282 



[chap. 4 



interesting to apply in the Munk-Stommel model of wind-driven ocean currents, 

 particularly since Munk's (1950) classical computations used stress consistent 

 with the earlier pressure configuration. 



From these figures Bjerknes deduced that the clockwise mean wind circula- 

 tion around the Atlantic anticyclone and its resultant stress on the sea had 

 appreciably strengthened between the two test periods. On this basis he ex- 

 plained the oceanic temperature changes. According to his argument, the 

 strong secular heating recorded at the Atlantic "polar front" or Gulf Stream 

 may be due to : (a) the increased warm water advection parallel to the front in 



AVERAGE ANNUAL PRESSURE 1926-331 



MINUS •• ■• ■• 1890-971 



ANNUAL SEA SURFACE ISOTHERMS IN °C 



Fig. 89. Change in mb of average annual sea-level pressure from the period 1890-97 to 

 that of 1926-33 (full lines) and average annual sea-surface isotherms (dashed lines). 

 (After Bjerknes, 1959, Fig 2. By courtesy of the author and the publishers.) 



the jet maximum ; (6) northward displacement of the front ; and (c) on the cold 

 side of the front a possible increase of average temperature due to amplified 

 meandering. 



The first test to seek in oceanographic data would be comparisons of mass 

 transport in the Gulf Stream for the two periods. Fortunately, hydrographic 

 sections exist from the Challenger Expedition of 1872-76 to compare with 

 nearly identical sections from the work of Iselin in the 1930's (Iselin, 1936). 

 Puzzlingly enough, both the positions and slopes of the important isotherms in 

 the stream's core are, as far as can be determined, unchanged over that 



