Density anomaly
Density anomaly, or negative thermal expansion, is the paradoxical tendency of water to expand during cooling. This behaviour manifests in anomalous decrease in water density when the temperature drops below 4°C. The units of measurement are kg/cm3 (kilograms per cubic centimetre).
It is not possible to measure the density anomaly directly. However, we can derive it from measurable properties: electrical conductivity, temperature, and pressure, obtained by your RBR CTD instrument, such as RBRconcerto³ C.T.D or RBRbrevio³ C.T.D. The RBRduo³ C.T will calculate density anomaly if you enter depth in the table under the Parameters tab in Ruskin. See Ruskin User Guide: Standard Instruments³.
We derive salinity first, via the PSS78 method, and then use it in the algorithm, often referred to as the UNESCO equation of state:
where V (S, t, p) is specific volume of seawater derived from salinity, temperature and pressure and 1000kg/m3 is density of freshwater.
See UNESCO (1981), Tenth report of the joint panel on oceanographic tables and standards for details.
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The UNESCO equation of state is applicable within these ranges: 2 < practical salinity < 42, −2°C< temperature < 35°C. If salinity values are lower than 2PSU (freshwater), density anomaly values will not be accurate. |