The Three-dimensional Temperature Distribution and its Variation in Time 93 

 Table 38. Heat budget of the total ocean (g cal/cm^ day^) 



Latitude 



10" 



20 "= 



30^ 40= 



50° j 60" 



70= 



Heat gain 



80° I 90= 



Direct solar radiation after 



allowing for cloudiness 

 Diffuse radiation 



Total heat gain 



39 

 36 



75 



Heat loss 



Effective back-radiation 

 Evaporation heat 

 Convection 



Total heat loss 



143 



160 



35 



338 



131 137 



6 



20 20 



157 i 157 



Gains-losses 



-72 -82 



In this heat budget it has been tacitly assumed that the heat exchange through the 

 ocean surface occurs independently for each separate latitude belt. Therefore no meri- 

 dional heat exchange (by ocean currents and by horizontal mixing) was allowed to 

 occur. 



The differences between heat gain and heat loss show that for lower latitudes north- 

 ward, until about 25° N. the gain in solar energy is greater than the loss, while between 

 about 45° latitude and the poles the back-radiation is dominating because only a 

 small consumption of heat occurs due to evaporation and convection. The excess of 

 the large tropical and subtropical area is, however, roughly equalled by the deficiency 

 of the higher latitudes so that, when the effect of meridional heat transport is taken 

 into account, it can be seen that with reasonable accuracy there is a heat equilibrium 

 for the entire ocean. 



This meridional heat transport is largely due to the turbulent motion in the ocean 

 currents through lateral mixing (in meridional direction) (see Chap. Ill, 2e). If the eddy 

 coefficient of lateral mixing is denoted hy Ay (g cm~^ sec"^), the meridional temperature 

 gradient by ddjdy, then the heat W carried towards the north through a unit vertical 

 area is given by the equation 



W = -c„A 



d& 

 dy 



The amount of heat transferred from south to north across latitude 25° is given in 

 Table 38 and it can be calculated from these values that for turbulence effective down 

 to a depth of 1000 m an amount W of heat, which is approx. 1 g cal cm-^ sec~S 

 will be transferred through a vertical area of 1 cm^. The mean horizontal tempera- 

 ture gradient at 25° latitude is about — 4°C per 10° of latitude which is —3-6 x 10-» 

 deg/cm. The above equation thus gives Ay ~ 3 x lO'^gcm"^ sec~^ This calcula- 

 tion for Ay is naturally a very rough one, but it gives a value for the lateral eddy 

 coefficient which corresponds rather well to more accurate other determinations. It 



