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



stratification of the sea is characterized by two principal layers. The upper layer ex- 

 tends from the surface down to about 600-1000 m and is termed the oceanic tropo- 

 sphere; its uppermost part down to about 100 m is subject to the direct influence 

 of the atmosphere. This is the layer of diurnal and annual convections originating 

 at the surface and it shows the strongest mixing due to the effects of the wind and 

 waves ; it can be designated as the layer of surface disturbances. The troposphere 

 shows the strongest temperature decrease with depth and in low and middle latitudes 

 forms an upper warm layer of water overlying the cold water masses underneath and 

 separated from them by a more or less sharply marked thermocline. 



Table 47. Mean vertical temperature (°C) distribution in the three oceans 

 between 40° N. and A0° S,. 



t Maximum 



The lower part of the thermal stratification is the oceanic stratosphere which extends 

 from the bottom of the troposphere (thermocline) down to the sea bottom; to it belong 

 the major water masses of the deep sea which are characterized by the very small changes 

 in temperature both in horizontal and vertical direction. Table 47 presents the mean 

 vertical temperature distribution in the three oceans for latitudes between 40° N. 

 and 40° S. and also the vertical temperature gradient at each depth in degrees 

 per 100 m. The approximate limits between the zone of disturbance, troposphere and 

 stratosphere are indicated in Fig. 53. This twofold subdivision in the thermal structure 

 of the ocean is limited to the tropical and subtropical parts of the ocean. As is shown in 

 Fig. 52 the troposphere becomes less well developed towards higher latitudes and the 

 stratosphere comes closer to the sea's surface. In the subarctic and subantarctic regions 

 (polewards of the oceanic polar front, see Chap. XIX) the troposphere disappears 

 and the cold-water masses of the stratosphere extend generally to the surface. The 

 water masses of the troposphere lie on top of and are embedded in the cold-water 

 mass of the stratosphere in tropical and subtropical areas, but thin out and disappear 

 in higher northern and southern latitudes. 



Because of the decrease of salinity with depth it can be expected, just for reasons of 

 stability, that the temperature must also decrease with depth. Solar radiation is con- 

 verted into heat in the upper layers and from here the heat spreads rapidly downwards. 



