Density of Water Masses in Ocean, Vertical and Horizontal Density Distribution 189 



T 25 26 27 28 of 



23 24 25 26 27 28 27 28 



500 



1000 



1500 



2000 



2500 



3000 - 



3500 



4000 



26 27 28 



Fig. 87. Vertical distribution of the density cr^ at some oceanographic stations in the Atlantic: 



found that resembles more closely that of the temperature at these depths. In the 

 subtropics of each hemisphere the lighter water extends down to great depths while in 

 the equatorial zone the heavier waters of the deeper layers extend higher upwards 

 to just below the strongly developed density transition layer. This gives rise to a 

 horizontal density gradient from the equatorial zone towards the two subtropical 

 regions, that is opposite to the surface gradient. This gradient remains unchanged in 

 direction, though becoming weaker and weaker down to about 2000 m below which 

 the meridional density differences are usually rather small. In all the vertical sections 

 there is, however, a weak density gradient from high northern latitudes across the 

 equator to as far as 40-50° S. which is connected with the oceanic circulation of the 

 deeper layers. 



It is readily understood that horizontal charts of a^-values show in principle the same 

 picture. A comparison of such charts with charts of the relative topography of the 

 isobaric surfaces (Helland-Hansen and Nansen, 1926) demonstrate that the course 

 of the isopycnals on the horizontal charts is in essential agreement with that of the 

 dynamic isobaths. The horizontal circulation of the water masses can thus be deduced 

 approximately from the horizontal distribution of the o-^-values. In that way stream 

 lines for the relative water flow are obtained (i.e., with reference to the lower layers). 

 Arrows showing the direction of flow are thus often inserted on the isohnes on 

 isopycnic charts of the upper layers to indicate the currents. These are subject to the 



