The Tropospheric Circulation 607 



currents during the spring. The large salinity diflferences which appear where the under- 

 currents of the Equatorial Current join off the Antilles soon disappear in the eastern 

 Carribean. There is a striking uniformity in the Caribbean and in the Yucatan Channel 

 due to lateral mixing. The weak inflow through the Windward Passage (sill depth 

 about 1600 m) makes little change. The differences in the Gulf of Mexico are larger. 

 The extended areas with vortices in the north-eastern and the western parts of the Gulf 

 which are very pronounced in the surface currents remain outside the circulation of 

 the tropospheric layers. Investigation of [r,5']-curves in the water masses of the South 

 Equatorial Current, the Sargasso Sea and the Yucatan Channel allows to estimate 

 how much of the inflow water through the Antilles takes part in the water passing 

 through the Yucatan Channel. Iselin (1936) found that of a total transport of about 

 26 million m^/sec through the Yucatan channel approximately 6 million originates in 

 the South Atlantic. For the deeper layers the eff'ects of the inflow through the Wind- 

 ward Passage and the Virgin Passage are of greater importance (see Pt. I, p. 133). 

 The uniformity of the distribution of the oceanographic factors over the area shows 

 the effect of the mixing processes which are stronger here than the pure transport 

 processes. Dynamic evaluation of the data for the latter should give greater informa- 

 tion (Parr 1937Z?). Figure 281 gives the dynamic topography of the physical sea level 

 relative to that of the 1200-decibar surface for the Caribbean and for the Cayman 

 Sea. The mean current core running from the Antilles through the Yucatan Strait to 

 the Florida Strait is clearly marked. The course of the dynamic isobaths shows that 

 the water flows uphill to reach the Yucatan Channel (see also Sverdrup, 1939). 

 Similarly as in both the Equatorial Currents, the water transport here is also largely 

 due to the air currents (prevailing wind to the east-north-east with a mean velocity 

 of 10 m/sec). Thus to a very large extent these currents are also gradient currents in a 

 baroclinic sea though they are subject to significant modification by the wind. 



{b) The Gulf Stream and its Internal Structure 



Although the Gulf Stream is the largest and the most important current of the 

 Northern Hemisphere a more dynamic investigation of its course has only recently 

 been started. The first current measurements in it were made by Pillsbury in 1885-9 

 from the "Blake" which was anchored in very deep water. Further investigations 

 were begun in 1914 by the oceanographic survey vessel "Bache" (Bigelow, 1917, 

 four transverse profiles through the Florida Current and the Antilles Current). More 

 recently a systematic survey has been started by the oceanographic survey vessel 

 "Atlantis" (Woods Hole Oceanographic Institution). 



The first dynamical evaluation of some transverse profiles in the Florida Current 

 was given by WtJST (1924) using the "Blake" measurements. This and subsequent work 

 have aff'orded a more or less complete description of the vertical structure of this 

 current from the Florida Strait to the Newfoundland Banks. Special mention should 

 be made of the work of Jacobsen (1929) on the Sargasso Sea using "Dana" observa- 

 tions and that of Iselin (1936) giving a detailed review of the comprehensive results 

 collected by "Atlantis". Dietrich {\92>lb, see also WiJST, 1930a) has given a detailed 

 analysis of numerous sections to show the process of formation and the dynamics of 

 the Gulf Stream. The thermo-haline structure of the Gulf Stream is immediately 

 apparent from the set of six success profiles given by WiJST (Figs. 282, 283). Profile I 



