northeast of Alenuihaha Channel the light water is actually to the left 

 of the current direction; in the northwest part the direction of flow is 

 perpendicular to that expected from the density distribution. 



The over-all surface density distribution in the survey region 

 leads to the assumption that the water generally flows in a southeasterly 

 direction, which is contrary to the surface dynamic heights. These 

 apparent discrepancies in the rule of direction of flow can be explained 

 by considering the subsurface distribution of mass. 



Subsurface Circulation 



A qualitative picture of the three-dimensional subsurface circula- 

 tion can be obtained by plotting variables on one of several systems of 

 coordinates. Each of these presentations will illustrate features which 

 are missed in others. The distribution of temperature, salinity, sigma-t, 

 oxygen, phosphate, or depth can be presented on vertical planes (sections). 

 A vertical section of sigma-t, for example, is most useful when it cuts 

 the direction of flow at right angles. It then gives an indication of rela- 

 tive current speeds. The distribution of parameters can also be plotted 

 on "horizontal" planes which are approximately parallel to the sea sur- 

 face (surfaces of constant pressure or depth). This method is particularly 

 useful in regions with a complex current system, such as is encountered 

 in the vicinity of the Hawaiian Islands. Its shortcoming is that vertical 

 gradients are not apparent. 



A useful variation of either method is the use of one of the para- 

 meters as a coordinate. Such a parameter is the potential density, which 

 is particularly useful since particle motion taikes place along surfaces of 

 constant potential density. (The potential density, approximately expressed 

 in units of signna-t, is the density a particle of water would have if brought 

 to the surface without loss or gain of heat. ) The deduction of the flow pat- 

 tern from the distribution of variables on these "sigma-t surfaces" is gen- 

 erally known as isentropic analysis (Montgomery 1938). 



On a horizontal frame of reference, then, one can plot the depth 

 of a sigma-t surface and obtain its bathymetric chart. For these depths 

 and on the Scime chart the salinity can then be plotted. Together, the 

 isopleths of depth and salinity referred to a surface of constant sigma-t 

 might give an indication of the direction of flow. 



The isobaths are generally parallel to the flow, provided that the 

 slopes of the sigma-t surfaces below are in the same direction. The 



