634 



The Tropospheric Circulation 



which result from interaction with the deeper water masses or are due to lateral mixing. 

 The current thus tends to become more and more superficial ; this process maintains 

 the high surface velocities. 



The cause for the horizontal meander-like oscillations of the narrow current band 

 of the Gulf Stream after leaving the continental shelf is not entirely clear. These 

 meanders occasionally become unstable and then complete cut-off vortices are formed ; 



Om 



lOOm 



200 m- 



300 m 



400m 



500m 



U 



600m 



mps 



1-0 o- 



Fig. 297. Transformation of a uniform current with a constant vertical density gradient into 

 a flow characterized by a minimum value of the momentum transfer. The initial uniform 

 velocity distribution is given by the heavy broken line, the final velocity distribution by the 

 heavy full line. The density distributions before and after adjustment are given by lines 

 marked by a (initial) and a (fmai)- Note that the depth of the final current is one half of the 

 initial depth. The total percentage density range has the value 00045. 



this has been discussed already on p. 616. Recent investigations on the vertical strati- 

 fication in the Gulf Stream (Arx, Bumpus and Richardson, 1954) using stations with 

 little distance from each other have shown that the narrow current band has a filamen- 

 tary structure. It is composed of thin layers of high velocity alternating with layers of 

 lower velocity. This extraordinary stratification is possibly connected with gliding 

 processes imposed by external circumstances on the individual water layers of the 

 Gulf Stream and can be assumed to be a consequence of turbulence processes, which 

 are imposed from outside. 



The meandering of the narrow current band of the Gulf Stream appears to be a 

 common phenomenon. These meanders show v/avelengths of about 200 km and their 

 speed of propagation is about 1 1 nautical miles a day, which is about a tenth of the 

 speed of the current itself. Stommel (1953) has given a simple meander theory for a 



