98 



BJ0RN HELLAND-HANSEN 



[REP. OF THE "MICHAEL SARS" NORTH 



the Baltic is carried away through Oeresund and The 

 Danish Belts which are comparatively very narrow and 

 shallow. The out-flowing water proceeds along the 

 southern coasts of Sweden and Norway. Owing to the 

 influence of the rotation of the earth the surface layers are 

 pressed to the right so that a solenoidal field is created. 



Fig. 38. An example of vertical distribution of Ct. Tlie water is 

 comparatively heavy at Stats. 1 and 3 and ligtit at Stat. 2. 



Fig. 38 illustrates the conditions in a section through 

 3 stations, numbered 1, 2 and 3. The section may, for 

 instance, have a direction N. — S. (cf. e. g. the lower 

 section on p. 91', Stats. 69, 67 and 66). The curved 

 lines represent the distribution of density (aj. The water 

 is comparativily heavy at Stats. 1 and 3, and light at 

 Stat. 2. The line p represents a certain isobar. The 

 distance in dynamic metres from the surface to this isobar 

 is greater at B than at A and C. On the assumption 

 that the velocities decrease from the surface downwards. 



as is usually the case, the current must, according to 

 the theory set forth above, run with a component east- 

 wards between Stats. 1 and 2 and westwards between 

 2 and 3. This means that the current runs in such a 

 direction that the heavy water is found to the left and 

 the light water to the right in the northern hemisphere 

 (reversed in the southern hemisphere). If occasionally 

 the velocity of the current increases with depth the light 

 water will be found on the left-hand side of the current 

 and the heavy water on the right-hand side in the northern 

 hemisphere. 



We may also draw another conclusion which is rather 

 important. The isopycnals usually run almost parallel to 

 the isotherms and isohalines. The isopycnals in Fig. 

 38 may, therefore, be considered as nearly equivalent to 

 isotherms or isohalines. At first sight it might seem that 

 the whole wedge-shaped body of water between stations 

 1 and 3 forms a characteristic current not only as regards 

 its physical properties, but also in having practically the 

 same direction of flow. According to what has been 

 said above the flow is, however, in opposite directions 

 at the two sides of the a.xis of minimum density (at Stat. 

 2). This axis is a critical one as regards the direction 

 of flow. Quite analogous conclusions hold good, of 

 course, when we have a section with an axis of maximum 

 density. 



In the ne.xt section we shall deal with the method 

 of making numerical calculations, and discuss some of 

 the results as regards the North Atlantic. We will only 

 mention here some questions in connection with a study 

 of the topography of isobaric surfaces. Having found 

 the dynamic depth from the sea surface to a certain iso- 

 baric surface by means of observations of temperature 

 and salinity at a number of stations, we can draw dy- 

 namic isobaths. The number of isobaths usually increase 

 with increasing pressure (depth). When the friction can 

 be neglected these isobaths will represent stream-lines for 

 the currents along the isobaric surface relatively to the 

 surface of the sea. Thus the conditions in the upper 

 water-strata influence the construction of the charts even 

 for great depths, which means that, for instance, the 

 seasonal variations in temperature have to be considered. 

 But we may also start from an isobaric surface at a 

 particular depth and make our calculations upwards to 

 the surface as well as downwards to greater depths. 

 If the velocities of the currents in one of the isobaric 

 surfaces are so small that they may be neglected, it 

 is convenient to start froin this surface. The topogra- 

 phical charts for isobaric surfaces higher up in the water 

 will then directly show stream-lines for the actual currents, 

 and the actual velocity will be inversily proportional to 

 the distance between the isobaths. In this case the sea- 



