114 Nonlinear Theories — Inertial 



With respect to this region, the theory need not involve an assumption 

 concerning lateral-eddy viscosity. The dissipation may be determined 

 dynamically, as in a hydrauhc jump in ordinary open-channel flow. 



THE REGION OF DECAY OF THE GULF STREAM 



After the Stream passes Cape Hatteras, it develops meanders, which increase 

 in ampUtude and eventually form detached eddies east of 60° W. longitude. 

 The interpretation of observational data beyond this point is quite am- 

 biguous, as has been mentioned in Chapter V. All indications are that the 

 Stream breaks down into fragments and eddies which cannot be adequately 

 described or surveyed by present means. Of all the regions of the Stream, 

 this region of decay is the most difficult to depict and to understand. The 

 region must be very extensive. The chart of the 10° C. isotherm depth 

 (fig. 66) demonstrates that the potential vorticity of the upper layer is not 

 even remotely uniform north of 35° N. latitude. The fragments, or, as they 

 used to be called, 'branches', of the Gulf Stream still discernible in the 

 North Atlantic Current (fig. 42) do not have the same potential vorticity, 

 although there does appear to be a fair degree of uniformity within and to 

 the south of any particular fragment. Apparently there are at least two 

 complicated processes at work in this region which are not easily susceptible 

 of simple theoretical treatment : 



i. The Stream becomes unstable and breaks up into fragments. Each 

 fragment seems to carry an equal share of the transport. The potential 

 vorticity of each fragment is uniform, but different from that of the parent 

 Gulf Stream. These fragments can be traced at least halfway across the 

 North Atlantic. They underlie the surface West Wind Drift. 



ii. Surface cooling and wind mixing profoundly modify the stratifi- 

 cation of the surface layers, especially in the wintertime. Since, on the 

 average, it takes more than a year for particles to pass entirely through 

 the region of decay, it is reasonable to suppose that the potential 

 vorticity of the waters in the region will be greatly changed by chmatic 

 factors. 



In the course of his fundamental studies of the atmospheric jet stream, 

 Rossby and his colleagues at the University of Chicago have developed 

 many interesting ideas which may have some bearing on the multiple- 

 stream structures in the region of decay (1947). Neumann (1952) has also 

 studied the region, and beheves that the multiple streams are not decaying 

 fragments of the Stream ; he attributes them to persistent nonuniformities 

 of the winds. My own impression is that wind patterns of the sort required 

 do not persist long enough to produce such eflects. 



