to those shown by Mazeika (1968). 



4. Mixed Layer Depth Analyses 



The mixed layer depth analyses shown in figure 22 were constructed 

 by using an empirical layer depth analysis model developed by the Naval 

 Oceanographic Office ASWEPS program. This model is water mass oriented 

 but also includes rules for layer depth analysis in areas of water mass 

 interaction and other peculiarities, i.e., sound channels and heat traps. 

 This model is presented by Thompson and Anderson (1965). 



Upon first inspection, the GILLISS layer depth data appear to be 

 random. When the two phases were taken separately and analyzed using 

 the sea surface temperature chart as a water mass guide, a reasonable 

 representation of the mixed layer depth was obtained. The data show 

 that layer depth patterns are not as continuous as previously expected, 

 but a reasonable pattern was deduced. 



Mixed layer depth is a function of the physical properties of the 

 water mass and the dynamic processes acting on and in the water mass. 

 The significance of each of the various processes varies with season, 

 locality, and synoptic weather situations. 



The GILLISS survey was made during a normally cooling period and 

 a period of transition from summer to winter oceanographic conditions. 

 Heat budget calculations for the survey period indicate an overall heat 

 loss from the surface layers. This factor alone indicates that mixed 

 layers would be expected to deepen. Figure 22 shows that mixed layer 

 depths became both deeper and shallower between phases. 



Further inspection of the various horizontal and vertical analyses 

 (figures 8 through 21) leads to the conclusion that layer depth changes 

 between surveys are primarily due to water mass advection. Two types 

 of advection changes of the layer depth are prominent. One is water 

 mass change with the layer depth characteristics of the new mass re- 

 placing those of the original water mass. An example of this change 

 is the deep layer of 120 meters located at 36°30'N,65°W in phase I 

 and at 37°N,66°W in phase II (figure 22). The second type of advective 

 influence on layer depth is due to layering of water masses at a given 

 location. Such conditions are usually associated with gradient zones 

 between water masses. The zero layers (shaded areas in figure 22) were 

 mainly formed by this phenomenon. 



The deepest layers during both phases are associated with the 

 warmest water, e.g., the Gulf Stream and eddies. The deep Gulf Stream 

 core (>60 meters) can be easily followed east-west across the center of 

 the area in phase II. The warming near 40°N,65°W is associated with deep 

 layers similar to those of the Gulf Stream, thus supporting the argument 



