22 PHYSICAL PROPERTIES OF SEA WATER 



Numerical Values of the Vertical and Horizontal Eddy Vis- 

 cosity AND Eddy Conductivity. For the determination of the eddy 

 viscosity, fie, in the ocean it is necessary to know the frictional forces 

 and the velocity gradients. The frictional forces cannot be determined 

 directly and must be obtained as the difference between other acting 

 forces, but the velocity gradients can be obtained from directly observed 

 currents. For dealing with wind currents, theories based on certain 

 assumptions as to the character of the eddy viscosity are helpful (p. 124), 

 and for considering currents close to the bottom, results from fluid 

 mechanics permit conclusions as to the eddy viscosity from current 

 measurements alone (p. 119). The uncertainties of the correct applica- 

 tion of theories, the difficulties in making current measurements in the 

 open ocean, and the even greater difficulties in determining the acting 

 forces, all introduce great uncertainties in the numerical values of the 

 eddy viscosity that have so far been determined. 



Table 1 contains results of such determinations. The specific methods 

 used cannot be discussed here. All values are from regions of moderate 

 or strong currents which have been measured with considerable accu- 

 racy or they have been derived from the effect of wind currents on the 

 surface layer. The values range from nearly zero up to 7500, but 

 some consistency in the variations can be seen. At the very bottom, 

 small values are always found, and at greater distances from the bottom 

 great values are associated with strong currents and small stability, 

 whereas small values are associated with weak currents and small stability 

 or with strong currents and great stability. The latter conditions were 

 found on the North Siberian Shelf, where the eddy viscosity approached 

 a small value in a thin layer of very great stability within which the tidal 

 currents reached their maximum values. 



The eddy diffusivity, Hs, can be derived from observed time and 

 space variations of temperature, salinity, oxygen content, or other 

 properties by computations which are based on equations for heat con- 

 duction or diffusion. In some cases the currents need not be known, 

 but in other cases only the ratio fXs/v can be determined, where v is the 

 velocity of the current. If this velocity can be estimated, an approximate 

 value of fis can be found. Temperature, salinity, and oxygen content 

 can be measured much more easily and with greater accuracy than cur- 

 rents, and data for studying the eddy diffusion are therefore more readily 

 obtained. As a consequence, many more determinations of eddy diffu- 

 sion have been made, and the results show greater consistency. 



Table 2 contains a summary of values derived from observations in 

 different oceans and at different depths between the surface and the 

 bottom. The values range from 0.02 up to 320, but by far the greater 

 number of values lie between 3 and 90. The range is therefore smaller 

 than the apparent range of the eddy viscosity. The highest value, 



