566 Basic Principles of the General Oceanic Circulation 



between highly saline and warm water from lower latitudes with weakly saline and 

 colder weater from higher latitudes lead to vortical movements of large extent. Similar 

 conditions are found in the subtropical convergence region of the South Atlantic. 

 There are rather different opinions about the question how far the West Wind Drift 

 reaches equatoward depending on whether the subtropical convergence is fixed 

 according to ship displacements or if it is derived by means of the distribution of 

 oceanographic factors. The position given by Deacon (1937), deduced mainly from 

 the temperature distribution, is always about 6° to 10° further south than that obtained 

 from current measurements. According to Bohnecke (1938, p. 201) the "subtropical 

 convergence" (of the currents) should be carefully distinguished from the "subtropical 

 boundary" (deduced from temperature and salinity). The former in a rather charac- 

 teristic way coincides with the tropic boundary and the latter with the polar boundary 

 of that large disturbance region which extends between the southern limit of the 

 Equatorial Current and the West Wind Drift (p. 564) as is found during the dynamic 

 preparation of serial observations. Also here it seems more appropriate to speak of a 

 convergence "region" between the two bordering water types being the place for 

 subtropical vortex formations. 



The Southern Hemisphere Polar Front (Antarctic convergence line) has been dis- 

 cussed on p. 549. The Northern Hemisphere Polar Front is sharply developed between 

 the Labrador Current and the Gulf Stream near the Newfoundland Banks but 

 gradually fades towards the north-east, reappearing again as a frontal zone between 

 the East Greenland Current and the Irminger Current. Larger and smaller vortex 

 formations with corresponding vertical movements are also found along this con- 

 vergence line. 



(c) Sea Surface Currents in the Indian Ocean 



Ships displacements available for other oceans are much less numerous than in 

 most parts of the Atlantic and current charts are therefore correspondingly more 

 uncertain. Reference to analogous conditions as in the Atlantic will usually permit 

 briefer description here, but the Indian Ocean has a single particular peculiarity in its 

 northern part where the wind system changes character completely every six months, 

 correspondingly causing similar changes of the ocean currents. This is the best 

 possible proof that the winds are decisive for the generation and maintenance of ocean 

 currents. A full cartographic description of the currents here requires monthly charts 

 (British Admiralty 1895; Deutsche Seewarte 1908; Dallas and Walker, 1908; 

 MoLLER, 1929) but charts for the summer monsoon and for winter are usually con- 

 sidered sufficient. 



The currents during the time of the north-east monsoon (north-east trades) corres- 

 pond best to the general system of ocean currents. They resemble those of the Atlantic 

 and the Pacific except that the Equatorial Counter Current lies between about T S. and 

 8° S., that the Northern Equatorial Current moves partially into the Southern 

 Hemisphere; during this part of the year the thermal equator is always south 

 of the equator. In the north the North Equatorial Current (monsoon drift) runs 

 almost due west. It is strongest to the south and south-west of Ceylon where the cross- 

 section through the current is narrow. In the Bay of Bengal there is an anticyclonic 

 vortex. The strong north-west to north-east winds over the Arabian Sea produce a 



