This new standard is called the Thermodynamic Equation of Seawater (2010), or TEOS-10. However, a new international standard for seawater density, as well as all other thermodynamic properties, has recently been developed. For many years the internationally accepted standard for seawater densities has been the 1980 International Equation of State, known by the acronym EOS-80. Density is usually calculated using a mathematical function of temperature, salinity, and pressure, sometimes called an equation of state. In addition to controlling physical properties, the variation in space and time of temperature and salinity are also important water mass tracers that can be used to map the ocean circulation. Important state variables measured for parcels of water in the ocean are therefore temperature, which is related to the heat content, salinity, which is related to the amount of dissolved matter, and the pressure (Table 1). Physical properties vary with the amount of heat and the amount of dissolved matter contained in the water, as well as the ambient pressure. Such a table or formula, when formally defined in a published document and endorsed by a scientific authority, is known as a standard. The table or formula that is used is usually derived from careful laboratory measurements. However, direct measurements can be complicated to carry out, especially in the field, and in many cases it is more convenient to measure a few important 'state variables' on which the properties depend, and then look up the desired property as a function of the measured state in a table, or calculate it using a mathematical formula. Physical properties can be measured directly. Density in particular is an important property in ocean science because small spatial changes in density result in spatial variations in pressure at a given depth, which in turn drive the ocean circulation. However, if the density signatures of all the end member water masses are known, this mixing can be "unraveled" to determine the proportions of their various source waters.The physical properties of seawater include both 'thermodynamic properties' like density and freezing point, as well as 'transport properties' like the electrical conductivity and viscosity. The information on these maps is important because the density characteristics of a parcel of seawater are determined while it is at the sea surface.Īs water parcels sink and move through the ocean, their densities will be modified by mixing with other parcels of seawater. Surface seawater temperature varies widely over the globe (-1☌ to 30☌), while seawater's overall salinity range is relatively small (see monthly salinity maps). Seawater density is determined by its temperature and salinity. In these maps, orange regions have higher density and purple regions have lower density (1030 to 1020 kilograms per cubic meter). These are produced from Aquarius sea surface salinity and ancillary sea surface temperature using TEOS-10 equation of state (McDougall and Barker, 2011).Ĭlick on the images (below) for a closer view. Monthly sea surface density map images, derived from Aquarius science products V5, are available here.
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