The Geostrophic Relationship 21 



of equal density, increasing downward. The spacing of these lines is also 

 vertically exaggerated, but not so much as the free surface. The difference 

 in level of the broken lines is about 700 m. across the Stream. 



The diagram extends to a depth of about 1000 m., the lower 3000 m. 

 being omitted. The arrows show the hypothetical magnitude and direction 

 of the horizontal pressure gradient across the Stream. Near the surface 

 the pressure gradient is controlled entirely by the shape of the free surface. 

 At depth, the slope of the density surfaces takes effect until at some depth 

 (in the diagram, 1000 m., but usually assumed^ to be 2000 m.) the hori- 

 zontal pressure gradients vanish. 



In order that such a distribution of density can long persist, these 

 pressure gradients must be opposed by an opposite and equal force. The 

 essence of the geostrophic method is that we suppose that these pressure 

 gradients are opposed by Coriolis forces acting to the right of the direction 

 of motion of the water. This impUes velocities in a direction perpendicular 

 to the plane of the figure, and directed into the page where the arrows 

 point left, and outward where they point right. Since these forces are 

 perpendicular to the direction of motion they neither drive nor brake the 

 motion. 



The Gulf Stream is not a river of hot water flowing through the ocean, 

 but a narrow ribbon of high-velocity water acting as a boundary that pre- 

 vents the warm water on the Sargasso Sea (right-hand) side from over- 

 flowing the colder, denser waters on the inshore (left-hand) side. 



' See footnote 2, on p. 163. 



