Water Bodies and Stationary Current Conditions at Boundary Surfaces 473 



basic current, the changes in time of the distribution of the isosteres must have been 

 quite considerable. Figure 216 shows these changes in the vertical current structure in 

 the two layers of the upper system: 5-15 m and 30-100 m. In the lower part of the 

 vortex the velocity is greatest between 2 and 3 h and at the same time least in the 

 upper part. During this time-interval there is thus an increase with depth of the velocity 

 of rotation. In the interval between 9 and 16 h conditions are reversed; at 10 h the 



8 16 24 32 



cm/sec 



Fig. 216. Changes in the vertical structure of the current of the upper system in the cyclonic 



vortex above the "Altair" submarine volcano. •« — , current in the layer between 5 and 15 m 



depth; <=, current in the layer between 30 and 300 m depth. 



top layer has the greatest velocity and there is thus at this time a decrease in the 

 rotational velocity with depth. This feed-back of these oscillations of the current field 

 on the mass distribution in the vortex must be extremely strong to give a complete 

 reversal of the current structure. At 10 h there must be an increase of the up-lift of 

 the isosteres and at 2 h an increased depression. These oscillations of the isosteric 

 surface about nodal lines at a certain distance from the vortex centre have been 

 demonstrated by observations of the anchor station. The isotherms and isohalines 

 oscillate around a mean position with an inertia period of 17 h, so that the anchor 

 station must be somewhat displaced towards the outer edge of the vortex, because the 

 isosteres are always lowered at 10-5 h and always lifted at 2 h. 



The oscillations in a circular two-layered vortex can be accounted for theoretically 

 (Defant, 1940 b) and an estimate can be made of the period of the free oscillations of 

 such a system. If the effect of centrifugal force is neglected (it is always small) then the 

 mean position of the boundary surface in such a vortex will correspond to the follow- 

 ing relation (z positive upwards; centre of the vortex at .v = 0; horizontal extent of 

 the vortex = 21): 



