When two water parcels with the same mass but different temperature and salinity mix together, the newly generated water parcel may have a density greater than the mean density of the original water parcels. In particular, if the original water parcels have the same density, then the newly formed water parcel may have a density greater than the mean density of the original parcels. As a result, the newly formed water parcel will sink. This process, called cabbeling in oceanography, is due to the nonlinearity of the equation of state of seawater, especially the increase of the thermal expansion coefficient with temperature.
As an illustration, if two water parcels, A and B, with the same volume and same density (a o = 26 kg/m3) mix, then the newly formed water parcel should have the mean temperature and salinity; thus, it is represented by point C in the T - S diagram (Fig. 2.16). It is clear that water parcel C is heavier than both A and B, so it will sink to the density layer below in the ocean. Cabbeling plays an essential role in forming the dense water sinking into the deep ocean, and is intimately connected with the formation of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW).
To overcome the problem associated with potential density discussed above, we can use the local pressure as the reference pressure to define the potential density, ar, where the subscript r indicates the local reference pressure r. Therefore, the so-called neutral surface is defined as a surface whose normal is in the direction of -a V© + jVS, where a and j are the coefficients of thermal expansion and saline contraction. If we can construct a surface
2.4.11 Neutral surface and neutral density
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