Main Features of General Oceanic Circulation and their Physical Exploration 703 



as used by Munk (1950) and Hidaka (1950) taking into account the lateral eddy 

 viscosity and obtain, for a zonal wind stress whose amplitude varies harmonically 

 with time, the variations in the strength of the currents and the phase lag behind the 

 wind in the individual circulation gyres. The results seem to be somewhat outdated by 

 the more recent developments in the theory of the general oceanic circulation. 



A new and rather important contribution to the effect of a time-dependent wind 

 on a stratified ocean has been made by Veronis and Stommel (1956). Now one deals 

 with non-stationary conditions, which stand in question and which can in general 

 be regarded as aperiodic disturbances across the given current field ; these disturbances 

 of rather different dimensions may therefore vary with both time and position. A model 

 was used in which the ocean was taken as horizontally unlimited — coastal effects 

 were thus disregarded — and it consists of two layers (an upper and a lower layer 

 separated by a boundary surface). The wind system introduced, however, is of a finite 

 size. In agreement with the theoretical work on the dynamics of ocean currents in the 

 central parts of the oceans (Sverdrup, 1947 and Reid, 1948) the lateral eddy viscosity 

 was disregarded. The theoretical investigation tends towards an understanding of the 

 way in which a two-layered ocean would react to changes in the wind field acting on it. 

 The main questions were as follows : 



(a) will the wind-generated current restrict itself to the top layers so that the hori- 

 zontal pressure gradients and the velocities in the deep layers could be neglected, and 

 how will the boundary surface and the physical sea level behave under these conditions ? ; 

 and 



{b) will the wind-driven current extend down to both layers, and is the horizontal 

 pressure gradient in both layers down to the sea bottom of the same order of magni- 

 tude?; or 



(c) will the wind influence cause combination of {a) and {b)l 



Movements of type {a) are called internal or baroclinic, those of type {b) external 

 or barotropic. This is illustrated by the scheme given in Fig. 338. It has often been 



Approximatly ^ 

 ctrest, ^~0 



Type (a): baroclinic 



Type (0 : barotropic 



Fig. 338. The type of motion in a two-layered ocean, (a) baroclinic or internal type, with 



a motion in the upper layer and a nearly motionless lower layer, (b) barotropic or external 



type. Horizontal pressure gradient nearly equal in both layers. 



