Large-Scale Features 31 



The density of sea water p is always a little greater than unity in the 

 c.g.s. system; therefore, in order to avoid having to write long decimals all 

 the time, the quantity (T has been introduced, and is defined in the follow- 



^S"^y= <r = (p-l)1000. (1) 



Thus, a density of 1-02721 is written as a cr of 27-21. 



The quantity cr^ is defined as the cr of sea water at atmospheric pressure 

 and at the temperature at which it was collected. Therefore, it differs 

 slightly from the a which would be computed from the potential density 

 of the same specimen, in that no account is taken of the adiabatic tem- 

 perature change of the specimen during reduction of the pressure to 

 standard pressure. This difference is very small. The essential effect of 

 compressibihty of sea water is taken into account by the use of Cj surfaces 

 on a T-S diagram, instead of the rigorously correct equal-potential-density 

 surfaces. 



The relation of density to temperature, sahnity, and pressure has been 

 the subject of elaborate and highly precise laboratory measurement. 

 Accurate tables have been prepared. They are discussed in more detail by 

 Sverdrup et al. (1942). A very much abbreviated table for (Xi is given in 

 Appendix III of the present study. 



Finally, it is necessary to make one further remark about mixing along 

 a surface of equal potential density. Since mixture on a T-S diagram 

 occurs along straight lines, whereas the Unes of equal potential density (or 

 equal <tJ are curved convexly upward, the mixture of two water masses of 

 the same potential density tends to increase the potential density of the 

 mixture. Although this effect, called cabbehng, should be borne in mind, 

 it is not of great importance in the crude qualitative analysis that is usually 

 done with T-S diagrams. 



North Atlantic water masses. — The water in the North Atlantic Ocean is 

 made up essentially of two water masses : one, the so-called North Atlantic 

 Central Water, and the other, the North Atlantic Deep Water. 



The North Atlantic Central Water is that water wdth a temperature 

 between 8 and 19° C. and a sahnity of between 35-10 and 36-70 %o. (The 

 symbol %o is read 'parts per thousand'.) This water mass is dehneated by 

 the two sloping broken Unes on the T-S diagram (fig. 8). As can be seen, 

 most of the soundings for temperature higher than 8° lie between these 

 narrow hmits. Isehn (1936, figs. 22 and 25) has drawn T-S diagrams for 

 water masses in the western North Atlantic which show even sharper 

 definition, indicating that over much of the Sargasso Sea the salinity of 

 water of a given temperature does not vary much more than the accepted 

 Hmits of error of sahnity determination accommodate. The sounding 

 which hes farthest from the mean in fig. 8 is that for Atlantis station 



