112 



Nonlinear Theories — Inertial 



of the top isothermal layers ( > 10° C.) is remarkably constant. Suppose, for 

 example, that the quantity Dq is the thickness of the layer between the sea 

 surface and the 10° C. isothermal surface. The depth of the 10° C. isotherm 

 is shown in fig. 66. Since the relative vorticity in central oceanic regions is 

 small, the reader may compute //Dq for the region between 10 and 35° N. 



Fig. 66. Depth of the 10° C. isothermal surface in the western North 

 Atlantic Ocean, according to Iselin (1936, fig. 47). Depths are given in meters. 



and convince himself that over this large part of the ocean the potential 

 vorticity so defined is actually uniform and nearly constant. From a 

 theoretical point of view it might be preferable to use density structure 

 (see figs. 10-15). 



Dr George Morgan has pointed out to me that the easiest way to see the 

 difference between the linear theories (the Munk theory, for example) and 

 possible nonUnear theories is to interpret the potential- vorticity equation 

 in the following way. The linear theory is steady, and essentially approxi- 



