The potential bottom temperatures, and consequently the winter- 

 time siu'faee temperatures required for vertical convection to bottojn, 

 were about 1.3° to 1.4° in the deeper parts of the section. In comiec- 

 tion with another study, the average April surface temperatures in the 

 five 1° rectangles bounded by 60° N., and 61° N., and 50° W., and 

 55° W., are available for comparison with such temperatures. For 

 the decades 1876-85, 1886-95, 1916-25 and 1926-35 the average 

 April surface temperature was warmer than 1.5°. The average April 

 surface temperature approximated the required temperature for the 

 decade 1896-1905, and was colder tlian required during the decades 

 1906-15, 1936-45 and 1946-55. The rectangles sampled were some- 

 what north of the Labrador Sea section under consideration and data 

 for some years are fragmentary, but April is probably later than the 

 season of minimum temperature, and it is considered that the his- 

 torical temperatures indicate that, in many winters, deep vertical 

 convective mixing is possible. 



Such an hypothesis requires a mechanism for tlie reestablishment of 

 conditions found during the summertime such as are shown in figures 

 23 through 26. The concept of mixing of adjacent waters of differing 

 T-S characteristics within a constant density surface to produce a 

 slightly denser mixed water which sinks to its appropriate new density 

 surface provides such a mechanism around the periphery of the 

 Labrador Sea. Here we have contrasts in temperature and salinity 

 between the Irminger Current component of the West Greeidand Cur- 

 rent and the adjacent water of the central I^abrador Sea and with the 

 adjacent East Greenland Current component of the West Greenland 

 Current. On the I^abrador side there are also smaller contrasts 

 between tlie warmer offshore part of the Labrador Current (derived 

 from the West Greenland Current) and the adjacent w^ater of the 

 I^abrador Sea offshore and again the inshore part of the Labrador Cur- 

 rent (derived from tlie Bafhnland Current). In each case horizontal 

 gradients of velocity provide the necessary shear for the mixing. 



Thus, after the deep convective mixing of wintertime ceases, sinking 

 of mixed water spreading southwestward from beneath the Irminger 

 Current component of the West Greenland Current provides the deep 

 salinity maximum-oxvgen minimum at 2,000 3,000 meters and sinking 

 of mixed water spreading northeastward from beneath tlie offshore 

 part of the Labrador Current provides the temperature nuiximum- 

 oxygen maximum at about 1,500 meters. In such a system the bottom 

 water, being least affected, is least altered from the winter picture and 

 retains the coldest temperatures. 



Witli water sinking from upper levels there is necessary an outflow 

 of water from the deeper levels of the Labrador Sea. If there is a 

 surface of no horizontal motion at intermediate levels, then the 

 inclination of the deep surfaces of equal density indicate an outflow 

 of deep and bottom water in the southwestern half of the section. 



41 



04538.3 — 63^—4 



