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The suppression of mixing in stratified water bodies between the surface layer and the water layer below the pycnocline can have serious consequences for the aeration of the lower layer, because this is where the most mineralization takes place and thus the oxygen demand is greatest. This is a nonlinear process as a result of the hydrostatic pressure differences (buoyancy principle of Archimedes) that dampen turbulent fluid fluctuations around the pycnocline the turbulent energy production needed for mixing increases as the stratification increases. When the production of turbulent energy in an estuary decreases, inflowing river water and seawater will be less mixed: vertical density differences force the water column into a more stratified state, with the vertical density difference concentrated around a transition layer, the so-called pycnocline, see Fig. Mixing is mainly caused by turbulence, which is generated by frictional shear stresses at the bottom and the water surface. This is the usual situation, also in cases where strong mixing occurs over the water column. Blue=lower density, red=higher density.Ī body of water is referred to as 'stable' when the density decreases from bottom to surface. Well-mixed (left) and stratified (right) water column. The balance between small changes in salinity and temperature has great importance for the large-scale ocean circulation.įig. Sinking of surface layers implies that subsurface layers of lower density will emerge this process is called 'overturning'. In both examples sinking is enhanced by cooling. Partial freezing of a low salinity surface layer may also induce sinking by increasing the salinity of the unfrozen water this occurs in the polar regions. Evaporation of a warm surface layer increases its salinity, in which case it may sink this occurs in the North Atlantic. Warm, low salinity water tends to stay near the surface whereas cool, high salinity water tends to stay near the bottom. (3), an increase of 1 g / kg in salinity has about the same effect on the seawater density as a decrease of 4-5 oC in temperature. Seawater density decreases with increasing temperature and increases with increasing salinity. In turbid coastal waters, the suspended sediment concentration in g / kg has to be added in this expression. This expression holds for the temperature range 0\lt T\lt 40 \ oC, the salinity range 0 \le S\lt 42 and for pressures lower than p\lt 10 bar. Source: NASA.Īn accurate empirical expression for the density-salinity-temperature relationship for coastal waters has been derived by Fonofoff and Millard (1983 ). Salinity-temperature dependence of seawater densityįig. Because of the primary focus of the Coastal Wiki on coastal waters, the dependency on pressure will be ignored in this article. Pressure only plays a role at depths much greater than those of coastal waters. The seawater density \rho in the ocean mainly depends on salinity S, temperature T and pressure p. Knowledge of the seawater density is therefore prerequisite for understanding and modelling marine processes. The same holds for the small-scale turbulent velocity fluctuations that are responsible for mixing processes in oceans, shelf seas and estuaries. The large-scale long-term mean currents in the ocean and in estuaries largely depend on the density differences between water masses. 3 Processes induced by density differencesĭensity gradients play a prominent role in marine hydrodynamics.2 Salinity-temperature dependence of seawater density.The pressures are stated in mega-Pascals, where a Pascal is a Newton per square meter, and as a multiple of standard atmospheric pressure. Water Vapor and Vapor Pressure Saturated Vapor Pressure, Density for Water Tempīelow are some selected values of temperature and the saturated vapor pressures required to place the boiling point at those temperatures.
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