30 Large-Scale Feattjres 



omitted because they usually show a great scatter, being variable for 

 difiFerent seasons. The depths of the shallowest points in the diagram are 

 indicated by numerals (meters), and the insert chart shows the positions 

 of the various stations. 



The T-S diagram is used to identify different kinds of water in the 

 ocean, to note similarities of properties between one region and another, 

 and to obtain a quahtative picture of the amount of mixing between water 

 masses of different geographic location. Four considerations must be kept 

 in mind in the use of T-S diagrams. 



First, potential temperature and salinity are conservative properties in 

 the ocean, except in a shallow homogeneous layer near the surface, where 

 evaporation, solar radiation, cooUng by back radiation, an exchange of 

 heat with the atmosphere, and similar changes occur. Sea water is com- 

 pressible; therefore, true temperature is not strictly conservative. The 

 change in temperature of a water parcel that is subjected to an adiabatic 

 vertical displacement of 1000 m. is roughly 0°1 C, and therefore, for many 

 practical purposes of graphing, the potential temperature and the true 

 temperature on a T-S diagram are interchangeable, although potential 

 temperature is preferable. 



Secondly, mixing processes in the interior of the ocean mix both tem- 

 perature and sahnity in the same way. Thus the mixture of two masses 

 of water represented by two points on a T-S diagram lies along a straight 

 line joining them. 



The third consideration is that the oceans of tropical and temperate 

 regions are stratified and vertically stable at all depths, with the possible 

 exceptions of the shallow homogeneous surface layer and of certain deep 

 isolated basins on the ocean bottom. Because of the damping of vertical 

 turbulence by the stability, there is a tendency for most major flows to 

 occur along surfaces of equal potential density. The potential density is 

 the density that a sample of water would have if it were brought adia- 

 batically to the surface at atmospheric pressure. It is important to make 

 the distinction between density and potential density, because of the con- 

 siderable compressibility of sea water. At a pressure corresponding to that 

 present at the average bottom depth of the ocean, 4000 m., water is com- 

 pressed about 2 per cent. Deep flows tend to occur along surfaces of equal 

 potential density; hence it is important that the potential density be 

 represented on the T-S diagram, to indicate the directions of preferred 

 flow and mixing. The family of smooth curved hnes on the T-S plane in 

 fig. 8 represent loci of constant (Xf, a quantity which, unfortunately, does 

 not have any special name, but which is very nearly the same as potential 

 density. Because of its importance to physical oceanography it must be 

 defined carefully. 



