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Parameter definitions
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Standard definitions for the parameters shown in this atlas are as follows. Further details can be obtained from the suggested references or from a standard textbook such
as Pond and Pickard (1995):
Potential temperature (°reeC) The potential temperature, Θ , is defined as the temperature that a sample of seawater would attain if brought adiabatically (without gain or loss of heat to the surroundings) from the pressure appropriate to its depth to the ocean surface (see e.g., Feistel, 1993). Salinity (PSS78 scale) The salinity, S, is essentially a measure of the mass of dissolved salts in one kilogram of seawater. Because the major ions in seawater are found in a constant ratio to each other, the salinity of a sample of seawater is now measured in terms of a conductivity ratio relative to a standard solution of potassium chloride. Thus salinity values according to the current definition of the Practical Salinity Scale of 1978 (PSS78) are dimensionless with no units. (See e.g., UNESCO, 1981). Neutral density (kg/m3) Neutral density, γn , gives a very close approximation to truly neutrally buoyant surfaces over most of the global ocean. γn is a function of salinity, in situ temperature, pressure, longitude, and latitude. (See e.g., Jackett and McDougall, 1997). By convention all densities are quoted as the actual density minus 1000 kg/m3. Potential density (kg/m3 ) The potential density, σ , is the density a parcel of water would have if it were moved adiabatically to a standard depth without change in salinity. σ0, σ2 and σ4 are the potential densities of a parcel of seawater brought adiabatically to pressures of 0, 2000 and 4000 decibars, respectively. (See e.g., Pond and Pickard, 1995). |
Oxygen (μmol/kg)
The dissolved oxygen content, O2 , can be used to trace certain water masses. Oxygen enters the ocean from the atmosphere, but is also produced in the surface layers by phytoplankton and is consumed during the decomposition of organic material. This leads to relatively large changes in concentration depending on depth, position and initial solubility (which is a function of temperature and salinity). (See e.g., Broecker and Peng, 1982). Nitrate, Phosphate and Silicate (μmol/kg) Nitrate, NO3 , Phosphate, PO4 , and Silicate, Si, are some of the main nutrients utilised by phytoplankton. They are also non-conservative tracers, but vary inversely with oxygen concentration in the upper- and mid-ocean. They are supplied mainly by river runoff and from sediments. (See e.g., Broecker and Peng, 1982). Chlorofluorocarbons (pmol/kg) Chlorofluorocarbons, CFCs, are anthropogenically produced chemicals that enter the ocean from the atmosphere. Since they have a time-varying atmospheric history, they can be used to deduce information on mixing rates in the ocean and to follow the movement of water masses forming at the sea surface (see e.g., Weiss et al., 1985). Total Carbon dioxide (μmol/kg) The total dissolved inorganic carbon content of seawater is defined as: TCO2 = [CO2 *] + [HCO3- ] + [CO32- ] where square brackets represent total concentrations of these constituents in solution (in mol/kg) and [CO2 *] represents the total concentration of all un-ionised carbon dioxide, whether present as H2 CO3 or as CO2 . (See e.g., DOE, 1994 for further details.) |
