1 1 6 The Three-dimensional Temperature Distribution and its Variation in Time 



variation penetrates almost to 300 m depth with an eddy coefficient of about 

 70 g cm~^ sec"^ at the surface, 30 and 27 at 50 m, and 200 m, respectively. 



For a selection of 1 and 2 degree squares in the area between the Faroes and the 

 Bay of Biscay, Neumann (1940, unpublished manuscript) using simple statistical 

 methods has derived the annual temperature variation for 20, 40 and 100 m and has 

 obtained rather similar results. The effect of stratification on heat transport in the 

 deeper layers of a water mass also appears in lakes in the same way as described above 

 and the annual temperature variation can be explained only under consideration of 

 these processes. In shallow seas (Shelf seas) it is possible to study more accurately 

 the penetration of heat and especially the eff'ect of turbulence generated by the wind, 

 and also to investigate the eddy viscosity caused by strong tidal currents at the sea 

 bottom. Recently Dietrich (1950, 1953, 1954) has given an instructive example of 

 the various possibilities by the use of isopleths of temperature in vertical cross-sections 

 in diff'erent shelf waters which illustrates the conditions present in the best possible 

 way. Figure 5\a shows the annual variation of temperature and salinity from the sur- 

 face to the bottom. 



31-1 35-0 35-1 



Fig. 5\a. Example of annual temperature and salinity variations from the surface down to 

 the bottom (according to Dietrich), a, Irish Sea, North-Channel; b. Central North Sea; 



c, Baltic, Bomholm deep. 



{a) In the Irish Sea, north channel, with strong tidal currents where even in summer a 

 thermocline cannot form and the strong turbulence evens out the annual temperature 

 variation in the whole mass of water from the surface to the bottom (extremely strong 

 heat transport from the surface downwards due to turbulence). 



{b) In the middle of the North Sea where the weak tidal current has little eff'ect. 

 The formation of a thermocline prevents the development of a more pronounced 

 annual temperature variation beneath it. 



(c) In the Bornholm deep in the Baltic, no noticeable tidal current and a strong 

 increase in salinity with depth (strong density stratification during the whole year) 

 so that the lower layer is isolated and shows no annual temperature variation. See 

 p. 115 and Munk and Anderson (1948 on the theory of the thermocline). 



The annual heat budget of a limited water mass can also be calculated without diffi- 

 culty if sufficient observational data are available. A number of calculations of this type 

 have been made for lakes and similar calculations have been carried out for more or less 

 enclosed seas. According to O. Pettersson ( 1 896) the Baltic gives off" 1 37-500 kg cal/m- 

 from August to November and a further 385-500 up to March, in total about 523-000 



