments situated along the coasts. References:
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