Chapter X 



Forces and their Relationship to the 

 Structure of the Ocean 



1. External, Internal and Secondary Forces 



(a) Of the external forces that give rise to or maintain the ocean currents, the most 

 important are the air currents, the changes in atmospheric pressure at the surface of 

 the sea, and the periodic tide-generating astronomic forces. These forces can also 

 initiate water movements in a homogeneous sea. The changes in atmospheric pressure 

 are transmitted through the entire mass of water down to the sea bottom and thus give 

 rise to horizontal pressure differences and the formation of gradient currents. The 

 effect of air currents is twofold: First, the tangential force of the wind on the surface 

 of the sea (wind stress) produces a surface current which is transmitted by the effect 

 of viscosity (turbulence) to the water layers beneath the surface. Secondly, the wind 

 produces waves at the surface of the sea and the pressure exerted by the wind on the 

 windward side of these waves also initiates water movements in the direction of the 

 wind (wind drift). These currents produced by the wind and by the changes in at- 

 mospheric pressure are considerably modified by the deflecting force of Earth rotation 

 and by the boundary surfaces of the sea (coasts, continental slopes and sea bottom). 

 The piling up of the water by coasts (Anstau) is by far the most important effect of 

 the external forces and is responsible for the formation and the maintenance of an 

 oceanic circulation in the deeper layers of the ocean. 



In a sea of homogeneous structure the external forces can produce no change in the 

 physical stratification of the water mass. In a non-homogeneous sea, however, the 

 water movements displace different types of water relative to each other, and thus 

 either directly or due to the boundary conditions produce changes in the thermo- 

 haline structure of the ocean. This upsets the system of internal pressures forces and 

 give rise to ocean currents. 



(b) The internal forces arise from the vertical and horizontal distribution of mass 

 within the ocean. These differences in the mass distribution (in horizontal and vertical 

 direction) are the consequence of changes in the heat content (temperature) and in the 

 salinity. If at first the water masses are in an internal equilibrium state, this equilibrium 

 can be disturbed by changes of this type, thereby initiating ocean currents which in 

 turn tend to restore the system to a new equilibrium. The principal sources for dis- 

 turbances in the mass distribution can be found at the surface of the sea, where solar 

 and atmospheric radiation and outgoing radiation first influence the ocean, and where 

 evaporation also takes place. At the other boundary surface of the sea (the sea bottom) 

 the intensity of the disturbances is small and usually of no importance in changing 



312 



