444 BOUNDARIES OF THE SEA 



the lower limit L of the halocline, and that the vertical transfer 

 here is unidirectional upward. Since the salinity is nearly constant 

 on this boundary, and it is well below the limit of recognizable 

 seasonal variations, the intruding sea waters may be identified 

 here by their temperature alone. 



Once they are entrained, these primary sea waters are mixed, 

 heated and cooled, and diluted with fresh water. Hence, as they 

 approach the surface their identities become obscured. However, 

 by recalling that the entrained sea water constitutes 95 to 98% 

 of the halocline and upper zone, and that the rate of vertical 

 transfer is of the order of 20 + 10 m per year, it is evident that 

 any change of the water on the interzone boundary presages 

 corresponding changes in the shallower waters, despite surface 

 effects. 



This interzone boundary may be defined in the logarithmic plot 

 of normal oceanographic data from serial observations. In this 

 definition, based on structure, all the data in the halocline and 

 lower zone are used. Minor variations in structure are smoothed. 

 The conclusions are objective. It can be applied in any region 

 where there is a halocline, whether or not the salinity at its lower 

 limit L is constant, e.g., between the inner and outer sub-Arctic 

 limits (Figs. 3 and 4). 



Northward of inner sub-Arctic boundary (Fig. 3) this procedure 

 may be approximated within small limits of error, by plotting 

 the temperature corresponding to the index salinity, Z3.^%,o- 

 This is permissible because of the constancy of the salinity at this 

 lower limit L. The required temperature is readily determined from 

 a temperature-salinity (T-S) diagram (Helland-Hanscn, 1916). 

 The typical relations in Figs. 8 and 9 show that the temperature 

 gradient is small in the vicinity of this value. Hence the error 

 associated with normal variation of the index salinity ( + 0.1%o) 

 is negligible. 



By using the approximate technique, the data from the series of 

 quasi-synoptic surveys have been plotted as shown in Fig. 10. In 

 these diagrams there are warm waters (warmer than 6.5°C) in the 

 Polar Front and near the coast (shaded), separated from a cool 

 water (cooler than 4°C) area in mid-ocean and the Gulf of Alaska. 



These may be compared to the conventional T-S analysis made 



