Basic Principles of the General Oceanic Circulation 557 



circulation in a meridional plane with only supplementary extensions to three- 

 dimensional space. This has no doubt been unavoidable in the past due to the lack of 

 sufficient observations, but a complete understanding of the oceanic circulation can 

 be obtained only in terms of spatial phenomena. The magnitude and the complexity of 

 the problems makes it understandable that a solution in full detail has not yet been 

 obtained and probably will not in the near future, but the accumulation of further 

 data and the advance of theoretical knowledge will lead closer to a comprehensive 

 elucidation of the mechanism of the general oceanic circulation which is the aim of 

 oceanography. 



The permanent oceanic currents can be divided into three groups according to their 

 genetic origin: 



(1) currents produced by thermo-haline convection, mainly due to cooling of 

 surface water in higher latitudes; 



(2) currents produced and maintained by the transfer of wind energy to the sea 

 surface ; 



(3) currents maintained by the excess of precipitation over evaporation, or vice 

 versa occurring in special oceanic regions. 



Each of these types of flow shows a different physical behaviour and acquires on 

 the rotating earth an individual form, which is also strongly influenced by continental 

 slopes acting as barriers for the oceanic movements. 



2. Oceanic Sea Surface Currents 



(a) Charts of Sea Surface Currents 



It has taken quite a long time until data on sea surface currents were that numerous 

 as to allow a reliable representation of the currents over the entire ocean surface. 

 Charts of currents presented in ordinary atlases are seldomly based on critically 

 tested observations and are often constructed making hypothetical assumptions. As 

 amount and density of the observational material (current measurements) increased, 

 charts of current conditions over smaller oceanic areas could gradually be extended 

 until finally world maps of ocean currents could be constructed. At the suggestion 

 of Neumayers (1898), Schott prepared a world chart of ocean currents. A new edition 

 of this was published in 1942 incorporating in an excellent manner the oceanographic 

 progress of the last 40 years. This chart (Schott, 1942), Deutsche Admiralitatskarte 

 no. 1947, 2 sheets, 1942) shows the total earth for the Northern Hemisphere winter 

 and an inset map for 30° N. to 20° S. shows seasonal variations for the tropics during 

 the Northern Hemisphere summer. North of 50° N. the chart represents more summer 

 conditions for which the data are more numerous. This current chart is reproduced in 

 Plate 8 on an equal area projection. The use of current arrows has been simplified in 

 places: velocities are indicated at \ knot intervals with a lower limit of 12 nautical 

 miles in 24 h and an upper limit of 36 nautical miles in 24 h. Differences in velocity are 

 indicated by the thickness of the arrows and the constancy of the current by the length ; 

 the last factor was expressed in four degrees : variable, fairly steady, steady and very 

 steady corresponding roughly to 25, 25-50, 50-75 and 75% flow displacement in the 

 direction of the arrow. Naturally in such large-scale charts only a somewhat general 

 representation of the currents can be given and some subjective interpretation is 

 always possible. Details in the infrequently navigated parts of the ocean are, of course, 



