44 



Atomic Radiation and Oceanography and Fisheries 



a decrease in temperature or an increase in 

 salinity, the surface water sinks and mixes with 

 deeper water. Convection is maintained by 

 (a) surface coohng due to long wave radiation 

 and heat conduction to the atmosphere, (b) the 

 loss of latent heat and water vapor in evapora- 

 tion, or (c) the increase of surface salinity from 

 free2ing of surface water. 

 2. Wind stirring: Vertical turbulence in the 

 upper layer results from wind action on the sea 

 surface. The extent of wind stirring depends 

 both on the magnitude and uniformity of wind 

 stress and on the vertical density gradient. Stir- 

 ring is effective only to a depth where there is 

 sufficient energy to overcome the effect of sta- 

 bility. Both the homogeneity and the depth of 

 the upper layer are affected by wind. Single 

 gales have been observed to deepen the surface 

 layer on the average by about 20-30 feet, 

 (Francis and Stommel, 1953). 



Rapid vertical mixing may be brought about 

 by other processes. Thus in shallow coastal 

 areas stirring by strong tidal currents is im- 

 portant. Stirring may also be accomplished by 

 the vertical component of currents, particularly 

 in regions of upwelling and sinking. 



It should be noted that even above a well- 

 developed pycnocline there is not complete ho- 

 mogeneity within the so called "mixed" layer. 

 Concentrations of those elements affected by 

 biological activity (such as oxygen and phos- 

 phorus) may show significant variation within 

 the euphotic zone. Even so-called "conserva- 

 tive" concentrations (temperature and salinity) 

 may not be uniform within the surface layer. 

 Such heterogeneity may be attributed to in- 

 complete vertical mixing or to vertical shear in 

 the surface layer. 



When radioactive materials are introduced 

 into the near-surface layer, they are transported 

 away from the area of introduction by surface 

 currents. These currents extend, in general, 

 through the entire depth of the upper layer 

 and seem to be driven, directly or indirectly, 

 by the wind. 



The average locations and velocities of the 

 important surface currents of the world ocean 

 have been studied for many years and are well 

 known (see, for example, Deutsche Seewarte, 

 1942; U. S. Navy Hydrographic Office, 1947 

 a and b, 1950; Sverdrup, Johnson, and Flem- 

 ing, 1942, ch. 15). This knowledge comes 



primarily from averages of countless ship-drift 

 observations, and from computations based on 

 the observed subsurface distribution of density. 



These calculations give mean speeds as high 

 as 193 cm/sec (90 miles per day) in the 

 Florida Current (Montgomery, 1938a) and 89 

 cm/sec (41 miles per day) in the Kuroshio 

 (Koenuma, 1939). The volume of water flow- 

 ing through the Florida Straits in 15 years is 

 about equal to that of the upper 500 meters of 

 the whole North Atlantic. Similarly, between 

 the northern Ryukyus and Kyushu, the Kuro- 

 shio transports a volume equivalent to that of 

 the upper 500 meters of the North Pacific in 

 about 50 years. It seems likely that there is 

 no area of surface water in the ocean that can 

 be considered as isolated from the remaining 

 surface waters. 



Recent intensive studies of the Gulf Stream 

 and other surface currents, using such modern 

 instruments as the bathythermograph, electronic 

 navigational aids, and geomagnetic electro-kine- 

 tograph (GEK), have revealed complicated fine 

 structures, with filamentous jets and counter- 

 currents not apparent in the average picture 

 (Fuglister, 1951). Characteristic maximum sur- 

 face velocities measured by GEK and Loran 

 dead reckoning in the Gulf Stream were found 

 to fluctuate between 150 and 300 cm/sec or 

 70 to 140 miles per day (von Arx, Bumpus 

 and Richardson, 1955). Thus, in estimating 

 the time at which radioactive materials will be 

 found at various distances from the area of 

 introduction, one must be cautious in the use 

 of average surface current speeds. 



Direct evidence of the transport of radio- 

 active materials by surface currents in the 

 western Pacific is given by "Shunkotsu-Maru" 

 survey (Miyake, Sugiura and Kameda, 1955) 

 and the "Taney" survey (U. S. Atomic Energy 

 Commission, 1956) four months and thirteen 

 months respectively after nuclear weapons tests 

 in the Marshall Islands in March, 1954. The 

 earlier survey found significant levels of radio- 

 activity at a distance of 2000 kilometers from 

 Bikini, suggesting a westward drift of more 

 than 9 miles per day (about 20 cm/sec) . The 

 later survey found significant levels of radio- 

 activity at least 7000 kilometers downstream 

 from Bikini ; this gives about the same minimum 

 westward drift. 



In addition to being drifted away from the 

 area of introduction, radio-active materials are 



