14 



Atojjik Radiation and Oceanography and Fisheries 



Circulation and mixing within the intermediate 

 and deep layer 



Within the pycnodine and for some distance 

 below it, it is believed that most of the motion 

 takes place along surfaces of constant potential 

 density, so that transport and diffusion in the 

 lateral direction are very much greater than in 

 the vertical. This belief has been confirmed by 

 experiments with radioactive tracers, reported by 

 Revelle, Folsom, Goldberg, and Isaacs (1955), 

 in which it was shown that the radioactive wa- 

 ter spread out over an area of about 100 square 

 kilometers while maintaining a thickness of the 

 order of a few meters. 



Much of our knowledge of deep and inter- 

 mediate currents has been inferred from the 

 distributions of properties. These indicate that 

 the average velocities of the deep currents are 

 only a few cm/sec. or less. However, Wiist 

 (1957) has recently made calculations on data 

 from the Atlantic which indicate velocities of 

 meridional currents of 3 to 17 cm/sec. in the 

 deep sea, along the western margin of the west- 

 ern trough, in depths between 3,000 and 5,000 

 meters. The calculated currents on the eastern 

 side of the deep South Atlantic, especially in 

 the region of the Angola Basin were, on the 

 contrary, very weak. Dietrich (1957) has like- 

 wise computed mean current velocities of about 

 10 cm/sec. for the deep Antarctic Circumpolar 

 Current, and for the Subarctic Bottom Current 

 in the northern North Atlantic, the latter in- 

 creasing to as much as 40 cm/sec. when flow- 

 ing across the Greenland-Scotland ridge. He 

 states, however, that in the largest part of the 

 ocean the bottom currents are below 2 cm/sec. 



Direct measurements of deep currents are 

 technically difficult. The few successful meas- 

 urements summarized by Bowden (1954) show 

 mean velocities from less than a cm/sec. to 13 

 cm/sec. Recently Swallow (1955 and unpub- 

 lished data) has measured subsurface currents 

 by tracking a neutrally buoyant float at a fixed 

 depth. His measurements in the North At- 

 lantic give mean resultant velocities of 1.7 to 

 9.1 cm/sec. Tidal currents of about 10 cm/sec. 

 have been obtained by Swallow and others in 

 deep water. It appears that the mean current in 

 many parts of the deep sea may be less than 

 the periodic variable currents. 



The turbulence of these variable tidal cur- 

 rents, especially near the bottom, contributes to 

 vertical and horizontal mixing in deep water. 



Mixing should also occur along the boundaries 

 of the rapid deep resultant currents indicated by 

 Wiist and Dietrich, where there must be con- 

 siderable shear. 



Dietrich (1957) also suggests that horizontal 

 spreading of near-bottom water may occur in 

 regions of turbidity currents, which occur es- 

 pecially on the continental slopes. 



Exchange between the open sea and coastal areas 



In coastal areas and estuaries where precipita- 

 tion and land runoff exceed evaporation, there 

 is a net seaward drift of dilute surface water 

 and an inshore drift of sub-surface water from 

 the open sea. This is superimposed on the flow 

 of wind-driven currents through the coastal 

 areas. 



Some idea of the average time involved in 

 interchange of coastal waters can be obtained 

 from the volume in and transport through vari- 

 ous areas along the American Atlantic seacoast. 

 Calculations give a mean age of 2^ years for the 

 waters over the Continental Shelf from Cape 

 Hatteras to Cape Cod, about 3 months for the 

 Bay of Fundy, and 3 to 4 months for Delaware 

 Bay (Wooster and Ketchum, Chapter 4) . 



Exchange between the deep and intermediate 

 layers and the mixed subsurface layer 



Evidence of local cross pycnocline interchange 

 was obtained from measurements of the vertical 

 distribution of radioactivity following the 1954 

 Pacific weapons tests (Japanese Fishery Agency, 

 1955 and Harley, 1956) ; it is not, however, 

 clear whether the observed phenomena were en- 

 tirely the result of physical exchange of the wa- 

 ter and its contents or were in part due to 

 settling of particles and to biological transport. 



The major exchange between the near-surface 

 and deeper waters takes place in the following 

 regions : 



In areas where the pycnocline is maintained, 

 by the distribution of mass related to the gen- 

 eral circulation, at a sufficiently shallow depth to 

 be eroded away by wind stirring. Such areas 

 exist near the equator, along the north edge of 

 the Equatorial Counter Current, and at the 

 centers of strong cyclonic eddies. 



In regions of upwelling, where vertical cur- 

 rents carry water toward the surface and stir 

 the surface and intermediate layers. Water 

 from as deep as about 500 meters may be 



