Fluctuations 



139 



misleading readings of velocity in the Florida Straits. Nevertheless, the 

 general fact is that a wide zone of anticyclonic vorticity (of approximately 

 — [0-5 + 0-1]/, where / is the local CorioUs parameter) seems to be 

 estabHshed and also requires explanation. It is, of course, impossible for 

 the variation of the Coriolis parameter with latitude to cause such a shear 

 over so short a distance as the length of the Florida Straits. 



At Miami the channel is only about one-half as deep and one-half as 

 wide as at Key West. The change in depth can have no important in- 



HAVANA 



Fig. 72. Hypothetical position of the interface in the Florida Straits. The 

 arrows indicate the magnitude of the current. In the actual Florida Straits 

 there is a right-angle bend in the channel between Key West— Havana and 

 Miami-Cat Key. Bottom topography is not indicated. From Stommel (1953, 

 fig. 4). 



fluence on the flow, since the bottom water does not have an appreciable 

 velocity at either section. The narrowing of the channel at Miami is very 

 important hydrographically, for in order to pass through it the water is 

 accelerated, and this requires a small drop in the level of the free surface 

 from Key West to Miami. Since the lower layers are at rest (except for 

 tides), the isopycnic surfaces must slope upward toward Miami to counter- 

 act the axial pressure gradient in the upper accelerating layers. Because 

 this slope must be some 500 times the drop of the free surface, it produces 

 a marked decrease in the thickness of the surface layers as Miami is 

 approached. By conservation of potential vorticity, this vertical shrinking 

 produces an anticyclonic shear and magnifies the transverse geostrophic 

 slope of the free surface. A crude quantitative analysis based on a 

 two-layer model is presented below. Fig. 72 illustrates the average 

 hydrographic structure of the Florida Current envisaged in this hypo- 

 thesis. 



