612 



The Tropospheric Circulation 



Coostal stream Gulf stream 



Fig. 284, Velocity profile (cm/sec) across the Gulf Stream off Chesapeake Bay, 20-22 April 



1932. 



south-west of the Newfoundland Banks. Eastwards from here there is a counter 

 current steadily broadening to the south. The absolute topography of the 500 decibar 

 surface still shows clearly the same pressure gradient as at the sea surface but it is 

 rather weakened. The 800 decibar surface shows a rise across the current of at the 

 most 20 dyn cm; and the pressure gradient has fallen to about a quarter. The 1400 

 decibar surface is almost plane and the lower limit of the current system must there- 

 fore lie between 1000 and 1200 m. 



A detailed analysis of the origin and the transformations of the Gulf Stream water 

 as it flows from the Florida Strait to the Newfoundland Banks were investigated by 

 Dietrich (1937) with the aid of distribution of oxygen content in numerous profiles. 

 He was able to show that the water masses of the Florida Strait and of the Antilles 

 Current to the north of the Lesser Antilles were made up partly of tropical South 

 Atlantic water and partly of subtropical water from the western North Atlantic. The 

 Gulf Stream water reaching Cape Hatteras has, however, undergone changes making it 

 almost completely identical in its properties with the water of the western North 

 Atlantic. This transformation was attributed by Dietrich to the transverse circulation 

 and to mixing. From the distribution of the oceanographic factors such a transverse 

 circulation seems not unlikely, but it is not possible to determine it from the pressure 

 field because of the low velocity and probably also because of its variability. 



The amounts of water and heat carried by the Gulf Stream are enormous. The 

 Florida section shown in Fig. 284 gives a water transport of about 25 million m^sec. 

 It can be assumed that this will also be the transport in the currents through the Carib- 

 bean and the Yucatan Channel, since the precipitation and the inflow of river water 

 (run-off) are small compared with this very large quantity. Some idea of the enormous 

 quantity of water involved is given by the estimate that it is twenty-two times as much 

 as is carried by all the rivers of the earth together. The amount of water carried by the 

 Gulf Stream further north is much larger than this and the transverse profile off 

 Chesapeake Bay gives a transport three times greater (82 million m^sec). It can be 

 assumed as a first approximation that the amount of water carried by the North 



