A great many variations of a general type of structure 

 frequently occur. For instance, type S-1A shown in figure 7 

 is a modification of type S-l, in which the subsurface tempera- 

 ture minimum is less well-defined. Other differences are 

 (1) the lack of a deep isothermal layer, (2) warmer water at 

 the surface, and (3) the general irregularities in the temper- 

 ature trace. Thermal structures shown in figure 7B and 7C 

 are characteristic of regions of lower latitudes. 



Another type of thermal structure, the S-2, is typical of 

 regions in which only a slight amount of heating has taken 

 place. Examples are shown in figure 8. This type represents 

 the summer transformation of type W-2, the structure near 

 the continent where winter cooling extends to great depths. 

 When heating at the surface does occur it is usually distrib- 

 uted over considerable depth by wind mixing. 



Heating of the surface layers is not necessarily continuous 

 throughout the summer season. Near the continent and in 

 other regions where drifting ice is moved into a previously 

 heated region, cooling takes place in the surface layers. This 

 results in irregular temperature structures with alternate 

 positive and negative gradients near the surface. In some 

 cases the positive gradients thus produced extend from the 

 surface to a depth of several hundred feet. Most frequently, 

 however, ice-cooled water appears as an isothermal layer 

 over warmer water with a sharp positive gradient between 

 the two layers. The magnitude of these increases of tempera- 

 ture with depth may be as much as 2 degrees C. Since, in the 

 summer season, cooling is the result of melting ice, the upper 

 cooled water will have low salinity; therefore the structure 

 remains stable. Such structures, designated here as type S-3, 

 are shown in figures 9 and lO.together with the localities where 

 they are found. 



15 



