550 



Effect of Wind on the Mass Field and on the Density Current 



Fig. 253. Vertical section of density (a,) in the Atlantic Ocean along 30" W. between 24° and 

 58° S. Above: topography of the physical sea level and of the 1000-decibar surface (relative 

 to the 3000-decibar surface assumed as plane). A.C., Antarctic convergence (oceanic polar 



front). 



(2) the continuous supply of water with low salinity which is produced by melting 

 of the northward drifting pack-ice. 



This second factor requires the presence of a thermo-haline circulation directed at 

 the surface from an area with high specific volume to another one with a low specific 

 volume. A circulation of this type is certainly present but the wind conditions are 

 probably the main cause (Deacon, 1934; Sverdrup, 1934Z)). In latitudes between 

 40° to 65° S. the prevailing wind is always westerly and gives rise to a drift current and 

 a consequent surface water transport to the north. According to meteorological obser- 

 vations the strongest surface wind in higher latitudes occurs between 50° and 60° S. 

 The water transport to the north is thus greatest between 60° and 50° S. and north of 

 50° S. is comparatively smaller. This gives rise to the formation of a convergence 

 line and a discontinuity layer in the mass field. The wind and its differentiation in a 

 meridional direction may also be considered the main reason for the intensification 

 and concentration within a narrow strip of the density current which would otherwise 

 spread out over a wider area. 



3. General Relationships Between Wind and Currents 



The investigation of steady currents produced by wind in a baroclinic top layer is 

 easily handled, since the deep water can be regarded as essentially motionless and the 

 wind field as quasi-permanent showing no changes with time or position. This allows 

 the eff'ects of both the vertical and horizontal eddy viscosities to be taken into account. 

 The equations of motion (XIII. 52) must then include terms for the horizontal eddy 

 viscosity, denoted briefly by h^ and h^. Integration of these equations over the entire 

 depth d and introduction of 



//. 



j: 



/^. dz, H, = 



h„ dz and P 



pdz 



(XVII.2) 



