Currents 



113 



water having properties that are controlled 

 by sea surface processes (Fig. 100). Basins 

 of the second group are in areas where the 

 surface waters are characterized by an ex- 

 cess of evaporation over runoff plus direct 

 precipitation, whereby the surface sea water 

 is made so salty and dense that it sinks be- 

 low the basin sill. Examples are the Med- 

 iterranean Sea (Maury, 1855, p. 136; Schott, 

 1928; Pollack, 1951), the Red Sea (Thomp- 

 son, 1939a, \939b), and the Persian Gulf 

 (Emery, 1956a). Surface sea water flows 

 inward and forms the surface layer. Beneath 

 it the denser basin water, which is of abnor- 

 mally high salinity and temperature, flows 

 outward when the interface is above the sill 

 depth. Oxygen content of the basin water 

 is generally high. The third kind of basin 

 occurs in areas where freezing forms sea ice 

 from surface sea water, rejecting much of 

 the salt in the process. The resulting cold 

 brine is denser than the original surface sea 

 water, so it sinks to form basin water. Ex- 

 amples are the Japan Sea (Suda, 1932), Baffin 

 Bay (Smith, Soule, and Mosby, 1937), and 

 the Norwegian Sea (Helland-Hansen and 

 Nansen, 1909). In such basins the surface 

 sea water flows inward and the denser basin 

 water flows outward, just as in basins of the 

 second type. Oxygen content is generally 

 high in the basin water. In basins of the 

 fourth group the density-depth relationships 

 are controlled by the influx of large quanti- 

 ties of fresh water from runoff'. The fiords 



of Norway (Strom, 1936), the Arctic Sea 

 (Shirshov, 1940; Sverdrup, Johnson, and 

 Fleming, 1942, p. 658), and the Black Sea 

 (Pora, 1946) are examples. The surface wa- 

 ter is of low salinity and of either higher or 

 slightly lower temperature than the under- 

 lying trapped basin water. Water of the 

 open sea flows inward at depth to make up 

 for losses of basin water through mixing with 

 the overlying fresher water. Oxygen in 

 many basins is low or absent, but in others 

 it may be high. 



Shelves 



Currents on the shelves cannot be com- 

 puted from dynamic topography as can cur- 

 rents in areas of deep water, for on the 

 shelves there is not a depth of no movement 

 (a horizontal pressure surface — see p. 99) 

 above the bottom. As a result, measure- 

 ments of temperature and sahnity can be 

 used for indicating only the approximate di- 

 rection of flow and not at all for the velocity. 

 Checks on direction can be made using drift 

 cards, drogues, and current meters. 



Water atop the shelves, like that in deeper 

 areas, has a mixed layer (often isothermal) 

 at the surface, a thermocline below it, and 

 colder denser water at greater depth. Close 

 to shore where the depth is only a few tens 

 of feet and where wind mixing is great, espe- 

 cially during storms, the mixed layer may 

 occupy the entire column of water. The 



Figure 100. Types of basins 

 and generalized cross-sectional 

 direction of flow of their waters: 

 1, Submerged dam. as for 

 southern California basins; 2, 

 evaporation greater than run- 

 off" plus precipitation, as for 

 Mediterranean Sea; 3, freezing 

 and rejection of cold brine, as 

 for Baffin Bay; and 4, evapo- 

 ration less than runoff" plus 

 precipitation, as for Black Sea. 



SEA- ICE 



ll/////J////f////////////////////^^/ ^////J/f7 



i i ( C ' 



EVAPORATION 



RUNOFF PRECIPI TA TION 



