The Three-dimensional Temperature Distribution and its Variation in Time 1 27 



then the equilibrium state is unstable in the vertical. If a small water mass in such a 

 thermal stratification is displaced downwards, it will remain colder than its surround- 

 ings in spite of adiabatic heating, and it will be forced down further and further from 

 its initial position. If it is displaced upwards then it will remain warmer than the 

 surroundings and will therefore continue to rise. If, on the other hand, the vertical 

 temperature gradient is less than the adiabatic, particularly if the temperature de- 

 creases with depth, then the potential temperature will also decrease with depth and 

 the stratification is stable. 



Table 54. Vertical distribution of potential temperature (°C) below 3000 m for several 

 stations in the western and eastern troughs of the Atlantic Ocean 



The vertical temperature distribution present in the ocean is such that the stratifica- 

 tion, in so far as it depends on the temperature, is stable. In the oceanic troposphere 

 the temperature decrease is so large that, in spite of the vertical decrease in salinity, 

 the equilibrium state remains quite stable. In the upper layers of the stratosphere 

 the stratification is still stable, however, it becomes continuously less stable with in- 

 creasing depth. Table 54 shows the vertical distribution of the potential temperature 

 below 3000 m for several stations in the eastern and western troughs of the Atlantic 

 Ocean which show these conditions rather clearly. The same is usually also found in 

 the open sea of the Indian and Pacific Oceans. 



At very great depths, below about 4500 m, especially in the more or less extended 

 deep-sea basins, the vertical temperature distribution approaches the adiabatic and 

 may even exceed it a little, so that there is an indiff'erent stratification at great 

 depths or sometimes it may even be slightly unstable. It is principally in the deep- 

 sea trenches of the Pacific and Indian Oceans that this occurs. In these there is nearly 

 always temperature increase, but it seldom exceeds the adiabatic gradient and if it 

 does then only by very little. Such a condition of indiff'erent stability is formed only 

 when there is an almost complete separation of the water mass from the surrounding 

 waters. More or less fully enclosed deep inland seas such as the individual basins 

 of the European Mediterranean and the North Polar Basin show this phenomenon to 

 a marked degree in their deeper parts. The classic example of conditions in a deep-sea 

 trench is the vertical temperature distribution in the Philippine Trench (Schott, 

 1914; SCHULZ, 1917; Wust, 1937; van Riel, 1934; Schubert, 1931). According to 



