Temperature is basic to any physical description of the oceans. It is the easiest and therefore the most common type of oceanographic measurement made. The exchange of heat between the ocean and the atmosphere depends strongly on temperature. The density field and resulting stratification of the ocean depend largely on temperature. The speed of sound in the upper layers of the ocean is most strongly dependent on temperature. Temperate further influences the kinds and rates of chemical reactions occurring in the ocean. The distribution of nutrients and other biologically important substances depends on temperature and the resulting density stratification.
Sea water is a binary fluid in that it consists of various salts in water. The presence of salts affects a number of oceanic parameters, including compressibility, sound speed, refractive index, thermal expansion, freezing point and temperature of maximum density. Salinity is a term used to measure the quantity of salts dissolved in sea water and is expressed in units of parts per thousand (%o or ppt). The precise definition of salinity is complicated. Fofonoff (1985) reviewed the development of the modern salinity scale and the equation of state for sea water. The Practical Salinity Scale 1978 was introduced to rectify shortcomings associated with the traditional chlorinity-conductivity relationship used to establish salinity (Lewis, 1980; Perkin and Lewis, 1980; Culkin and Ridout, 1989). In the new scale, the existing link between chlorinity and salinity was broken in favor of a definitive salinity-conductivity relationship. The new practical standard is IAPSO (International Association for the Physical Sciences of the Ocean) Standard Seawater, produced and calibrated by the IAPSO Standard Seawater Service. Salinity is now a dimensionless quantity (psu or practical salinity unit) because the algorithms in the new scale were adjusted to eliminate the %o (or ppt) used in previous scales.
The density of sea water is related to temperature, salinity and pressure (which is nearly proportional to depth) through the equation of state (e.g. Fofonoff, 1985). Density provides a measure of the hydrostatic stability in the ocean. Specifically, a stable water column is one in which density increases monotonically with increasing depth. Sea water is compressible, although less so than pure water. The compressibility of sea water can be expressed by the coefficient of compressibility, which relates fractional changes in water volume to the corresponding changes in pressure (e.g. Apel,
Compressibility of sea water is an important factor in several applications: the precise determination of the density of sea water, particularly at great depths; the computation of adiabatic temperature changes in the ocean (in an adiabatic process, compression results in warming, while expansion results in cooling); and most importantly, the computation of sound speed in sea water.
The speed of sound (c) in sea water is related to the isothermal compressibility (K) as where y is the ratio of specific heats of sea water at constant pressure and constant volume and p the density of sea water. The isothermal compressibility is easier to measure experimentally than is the adiabatic compressibility.
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