The Three-dimensional Temperature Distribution and its Variation in Time 129 



layers are not at complete rest, and that due to the exchange produced by turbulent 

 motions in these layers the heat is more rapidly dissipated than by normal conductivity. 

 An exchange of about 4 g cm-^ sec~^ would be sufficient to account for the 

 observed slightly superadiabatic temperature gradient. This appears, however, 

 not entirely conclusive. Even when the water masses in these more or less enclosed 

 deep-sea troughs and trenches do not participate in the general horizontal circula- 

 tion of the deep sea and can therefore be regarded as motionless in horizontal direc- 

 tion, there may still occur vertical convection currents produced by the continuous 

 influx of heat from the Earth which will carry this heat to the layers above. Such a 

 convection will be effective if there is the smallest vertical instability. Once such 

 instability exists in the bottom layer there will be a steady interchange of small water 

 quanta rising and sinking, and this convectional circulation will be maintained by the 

 steady inflow of heat through the sea bottom. In the water masses above an adia- 

 batic temperature gradient will be established; a gradient greater than the adiabatic 

 can, however, form only in the very bottom layer, though even here it will be scarcely 

 possible to detect it by physical measurement. It is required here in order to maintain 

 the vertical circulation against the internal viscosity. This might be the cause that the 

 water masses of the deep-sea trenches and the deep basins in the ocean show a vertical 

 stratification approximating closely indifferent equilibrium state. 



{d) The Vertical Temperature Distribution in Adjacent Seas 



While a steady decrease in temperature with increasing depth is characteristic for 

 the open oceans, in adjacent seas connected with the open ocean over shallow sills 

 the temperature below a certain depth is almost constant no matter how deep they 

 may be. The adjacent seas can be divided into two groups according to their tempera- 

 ture stratification : the first includes all those adjacent seas where the surface water in 

 winter cools to a temperature which is lower than that of the open ocean at the greatest 

 depth at which they are in communication (sill depths). Provided there is an almost 

 homo-haline structure in these adjacent seas, the autumn and winter convection causes 

 the cooled surface water to sink to the bottom, and the deeps in these adjacent seas 

 are thus filled with water masses at approximately the lowest surface temperature 

 occurring during the coldest month of the year. The deep layers in this show roughly 

 the winter temperature of the region concerned, provided the convection is not pre- 

 vented from reaching the greatest depths by irregularities in the thermo-haline struc- 

 ture of the surface layers, for instance, by a layer of low salinity. 



Examples of this type of adjacent sea are the Red Sea and the European Mediter- 

 ranean. In the first case, in the Straits of Bab-el-Mandeb (north of Perim island), the 

 sill depth is 1 50 m; in the second, in the Straits of Gibraltar, about 350 m. In the 

 Mediterranean during the summer there is a pronounced anothermal stratification 

 in the upper layers, while depths below about 300-400 m are essentially homo- 

 thermal. Towards the end of the winter this homo-thermal state extends upwards to 

 the surface. The temperature of this deep layer is thus about 12-9-1 3-2 °C in the 

 Balearic Basin and in the Tyrrhenian Basin, and about 1 3-6-1 3-9°C in the Ionian 

 Basin and in the eastern basin near the Syrian coast. The northern Adriatic Sea shows 

 values near to 12°C. These temperatures are all in good agreement with the winter 

 temperatures in these regions (Table 56). 



