Chapter XIX 



The Tropospheric Circulation 



1. The Position and Structure of the Oceanic Troposphere 



The important subdivision of the oceanic space into troposphere and stratosphere is 

 due primarily to the climatic influence of the atmosphere on the water masses of the 

 uppermost ocean layers. More or less constant conditions in weather and radiation 

 at the ocean surface give rise to the development and maintenance of water types of 

 diff'erent character in different climatic zones. Broadly speaking there are two principal 

 water types which are constantly being formed in large quantity and with a rather 

 constant internal structure; they correspond to the two great zones of contrasting 

 climate, the tropical and subtropical regions, and the polar regions. These two water 

 types are: 



(1) the tropical-subtropical water type which is warm due to the excess of incoming 

 radiation and has a high salinity due to evaporation, and 



(2) the cold weakly saline water type of the subpolar and polar zones. 



The former is lighter, the latter heavier, and this difference is the cause of con- 

 tinuous large-scale movements. These movements follows the fundamental principle 

 that each water type tends to flow by the shortest route, by vertical or horizontal dis- 

 placement to the depth in the ocean at which it will be in a stable equilibrium corres- 

 ponding to its density; here it will spread out as a layer. The heavier subpolar water 

 type therefore sinks to greater depths, and spreads more or less horizontally to fill in 

 this way the deep lower layers of all the oceans. The lighter tropical and subtropical 

 waier type, on the other hand, remains in the upper layers of its original zone as the 

 lightest water type. The subdivision in the structure of the oceans is thus a con- 

 sequence of circulation. It is to be expected already from the history of formation of 

 the two main oceanic subspaces, that they will have essentially separate circulations; 

 these will be called tropospheric and stratospheric. This does not imply that there is 

 no connection between the two circulations; on the contrary, at certain places inter- 

 actions occur and the water masses of both type undergo transformation by turbulent 

 mixing and manifold atmospheric influences so that tropospheric water becomes 

 stratospheric and vice versa. 



The thermo-haline structure of the troposphere has been explained in pt. I, Chapter 

 III, §4, p. Ml et seq. and IV §3, p. 165 et seq. The most important phenomenon is the 

 layer of discontinuity in the vertical distribution of temperature and density which is 

 always sharply defined in the tropics and subtropics and is associated with a charac- 

 teristic salinity distribution. An example is shown in Fig. 70 of pt. I. Beneath the dis- 

 continuity layer which acts as a barrier to upward and downward movement, is the 

 subtroposphere which is occupied by little differentiated and nearly motionless waters. 



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