mixed (ni a signia-t surface will seek a slightly 

 deeper density surface. 



It is eniphasized here that the basis of isen- 

 tropic analj^sis is the deductidn of flow patterns 

 from the distribution of dissolved substances 

 which are independent of the density determina- 

 tions. Furthermore, flow patterns are independent 

 of the assumptions lor geostrophic movement and 

 hence can \-erify or disclose any deviations from 

 computed gradient flow. Oxygen and other chem- 

 ical properties, due to then- independence from 

 density computations are ideal flow indicators on 

 the density surfaces when due consideration is 

 gi^'en to biological changes. However since these 

 values are not always available, either tempera- 

 ture or salinity may be used. Parr (1938b) fa- 

 vored temperature because he felt the interpolated 

 values are more certam than those of salinity. 

 Montgomery (1938) on tlie other hand takes the 

 view that salmity is superior due to the strong 

 vertical temperature gradients and the actual 

 temperature on the sigma-t surface would not be 

 representative. For Ice Patrol requirements, the 

 wide range of temperature compared to the 

 accuracy of detennination provides for very great 

 property variation on the sigma-t surface which 

 is the best definitive characteristic of tlie current 

 system. It would be redundant to contour both 

 temperature and salinity since they are used to 

 define the sigma-t surface initially. 



Chiirt Preparation 



Charts of the temperature distribution on se- 

 lected sigma-t surfaces in the Grand Banks area 

 during the first and third surveys, were prepared 

 from the plots of temperature versus sigma-t at 

 each station. These are shown hi figures 15A to 

 23A. The depth of the surface at each station 

 has been indicated and a comparison with the 

 surface dynam.ic height contours nuiy be made 

 with figures 2A and 5A. On each chart, the 

 intersection of the sigma-t surface witli the sea 

 surface is shown as a broken luie and, \\hen it 

 occm-s, the intersection with the bottom as a 

 dashed luie. 



Chart Analysis 



An examination of tlie isentropic charts, figui'es 

 15A to 17A, reveals that the distribution of the 

 1° C. isotherm clearly delineates the boundary 

 of the Labrador Current. A comparison with 

 the 25 and 50 meter horizontal isotherm charts 

 of figures 24A and 25A for the same survey shows 

 that the horizontal isotherm charts do not clearly 



display the water tongues characteristic of the 

 Labrador cold core. However on the sigma-t 

 surface charts the tongues of water are very nicely 

 demarked, therefore, several levels were prepared 

 for analysis of the water movement. 



The sigma-t surfaces of 26.7, 26.8, and 27.0, 

 figures 15A to 17 A, from the first survey show a 

 severe downward slope towards the Banks. This 

 slope would tend to reduce or even eliminate 

 horizontal mixing because a crossing of strong 

 horizontal density gradients would be required. 

 The third survey isentropic charts, figures ISA to 

 20A, show a definite leveling of these surfaces and 

 a slight increase in depth. This is probably due 

 to the greater volumes of lighter water in the area 

 since a 26.6 surface can be drawn for the third 

 survey, whereas, on the first survey, water of a 

 sigma-t of 26.7 was the lightest water present in 

 abimdance. If a comparison is made between the 

 isotherm charts, plotted on the 25- and 50-meter 

 levels for the first survey, and the isentropic tem- 

 perature distribution charts, definite differences 

 can be seen. As pointed out above, the tongues 

 of water cannot be observed on the constant depth 

 charts, figures 24A and 25A, even though these 

 levels are in the depth range of the sigma-t charts 

 which clearly describe the boundaries of the tem- 

 perature tongues. This is apparently because 

 these tongues of water are moving along steeply 

 sloped surfaces and horizontal slices at specific 

 levels fail to capture them. A comparison at the 

 third survey sigma-t charts and the horizontal 

 presentation at 75 meters, figures ISA to 23A and 

 26A show that the isotherms appear quite similar 

 in their distribution on both types of presentation. 

 Horizontal mixing can now take place due to the 

 lessened horizontal gradient and the tongue like 

 property distribution is destroyed as the Labrador 

 Current flows south. Both presentations of the 

 third survey are similar with the water grading 

 from cold to \\ arm toward the east as the influence 

 of the Atlantic Current begins to dominate the 

 temperature distribution. This greater horizontal 

 mi.xing is obvious from tlie north-south tempera- 

 ture distribution of the Labrador where the third 

 survey shows the arrival of colder water at the 

 head of the system than found on the first survey. 

 In addition, this colder water appears to be 

 warmed faster as it travels south thus indicating 

 more active mixing is taking place with tlie 

 warmer Atlantic Current. 



The unique display of tongues of cold water on 

 the sigma-t surfaces leads to the question as to 



