Salinity of the Ocean, its Variation in Oceanic Space and in Time 173 



is based on the "Carnegie" observations (Sverdrup) and south of 40° S. on the 

 "Discovery" observations (Deacon, 1937). Longitudinal sections through the western 

 and central parts of the Pacific Ocean have been given also by Wust (1929). 



(d) Subpolar Intermediate Water 



At 800-1000 m there is a characteristic lovv^ salinity zone extending across almost 

 the entire ocean though not always equally well developed. In the south it begins 

 always just south of the oceanic polar front where this special water mass sinks rapidly 

 from the surface to a depth of 800 m and spreads out from here with decreasing vertical 

 thickness and decreasing salinity in its core into the Atlantic across the equator to 

 about 20° N. It can still be traced north of here until it joins the deep and saline 

 water accumulations of the subtropics. There is little to be seen from an Arctic counter- 

 part to this subantarctic intermediate water. Only in the western section weak indica- 

 tions of such arctic intermediate water may be found as far as the Newfoundland rise. 



Also in the Indian Ocean this intermediate water is found everywhere underneath 

 the high saline water mass south of the subtropics as an intrusion of low saline water 

 with its core somewhat deeper than in the Atlantic (approx. 1000-1200 m). In the 

 Pacific tongues of low saline polar water spread out below the high saline tropo- 

 sphere almost to the equator, from both north and south. The Antarctic branch of 

 low saline water forms just south of the oceanic polar front at 50°-60° S.; the arctic 

 branch formed in the area of the Okhotsk Sea is weaker; in the western and central 

 parts of the Pacific Ocean it can be followed to about 10° N. It is completely absent 

 in the whole of the eastern part of the Pacific and there is thus an asymmetry in the 

 salinity distribution similar to that in the Atlantic Ocean. 



The vertical thickness of the subantarctic intermediate water is about the same in 

 all the three oceans (about 600 m) and it is separated from the troposphere above by a 

 sharp salinity (and density) transition layer. It is of particular interest that the inter- 

 mediate water is found with the same characteristics and thickness across the entire 

 transverse section of the ocean, especially in the Atlantic. Evidence for this is given in 

 Fig. 77 which gives a cross-section of salinity through the Atlantic at about 22° S. 

 This uniformity of this water across the total cross-section can be regarded as a conse- 

 quence of strong lateral mixing which leads to an equalization of all existing major 

 horizontal salinity differences. 



A detailed investigation of conditions in the subantarctic intermediate water and 

 its meridional spreading in the Atlantic has been given by Defant (1936). The vertical 

 salinity distribution in successive cross-sections normal to the main direction of 

 spreading is best characterized by the dimensionless quantity {sq — s)I{sq — s^, 

 where ^o (=34-85%o) is the salinity which the subantarctic intermediate water takes on 

 by continuous mixing with the surrounding water and s,n (=34-19%o) is the salinity of 

 the subantarctic intermediate water in its region of origin before spreading out 

 towards the north. The quantity {sq — s,n) corresponds to a potential difference present 

 between the two oppositely moving types of water which is finally eliminated by mix- 

 ing. Determination of this quantity in cross-sections, 500 km apart from each other, 

 for the core layer (salinity minimum) and for several layers above and below this core 

 allows of construction of lines of equal values of the quantity (5'o — 5)/(^o~ ■5' m) expressed 

 in percentage of intermediate water. These lines then illustrate the mixing process 



