22 



Current velocity measurements by the G.E.K. (von Arx, 1950a) suffer 

 in magnitude when the current system extends downward through more than a 

 negligible fraction of the total depth of water (Stommel, 1948). This may be a 

 useful limitation in connection with studies of the wind-induced surface currents 

 generated in the open sea by passing storms since Stern (Malkus and Stern, 1952) 

 has shown that the signal due to local accelerations of the surface circulation by 

 the wind is added directly to those produced by steadier geostrophic circulation 

 at greater depths. Thus, if the background level at any station could be estab- 

 lished in a period of calms, the accelerations of the surface layers due to wind 

 at this place may be observed. A preliminary trial of this idea has been made 

 from one weather ship on station and Bowden (1953) in England. 



Another possibility in which the limitations of the G.E.K. can be turned 

 to good use has been pointed out from theoretical considerations by Malkus and 

 Stern (1952) who have reinvestigated the nature of the electric potential fields 

 associated with motion of the water of the deep sea, taking account of both the 

 horizontal and vertical components of magnetic flux. 



According to the results of this study, the difference between the inte- 

 grated surface current detected by the lateral drift of a ship and the integrated 

 lateral component of current observed by G.E.K. is equal to the mean velocity 

 of water beneath the line of traverse. From this and a knowledge of the aver- 

 age depth of water under the traverse, the volume transport may be computed. 

 In mid-latitudes and in the vicinity of the strong western-ocean currents the dif- 

 ferences may be large enough to be measured with useful precision with existing 

 navigational and G.E.K. equipment. If means can be developed to make pre- 

 cise the necessary measurements of small differences between large quantities, 

 this technique may be a valuable one since it provides a naeans for making ob- 

 servations of the mean velocity and of the volume transport independently of the 

 geostrophic assumption. 



DEEP CURRENTS 



Currents in the very deep parts of the oceans present special problems 

 of measurement, not so much due to their inaccessibility but to their very small 

 speed. If Kulp's figure of 1, 750 years is a representative time during which 

 this deep water may remain submerged, it then seems less likely that the cir- 

 culation can be actively affected by the surface circulation through other than in- 

 direct processes. For example, if the thermocline rises and falls slowly in 

 response to prolonged changes in the average wind stress, its variation in depth 

 may be due either to a change in the overburden of light surface water, or the 

 mechanism may be more directly related to the downward turbulent flux of heat. 

 In the first case the thermocline might act as a diaphragm which shifts by dis- 

 placement the very deep water masses below, while in the second case the ther- 

 mocline would move but the water would not. If the first possibility is correct, 

 and the sea level is not changed, a deepening of the level of the thernnocline be- 

 low the center of a light mid -ocean water mass due to increased average wind 

 stress would be compensated by shoaling the level of the thermocline around its 

 perimeter. The reverse effect might be expected if the average winds dimin- 

 ished. Such compensating motions of the thermocline would be peculiar to the 

 diaphragm effect and thus distinguish it from the heat flux mechanism. Were 

 the vertical motions or packing of iostherms in the thermocline to be studied 

 synoptically by means of a grid of anchored buoys, such as those described ear- 

 lier, the nature of some of the deep motions of the sea might be suggested. 

 Still, direct measurements of the small motions at great depth would always be 

 desirable. 



