94 The Three-dimensional Temperature Distribution and its Variation in Time 



is, however, certain that lateral mixing in ocean currents is a factor of considerable 

 importance for the horizontal distribution of the heat in the ocean and thus plays an 

 important role in the heat budget. 



2. Heat Transport in the Sea : Absorption, Conduction, Thermo-haline and Dynamic 

 Convection (Turbulence) 



The previous section gave an outline of the average heat amounts reaching the upper- 

 most layer of the ocean ; the question of what happens to this energy shall now be 

 considered. First of all, it can be expected that the radiation energy absorbed will 

 manifest itself as a rise in temperature. 



{a) Temperature Change Caused by the Absorption of Radiation 



The almost complete absorption of the solar radiation (direct and diffuse), and also 

 of the long-wave radiation of the atmosphere in the uppermost layers of the sea, must 

 cause large daily and annual variations in temperature if this heat is not conducted 

 in some way to the deeper layers. As given in Table 37, middle latitudes receive about 

 300 g cal cm~- per day from direct and diffuse solar radiation. 120 g cal of it would 

 be required for the evaporation of about 2 mm of water so that there would remain 

 approximately 1 80 g cal for heating the water mass and for producing a daily tem- 

 perature cycle. Of this amount the uppermost layer of 10 cm thickness absorbs 

 about 81 g cal, according to Table 20, while the top meter absorbs 1 15 g cal per day. 



Table 39. 



Fore- After- 



Night Total 



Under stationary conditions this energy gain must be re-radiated during daytime by 

 the water. The partition between day (incident and back-radiation) and night (back- 

 radiation) for the 10 cm layer will be roughly as shown in Table 39. The rise in tem- 

 perature of the top 10 cm of water, during the forenoon until the temperature maxi- 

 mum, is caused by the absorption of 41 g cal, while the rise for the top meter (100 

 cm^ of water) is derived from the absorption of 57 gcal; these amounts correspond 

 to a temperature range of 4-1 ° and 0-57°C. Therefore, the diurnal temperature changes 

 in the surface layer of the sea (and in lakes) may remain very small and are much less 

 than that of the land and the air immediately above it. During the summer half of the 

 year the gain during the day is greater than the loss during the night and heat is 

 accumulated in the uppermost layer. 



These considerations raise the question of a possible radiational equilibrium within 

 the uppermost layers of the sea; only in this way can there be an appreciable absorp- 

 tion of radiation. In layers that are not too thick, water is somewhat more transparent 

 for short wave than for long wave radiation (see p. 52). Since the absorption 



