1 70 Salinity of the Ocean, its Variation in Oceanic Space and in Time 



taken as the upper and lower limit of the layer with maximum salinity it has an average 

 vertical thickness of only 50 m ; it stays about the same thickness over its long course 

 to well within the Gulf of Guinea, and the salinity of the core layer changes very little 

 after it has lost its tongue-like form along the first half of its route. A comparison of the 

 salinity section with a corresponding density section shows that the position of the 

 salinity maximum along the greater part of the cross-section coincides with the 

 strongest vertical concentration in the density field. The very saline water thus extends 

 in a thin layer along the thermocUne itself. The spreading in this layer is caused by 

 advection and turbulence but the latter factor must be of very little effectiveness, be- 

 cause of the almost unchanged character in this remarkably thin layer over such large 

 distances. It must be supposed that above and below the thermocline the transport of 

 water with maximum salinity is accompanied by strong mixing with the water above 

 and below, but that in the thermocline itself the stability strongly suppresses turbulence, 

 so that the almost horizontal spreading takes the character of a laminar flow. This has 

 been confirmed by calculations of the vertical exchange coefficient in the area of the 

 Equatorial Countercurrent by Montgomery (1939), who found /i^ = 0-4 g cm~^ sec~^ 

 along the axis of the Countercurrent. Since lateral mixing was neglected in these 

 calculations the value found will be a maximum value; the true one must approach 

 rather closely the molecular diff'usion coefficient for salt in water (0-011). As men- 

 tioned above, the spreading must therefore be of laminar character. However, in 

 horizontal direction lateral mixing is very eff'ective and the lateral exchange coeffi- 

 cient Ay reaches the value of 4 x 10^ g cm~^ sec"S generally found. 



From the deep-reaching accumulations of warm and saline water in the subtropics 

 there is not only a flow of this water towards the equator but also towards the poles 

 in somewhat deeper layers. Thus at depths only a little below the upper layer, and the 

 almost homo-haline top layer which shows decreasing salinity towards the pole, there 

 is a secondary maximum in the vertical distribution of the salinity. In the Southern 

 Hemisphere this poleward flow of highly saline water occurs first at a depth of 100 m, 

 but descending to a depth of 150 m or more, and continues on over a very broad 

 front across the entire ocean; however, the energy of this outflow is soon dissipated 

 and the maxima disappear due to mixing. In the Northern Hemisphere this maximum 

 is associated with the Gulf Stream and its continuation (the Atlantic Current) and it 

 can be followed across the entire Atlantic Ocean into the Norwegian Sea and further 

 polewards. Figure 74 shows a longitudinal salinity section given by Schott (1942) 

 through the Atlantic Current from the Wyville-Thomson Ridge past the Shetland 

 Islands as far as Spitzbergen. The Atlantic water soon descends underneath the cold 

 and low saline polar water of the surface layer. Although the salinity maximum is 

 decreased by mixing it can still be traced in the North Polar Basin and into the Barents 

 Sea. Its occurrence here was discussed in connection with the description of the vertical 

 temperature distribution in the North Polar Basin (see p. 133). An interesting and, 

 from the point of view of the method used, important study of this spreading of At- 

 lantic water {§ = 10-2°, S = 35-45%o) in the northern part of the North Sea, in the 

 Norwegian Sea and in the Barents Sea and its mixing with the surrounding water 

 {d = 2-5°, S = 34'90%o) made by Jacobsen (1943) should specially be mentioned 

 here. 



From our knowledge of the tropospheric salinity maxima of the Pacific and the 



