Oceanography

Distribution of temperature and salinity

Temperature is one of the most important physical properties of sea water. Salinity and temperature together control the sea water density. Sea water temperature controls the distribution of marine organisms and fish. Because of the influence heat has upon chemical reactions (metabolism) sea water temperature has profound effects on the life processes of fish. Sea water temperature has profound effects on the life processes of fish. Sea water temperature influences the properties of surface air over the ocean. The vertical temperature structure affects the propagation of sound in the sea and is therefore important for anti-submarine warfare. Sea water temperature is indicative of other changes and conditions in the sea such as upwelling intensities, current and water mass boundaries.

The sun provides 99.97% of the heat energy required for the various physical processes taking place in the earth-atmosphere system. The other energy sources, such as heat from the interior of the earth, radiations from other Celestial bodies, energy from the gravitational attraction of the sun and moon are negligible compared with the radiant energy received from the sun. The sun, which is a luminous gaseous sphere, has a surface temperature of 6000°C and emits energy in electromagnetic waves. The maximum emission occurs at relatively short wave lengths in the visible portion of the spectrum, between about 0.4 and 0.7µm (Trewartha and Horn, 1980). According to Sellers (196%), a breakdown of the spectral composition of solar radiation indicates that 9% is in the ultraviolet (less than 0.4µm), 45% is in the visible(between 0.4µm and 0.74µm), and the remaining 46% is in the infrared (more than 0.74µm). The amount of solar radiation incident at the top of the earth’s atmosphere depends upon the time of the year, the time of the day and the latitude (Ayoade, 1983).However, all of the radiation incident on the top of the atmosphere does not reach the surface of the earth. The atmosphere absorbs, reflects, scatters and re-radiates solar energy. On a global basis of the above radiation, 6% is reflected and scattered by atmospheric. Cosntitutents to space, 24% is reflected and scattered to space by clouds,,3% is absorbed by clouds, 14% is absorbed by atmospheric constituents, 22% reaches the surface of the earth as diffuse sky radiation, and 31% reaches as direct beam solar radiation on the surface of the earth. Thus, the radiation reaching the surface of the earth is composed of direct beam solar radiation and the diffuse sky radiation.

The radiation that actually reaches the surface of the earth, i.e., the insolation is not all absorbed. Certain amount of insolation is reflected by the earth’s surface. The percentage of the insolation reflected by a surface is called the “albedo” of that surface. Fresh snow albedo is nearly 80%, from which it is clear that fresh snow absorbs only a comparatively small proportion of the isolation. The albedo of water is generally less than that of most surfaces on land. Due to several reasons, sea water and land surfaces behave differently to insolation. First, the albedo of sea water surface is generally smaller than that of land suface. Second, sea water surface is transparent and hence sun’s rays can penetrate deeper than in the case of land surface which is comparatively opaque. Third, heat transfer in sea water is mainly by convection process, which is a quicker method for heat transfer than the slow conduction process through which heat is transmitted within land. Fourth, the specific heat of sea water is greater than that of land. Sea water required a greater quantity of heat per unit mass to change its temperature than that of land. Lastly, as water is always avaialbe for evaporation on the sea water surface, evaporation is continuous there, while on land evaporation takes place only if the soil is moist. As evaporation is a cooling process, it must be considered while comparing the thermal properties of land and sea water surfaces.

Factors Influencing Sea water Temperature

Sea water temperature varies with respect to space (horizontally and vertically) and time. Major variations in temperature are very common in the upper layers of the ocean because of the influence of external environmental factors. The factors which affects sea water temperature and its variation can be broadly classified into two groups: (1) energy transfer processes taking place below the sea surface and (2) advective transfer processes taking place below the sea surface. The first group is concerned with the factors associated with the transfer of solar radiation to the sea and with the factors associated with the removal of heat from the sea. The chief energy transfer processes that produce heating of the sea surface are the absorption of radiation from sun and sky, the convection of sensible heat from the atmosphere, and the condensation of water vapour. The energy transfer processes that produce cooling of the sea surface are the back radiation from the sea surface, the convection of sensible heat to the atmosphere, and the evaporation. The second group is concerned with the external and internal forces which cause movement of water without transfer of heat to or form the atmosphere (LaFond, 1954a). These forces produce tide, current and wind etc. which transport water from one place to another in the ocean. If the advected water differs in temperature from the adjacent water in its new region in the ocean, a temperature gradient horizontal or vertical, will be produced.