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Ocean Currents in a Non-homogeneous Ocean 



of stations. However, it requires laborious calculations since the errors occurring with 

 each triangle computation, although not large, must be eliminated by a smoothing 

 technique from triangle to triangle. Neumann has in some way modified this method 

 for practical use by taking all stations with the same reference level depth together, 

 thus obtaining a series of pairs of stations with constant reference level depth. For 

 these, however, the previous simple procedure is applicable. To connect one series of 

 station pairs to the next requires only one station triangle, and this can be selected in 

 the most favourable position where the triangle errors are small. In this way each 

 station series can rapidly be connected to the next with minimum error thus over 

 coming the difficulties otherwise occurring for a varying depth of the reference level. 



(c) Consideration of Stations in Shallow Waters 



In shallow parts of the sea the reference-level is usually found below the sea bottom 

 and the method described above can no more be used. It is, however, desirable to know 

 the absolute topography of the pressure surfaces in these shallow waters also, especially 

 as the most intense currents are often found here. Jacobsen and Jensen (1926), as 

 well as Helland Hansen (1934), have devised methods for calculation in this case. 

 It is necessary to preassume for these that the internal friction can be left out of con- 

 sideration, and the velocities as well as the horizontal pressure gradients at the sea 

 bottom should be zero. The method proposed by Helland- Hansen is based on the 

 following reasoning : 



Figure 231 shows a dynamic section starting at a coastal point E across a shelf con- 

 taining station D, C, B and ending at station A out in deep water. The thin lines are 



Fig. 231. To the method of fitting shelf stations together with deep-sea stations. 



isosteres. In the sea between A and B the dynamic reference-level runs along the thick 

 dashed line. In the shelf area BCD the depth of the sea is less than the depth of the 

 reference-level in the deep water. In order to obtain the deviations of the dynamic 

 topographies of the isobaric surfaces we can imagine for the shallow part a fictive 

 vertical section from B to D below the level of the sea bottom, assuming the velocities 

 to be zero at the sea bottom. The isosteres in this imaginary section are horizontal and 

 therefore there is no motion in this part. The actual movements in the real section 

 and that in its imaginary extension will thus be the same. The latter can, however, be 

 used to extend the topography of the pressure surfaces above the shelf as far as the 

 coast. 



