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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



through the Hawaiian Islands. Three density 

 layers can be readily distinguished. The first is 

 the mixed surface layer with neutral stability 



( -T^=oV Tlie second, the pycnocline, is a layer 



within which there is a rapid increase in density 

 with depth. The third, in which the density 

 increases very slowly with depth, is below the 

 pycnocline. 



Approximate stabilities, £'= 10"' --r~ (Sverdrup 



et al. 1942: 417), in the three layers are as follows: 

 in the mixed surface layer £" = 0: in the pycno- 

 cline W-^-p ranges from 3400 m.-' to 5000 m."', 



1400 m.-' to 2300 m."', and 350 m.-' to 2500 m.-' 

 at 10° N., 20° N., and 30° N., respectively. These 

 compare with stabilities of 120 m.~' at 10° N. to 

 170 m.~' at 30° N. in the deeper layer below the 

 pycnocline. Thus, stability changes from layer to 

 layer are large. In addition, whereas the change 

 from the second to the third layer is gradual, the 

 change from neutral stability in the mixed surface 

 layer to high stability in the pycnocline is abrupt. 

 The bottom of the mixed surface layer can there- 

 fore be regarded as a surface of dynamic signifi- 

 cance. Above this surface, parcels of water can 

 move up and down with a minimum expenditure 

 of work. In the pycnocline, a comparatively 

 large amount of work must be done to displace 

 water vertically. For this reason, the depth of the 

 effective layer through which energy changes due 

 to processes at the sea surface are distributed is 

 assumed to be the mixed surface layer. 



In order to measure the depth of the mixed sur- 

 face layer and determine its distribution, it is 

 useful to remember that the density of the surface 

 sea water is a function of both the temperature 

 and the salinity. These were also drawn in figure 

 1 to such a scale as to demonstrate their relative 

 effect on the density. It is apparent that the 

 vertical density structure is primarily a function 

 of the temperature and that the salinity in the 

 latitudes under consideration is of less importance. 

 It is also apparent that the top of the pycnocline 

 corresponds approximately with the top of the 

 thermocline and that the surface layer in which 

 E=0 corresponds approxunately with the layer 

 in which the vertical temperature gradient is also 



zero (^=oV The depth of the mixed surface 



layer can therefore be determuied from tempera- 

 ture-depth curves in the Hawaiian region. 



In practice it is not always as simple to deter- 

 mine the depth of the mixed layer from the vertical 

 temperature distribution as indicated in the ex- 

 amples of figure 1. It will therefore be useful to 

 briefly review features of the vertical temperature 

 distribution which are discussed in detail in the 

 Apphcation of Oceanography to Subsurface War- 

 fare (National Defense Research Committee, 

 1946). Tlie most important of these is the ther- 

 mocline, in which the maximum temperature 

 change occurs. It is between the surface layer of 

 constant temperature and a deeper layer in which 

 the vertical rate of change of temperature is small. 

 In the Hawaiian region, this thermocline, which 

 we call the main or permanent thermocline, is 

 always present. Below the mixed layer, the aver- 



,. d(9 . . , 



age temperature gradient, ^) is approximately 



15° C, 6° C, and 5° C. per 100 m. at 10° N., at 20° 

 N., and 30° N., respectively. Superimposed on the 

 permanent thermocline may be a seasonal thermo- 

 cline. This is illustrated by the vertical tempera- 

 ture distribution at about 30° N. in winter and 

 summer (fig. iC). In addition to the permanent 

 and seasonal thermocline, there may be a diurnal 

 thermocline, defined as a small rise in surface 

 temperature of the order of 1° C, which may ex- 

 tend to a depth of 10 m. Finally, the stable layer 

 may extend to the sea surface; i.e., the constant 

 temperature surface layer may be absent. 



The last two rarely occur in the area under 

 investigation and the practical problem remains 

 one of determining the depth of the mixed surface 

 layer with consistency from .temperature-depth 

 curves. In low latitudes the depth of this layer 

 is generally well defined by what Munk and Ander- 

 son (1948) called the "knee." However, more 

 than one knee may occur, as at 10° N. in August, 

 illustrated in figure lA. In examining tempera- 

 ture-, salinity-depth curves in the Hawaiian region, 

 one finds that the first knee, after excluding the 

 diurnal thermocline, also defines the layer of con- 

 stant salinity, whereas deeper knees are in more 

 saline water. 



Difficulty also arises when the depth of the 

 knee is not well defined and may actuall.y lie 

 within a layer of changmg salinity with depth, 

 the halocline. In other words, the temperature 

 decreases gradually below a layer of constant 



