28 



Large-Scale Feattjkes 



northern half of Block 3, probably mostly in the -wTntertime. The actual 

 transports of water at various locations in the Atlantic, as inferred from 

 geostrophic calculations based on actual data, are discussed in more detail 

 in Chapter XI. The thickness of the intermediate layer is remarkably con- 

 stant throughout the whole North Atlantic Ocean ; this is in marked con- 

 trast to the thickness of the surface layer (water at a temperature higher 

 than 17° C), which varies widely from place to place. It seems reasonable 

 to suppose that the water sinking into this intermediate layer in northern 

 latitudes (where it reaches the surface in the winter) is eventually mixed 

 upward into the surface layer in subtropical latitudes. This mixing occurs 



45 N 



2 /v 



Fig. 7. A dissected-block diagram of the thermal structure and circulation 

 of the North Atlantic Ocean, as viewed from a great height over the Gulf of 

 Mexico. The unshaded part is the warmest water; the lightly shaded part in- 

 dicates the water of the thermocline; and the heavily shaded part represents 

 cold deep water. The smooth curved arrows indicate the direction of flow of the 

 horizontal currents of the surface and thermocline; the zigzag arrows indicate 

 hypothetical slow vertical flows. A description of the individual blocks is 

 given in the text. 



mostly in the faU and Avinter of the year, when cooling and wind stirring of 

 the surface layer are at a maximum. To complete the cycle one must sup- 

 pose that part of the surface water eventually finds its way back into the 

 northern half of Block 3 through eddies and multiple streams and that 

 it is there reconverted into intermediate water. A large fraction of the 

 surface water, however, circulates ^vithout transformation in the hori- 

 zontal wind-driven surface gyre. 



Water-mass analysis. — Sverdrup, Johnson, and Fleming (1942) have dis- 

 cussed, in very concise form, the nature and probable origin of various 



