Ejfect of Wind on the Mass Field and on the Density Current 549 



Ekman (1931) has drawn attention to a special effect of the wind on a given solenoid 

 field. In a top layer (the place where density currents occur) the isosteric surfaces are 

 assumed to rise from south to north (Northern Hemisphere; approximately the 

 conditions found in the Atlantic between 40° to 50° N. and 30° to 40° W.). In the 

 absence of wind there will be a density current directed towards the east. If now a 

 steady persistent wind gives rise to a drift current, thus altering the mass field, then, for 

 a northerly wind the total transport of the drift current will be directed to the west and 

 for a southerly wind to the east. The basic current therefore will be either retarded 

 or accelerated. An east wind blowing against the current will produce a transport of 

 the upper water to the north and will thus tend to even out meridional density differ- 

 ences, and in this way to decrease the velocity of the density current. If the wind blows 

 towards the west (as in the Atlantic over the Gulf Stream), then the upper layer will 

 be driven towards the south and the slope of the isosteric surfaces will increase. As 

 long as only the total system of surfaces without internal change is displaced towards 

 the south the strength of the density current, which is largely fixed by the horizontal 

 distances between the isosteres, will remain unchanged; however, under certain con- 

 ditions changes in inclination of these surfaces will also occur and the density current 

 will increase its strength. This is especially the case when the upper lighter water is 

 displaced by the wind, while the lower one remains unaffected. The wind blowing in 

 the direction of the density current, in addition to the generation of a drift current, 

 also has the effect of localizing the density current and may transform an otherwise 

 broad and slow current into a narrow rapid one, still with the same transport. Ekman 

 saw in this process an explanation for the narrowness to which the Gulf Stream is 

 confined in this part of the Atlantic. This peculiar phenomenon of a "river in the sea" 

 is in any case an argument in favour of such wind effects. 



Another example of wind effect on the mass field is the boundary surface found 

 throughout the interior of the entire Antarctic Ocean which appears at the sea surface 

 of the ocean as the Antarctic Convergence Line (Southern Hemisphere Polar Front). 

 This boundary surface separates the heavier, colder, Antarctic water to the south 

 from the lighter but more saline water of the oceanic troposphere to the north. The 

 boundary surface has a slope corresponding to the density and current conditions. It 

 behaves like a solid wall (continental slope) and makes an Antarctic vertical circulation 

 possible. Figure 253 (Sverdrup, 1933a) shows a meridional density section at 30° W. 

 derived from the observations of the "Discovery" expedition. The boundary surface 

 meets the sea surface at 50° S. in the Antarctic convergence line. The topography of 

 the physical sea level and the 1000 decibars surface (both relative to the 3000 decibars 

 surface) are shown in the diagram above. These isobaric surfaces slope downwards 

 from north to south corresponding to the current flowing eastward in both water 

 bodies; this current must be stronger on the northern side than in the Antarctic water 

 to the south. 



The cause of the formation of a discontinuity surface is not immediately apparent, 

 since the current flows exactly towards east in all latitudes and meridional current 

 components are required in order to produce and to maintain it. 



Two factors favour the occurrence of a northward component in the Antarctic 

 water. 



(1) The prevailing westerly winds, and 



