direction can be obtained by the use of the rule previously stated. Deep 

 isobaths represent colunnns of lighter water below the surface and there- 

 fore will be to the right of an observer facing in the direction of flow. 



Flow takes place along the sigma-t or quasi-isentropic surfaces 

 (Sverdrup et al. 1942:414-416), across which mixing is at a minimum. 

 Thus, identifying properties of a water mass, such as salinity and tem- 

 perature, would be retained longest in the direction of flow or, conversely, 

 the direction of flow would be along the minimum change of the identifying 

 property such as salinity. 



These principles will be used in an attempt to interpret the 

 subsurface circulation. 



Figure 6 shows the sigma-t distribution plotted on a vertical 

 plane extending from west to east through stations 1, 10, 14, 22, 21, 

 and 30. The dominating feature of the section is the wave-like distri- 

 bution of sigma-t west of station 22. The rising isopleths east and west 

 of station 14 are associated with the "doming" of the sigma-t surfaces 

 which characterize the counterclockwise vortex mentioned before. The 

 rule of direction of flow indicates a current setting north between sta- 

 tions 22 and 14 and one setting south between stations 14 and 10. 



The section between stations 1 and 10 demonstrates that the 

 surface density distribution may give a misleading picture of the surface 

 current pattern. The 23.0 sigma-t isopleth slopes from 20 m. at sta- 

 tion 1 toward the surface at station 10, whereas the 23.2 sigma-t isopleth 

 and all those below its depth slope down. Thus, the surface density indi- 

 cates a southerly component of flow when in reality there exists a strong 

 northerly component. 



East of station 22, the varying slopes of the sigma-t isopleths 

 make estimation of the surface current speed and direction difficult 

 without actual dynamic computations. In Alenuihaha Channel (stations 

 22 to 21) the absence of a pattern associated with well-developed currents 

 is to be expected, since a strong flow normal to the channel direction is 

 not likely. 



The depth variations of the 27.0 sigma-t isopleth west of station 

 22 shows that the distribution of mass associated with the counterclock- 

 wise vortex extends to depths greater than 600 m. It also indicates the 

 possibility that currents at that depth are not negligible. But since the 

 direction of slope of the isopleth is the same as those above, the error 



