1 34 The Three-dimensional Temperature Distribution and its Variation in Time 



depth. If this condition is not satisfied the convection processes in the autumn and 

 winter will not be able to extend to the bottom. The consequence of this limitation of 

 the convection to a surface layer of greater or lesser thickness is a dichothermal 

 temperature stratification during the warmer period of the year. There is a colder 

 intermediate layer situated between a warmer upper and a warmer lower layer, which 

 can be interpreted as the remainder of the convectional flux extending to this depth 

 during the cold period of the year. 



1000 



100 

 Nautical miies 



Fig. 56a. Vertical distribution of the potential temperature beneath 1000 m over a vertical 



section through the North American Basin, the Anegada-Virgin Passage and in to the 



Caribbean Basin (according to Dietrich). Vertical enlargement by 1 : 1500. 



This cold intermediate layer is typical of the whole of the open Baltic Sea during 

 the summer. The approximately homo-haline top layer heated by solar radiation 

 extends down to about 30-50 m depth; underneath a depth of 50-80 m there is 

 a core of relatively cold water with a temperature of 2-3°C, while still further down to 

 the bottom the temperature gradually rises to 4-5 °C. This cold intermediate layer re- 

 sults from cooling of the surface water during winter. The temperature distribution of 

 the top layer during this time shows an almost isothermal state due to mixing by turbu- 

 lence and convection, whereby at the same time the temperature at the surface may 

 fall to near or sfightly below the freezing point (see Fig. 51^, c; p. 116). Similar 

 conditions can be found in the Black Sea. For further detail see Skorzow 

 and NiKiTiN (1927) and especially a monograph on conditions in the Black Sea by 

 Neumann (1944). 



During the summer the water masses in the polar waters may also show a similar 

 temperature distribution in the upper 100-150 m. Conditions in this layer at the end 



