The Tropospheric Circulation 



625 



deeper reference level changes the results very little. There is thus a contradiction 

 between the "geodetic" and the "oceanographic" levelling which requires explanation. 

 A plausible explanation was indicated by Sverdrup (and co-workers 1946, p. 578) 

 based on the following assumptions. 



(1) That the geodetically determined gradient of the sea level is actually present in 

 the coastal waters just off the coast and that corresponding to this there is a coastal 

 current flowing southwards. 



(2) That in the neighbouring waters the physical sea level slopes down seawards 

 until the left-hand edge of the Gulf Stream which causes a current to flow southward 

 due to the piling up of water. This gradient current would be one part of the large 

 elongated vortex on the left-hand side of the Gulf Stream while the second part flows 

 along the left-hand edge of the main current and in the same direction. 



(3) Corresponding to this current and the adjoining Gulf Stream, the physical sea 

 level rises steeply seaward from the coast (p. 607). The depression showing the deepest 

 water level thus would follow the continental slope rather closely so that the south- 

 ward flowing branch of the vortex lies over the shelf. The topography in a transverse 

 section across the Gulf Stream thus has some similarity with that shown in Fig. 203 



1231 1230 1229 1228 1227 



1226 



500 



1000 



1500 



2000 



„Atlontis" 

 _ April 1932 



Fig. 294. Density distribution (at) and position of the lower limit d of the current system 

 in a cross-section through the Gulf Stream. "Atlantis", April 1932 (Chesapeake Bay, 



Bermuda). 



(p. 460) south of the Newfoundland Banks where on the other side of the depression 

 in sea level the Labrador Current flows eastward. According to the Rossby theory the 

 elongated vortex between the Gulf Stream and the continental slope is a dynamic 

 necessity. The Gulf Stream now would flow downhill in accordance with its mass 

 structure and the surface slope would be directed southwards only at the coast. This 

 piling up of water over the continental shelf was regarded by Sverdrup as due to the 

 prevailing wind over the North Atlantic. The south-west wind over the northern part 

 of this ocean maintains a high water level along its northern borders and maintains 



2S 



