46 



Atomic Radiation and Oceanography and Fisheries 



surface and deeper waters is most likely to take 

 place under the following conditions: 



1. In regions where the pycnocline is suffi- 

 ciently shallow to be eroded at the top by wind 

 stirring. In coastal waters the pycnocline is 

 usually shoaler than in midocean, and shallow 

 pycnoclines may also be found in high lati- 

 tudes, at the equator, along the north edge of 

 the Equatorial Countercurrent, and at the cen- 

 ter of strong cyclonic eddies. This process is 

 not effective to great depths, but could serve to 

 bring radioactive materials into the surface wa- 

 ters from the pycnocline layer. 



2. In regions of up welling, where the pycno- 

 cline is relatively weak and where vertical cur- 

 rents not only carry water toward the surface 

 but also stir surface and deeper waters. It is 

 unlikely that water from depths of more than 

 500 meters is ever brought to the surface by 

 this process. Upwelling is common along west- 

 ern coasts of continents in the trade wind belt, 

 such as the coasts of Peru and Northern Africa. 

 In a simple sense, the persistent trade winds 

 blowing parallel to or offshore develop an off- 

 shore component of transport in the surface 

 waters, and deeper waters upwell to maintain 

 the volume continuity. Upwelling may also 

 occur along other coasts when the winds are 

 suitable. The process has been extensively stud- 

 ied along the coast of California where it is 

 not continuous because of the variability of the 

 winds (Sverdrup et al., 1942, p. 725). The 

 speed of coastal upwelling has been variously 

 estimated as 0.6 m/day (McEwen, 1934), 2.25 

 m/day (Saito, 1951) and 2.7 m/day (Hidaka, 

 1954) . However, since these estimates are theo- 

 retical mean values, they may differ significantly 

 from actual instantaneous upwelling rates. 



Midocean upwelling, associated with diver- 

 gence of the surface currents, occurs in a band 

 along the equator in the eastern and central 

 Pacific Ocean (Cromwell, 1953). Observations 

 indicate that the effects of this upwelling ex- 

 tend to 50 meters in the eastern Pacific and to 

 100-150 meters in the central Pacific (Wooster 

 and Jennings, 1955). Similar but less pro- 

 nounced upwelling has been observed in the 

 equatorial Atlantic (Bohnecke, 1936) . 



3. In regions of surface convergence, where 

 sinking waters may fill the depths of the ocean, 

 or may spread at intermediate depths according 

 to their density. In tropical and temperate 

 latitudes such sinking is confined to the surface 



layer. In such regions mixing in the upper 

 layer may be facilitated but exchange across 

 the pycnocline probably is not, since the sinking 

 water tends to increase the density gradient in 

 the pycnocline. 



In high latitudes, on the other hand, sinking 

 waters may reach great depths, and it is in 

 such regions that most of the intermediate and 

 deeper water masses of the ocean are formed. 

 The most extensive and pronounced of these 

 convergences is the Antarctic Convergence which 

 occurs at 50 to 60° S in a band around the entire 

 Antarctic Continent. The cold, low-salinity 

 water which sinks there forms an identifiable 

 water mass, the Antarctic Intermediate Water, 

 which spreads at depths between 800 and 1200 

 meters in all southern oceans. This water can 

 be identified everywhere in the South Atlantic 

 and extends across the equator as far as 22 °N 

 in the North Atlantic (Deacon, 1933; Iselin, 

 1936). 



In the Irminger Sea, between Iceland and 

 Greenland, and in the Labrador Sea, warm high 

 salinity water of the Gulf Stream is partly mixed 

 with cold low-salinity water flowing out of the 

 Arctic Ocean. The resulting mixture may spread 

 in small quantities as Arctic Intermediate Water, 

 or when sufficiently dense may form the deep 

 and bottom water of the North Atlantic (the 

 possibility that the formation of this deep water 

 is not a continuous process is discussed later) . 

 Intermediate waters of the North Pacific are 

 probably formed in winter at the convergence 

 between the Kuroshio Extension and the Oya- 

 shio (Sverdrup et al, ch. 15). There is ap- 

 parently no deep or bottom water formed by 

 this process in the Pacific. 

 4. In regions where the density of surface 

 waters is so increased by evaporation, cooling 

 or freezing, that they sink to intermediate or 

 greater depths. Active formation of Antarctic 

 Bottom Water takes place in the Weddell Sea 

 due to the freezing of high salinity surface 

 waters. In the Mediterranean and Red Seas, 

 bottom water is formed by winter cooling of 

 waters whose salinity has been greatly increased 

 by evaporation. Mediterranean water flows out 

 into the North Atlantic at depths of 1000 to 

 1500 meters and can readily be identified near 

 Bermuda, 2500 miles from its source. 



In summary, exchange between near-surface 

 and deeper waters takes place most commonly 

 (1) in high latitudes, (2) along the equator. 



