Cliapter XVIII 



Basic Principles of the General Oceanic 



Circulation 



1. Introduction 



The ultimate cause of all movements in the sea is the supply of energy by solar 

 radiation. The meridional variations in the energy supplied lead to regional differences 

 in the structure of the oceans. The oceanic circulation modifies, however, the distri- 

 bution of temperature and salinity, which are basically determined by the climate, 

 and also affects the distribution of dissolved gases in the sea; it therefore has an 

 indirect influence on the distribution and accumulation of marine life. The general 

 oceanic circulation is therefore the fundamental problem of oceanography. 



The transformation of solar radiation into heat in atmosphere and sea takes place 

 mainly in the layers close to the interface between air and land, between air and water, 

 respectively. Other important influences from the hydrosphere on the atmosphere and 

 the reverse are also localized at the sea surface and in this way the sea surface becomes 

 one of the most important interfaces of the earth; it is the starting point of both the 

 atmospheric and the oceanic circulation. The principal factors involved in these, such 

 as the solar and sky radiation, outgoing radiation, evaporation, precipitation, 

 melting of ice and the wind stress on the water exert their major effects here. In com- 

 paring the atmospheric and oceanic circulation the special circumstance should be 

 kept in mind that the interface (sea surface) which is decisive for the initiation of 

 vertical motions is situated below the atmosphere but above the sea. Therefore, in 

 order to start a vertical circulation in the atmosphere air must be lighter than the 

 surrounding air masses (rising motion), while in the ocean water as compared with 

 the surrounding waters must be denser (sinking motion). The variable position of 

 this interface, from which the vertical circulations originate, causes corresponding 

 differences of the circulation system (Defant, 1929). 



According to the general causes, mentioned above, of steady water movements in 

 the sea, two fundamental factors stand in question: 



(1) the internal field of force of the mass structure, and 



(2) the external field of force due to the winds. 



Other less important external forces such as the supply of water by precipita- 

 tion or its removal by evaporation are less effective than the wind forces (see 

 p. 572). 



These two basic factors act quite differently on the water movements and an under- 

 standing of the general circulation can only be based on the resultant of the two 

 effects. Most investigations have been limited to the components of motion of the 



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