Concluding Remarks 175 



phenomenon occurring at a certain boundary of the oceans where high- 

 order derivatives in the governing equation (elsewhere neghgible) are 

 locally important. In this sense, the Gulf Stream 'exists' as a physical 

 entity in the same sense as the boundary layer next to an aerofoil exists : 

 it is a recognizable part of a larger physical system. 



The idea of a western boundary current — whether it be viscous or 

 inertial — is of basic importance because it simplifies the physics and 

 mathematics of the central regions of the ocean. The higher-order terms in 

 the vorticity equations are restricted to a narrow coastal region, and the 

 remainder of the ocean can be studied with very simple equations (as we 

 have already seen in Chapter XI). This possibihty of emancipating the 

 interior solution from boundary conditions at the west may very well be 

 the most important lesson to be learned from the study of the Gulf Stream. 



CLIMATE AND THE GULF STREAM 



There is scarcely any more firmly rooted idea in the mind of the layman 

 than the notion that the Gulf Stream keeps the European chmate warm. 

 So long as it was believed that the Gulf Stream was a river of warm water, 

 this idea did make sense. It is no longer possible to be so certain of the 

 direct climatological influence of the Gulf Stream, for it now seems that it 

 is not so much the Stream itself that is important, as the position and 

 temperature of the large mass of warm water on its right-hand flank. 



Any argument which purports to relate cUmate to the transport of the 

 Gulf Stream must include some account of the physical relations between 

 the Stream and the density (and thermal) structure of the water on either 

 side. In fact, Isehn (1940) went so far as to speculate that warming of the 

 European chmate might actually be least during periods of increasing 

 transport of the Gulf Stream. This idea was based on the hypothesis that 

 the processes which produce the warm surface masses of the Central 

 Atlantic Water are more or less constant in time, and hence, owing to the 

 geostrophic relation, an increasing transport of the North Atlantic Gyre 

 must be accompanied by simultaneous deepening of the thermocHne in the 

 Sargasso Sea and a radial shrinking of the current system. In this way, 

 warm surface water would be withdrawn from the north, and the European 

 cUmate might be colder. Conversely, a weakening of the transport of the 

 current system would, Isehn reasoned, be accompanied by a rising thermo- 

 cHne throughout the Sargasso Sea, and the excess warm water would force 

 the current system radially outward and farther northward, and might 

 even succeed in discharging quantities of surface water to high latitudes, 

 thus warming the European chmate. There is no convincing evidence to 

 prove that this (or any other) sequence of events actually takes place, nor 



