644 The Tropospheric Circulation 



which was not enough to afford any insight into the inner mechanism of this phenome- 

 non. Some data for the area off Chile and Peru have been obtained by the last "Car- 

 negie" cruise (Sverdrup, 1930) and the "Meteor" expedition during the spring of 

 1937 made six profiles at right angles to the coast with the objective to study the upwell- 

 ing water phenomenon off the north-west coast of Africa (Defant, 1936a). Detailed 

 systematic investigations of the strong upwelling phenomena off the Californian coast 

 have been made since 1937 by the Scripps Institution of Oceanography (Sverdrup, 

 1938a, Sverdrup and Fleming, 1941). These cover the development of upwelling 

 phenomena in successive surveys and have provided some understanding of the 

 dynamics of the upwelling process. Some comments might be made here on individual 

 regions with upwelling. A summary for the oceanic regions off south-west Africa has 

 been given by Defant (1936a), see also, Bobzin, 1922). The surface temperature 

 conditions are given in the charts of the "Meteor" Report, vol. v. Atlas. In all months 

 the low temperatures occupy the total width of the shelf (about 100 nautical miles), 

 at the continental slope occurs the rapid rise to the higher temperatures in the west. 

 During every month the temperature anomaly is highest at the coast with maximum 

 values of — 8°C to — 10°C. The area of maximum anomaly moved in a meridional 

 direction during the course of the year: in the summer (January) it occupies its 

 southernmost position and is strongest between Table Bay and Luderitz Bay (32° S to 

 23° S.); in winter it moves furthest to the north (between the Luderitz Bay and Walvis 

 Bay, 27° to 14° S.). During the entire year the current system of the sea surface shows 

 a particularly characteristic one-sided divergence line which extends along the coast 

 from about 30° to 20° S. or even more. In the south its distance from the coast amounts 

 to about 160 nautical miles; in the north, however, 300 to 360 nautical miles. The region 

 to the east of this divergence line is the region of cold upwelling. Where the unilateral 

 divergence is most strongly developed, also the temperature anomaly is greatest. 

 The anomaly at the coast vanishes north of 20° S., where the divergence with a de- 

 creasing intensity turns westwards and gradually fades away. The uniform rise of the 

 isopycnals from west to east (towards the coast) is a particularly marked feature of the 

 thermo-haline structure of the upwelling region. Off the coast especially in the north 

 there is a well-developed transition layer, and all the isolines immediately beneath 

 this transition layer off the coast show a surprisingly sharp downward deflection to a 

 depth of 350 m. This is only explicable as an effect of piling up of water at the conti- 

 nental slope whereby in the depths lower than 30 or 40 m the water masses are pressed 

 downwards. 



Similar conditions apply also to regions with cold water upwelling off the north- 

 west coast of Africa. From January to May especially this region can be visualized 

 by a tongue of cold water extending from higher latitudes southwards along the coast. 

 Figure 301 shows this temperature anomaly for April ; it occupies the entire area between 

 the Canaries and Cape Verde in which the anomaly already on the average is increased 

 to almost — 7°C just off the coast and for individual cases reaches values of — 10°C 

 or more (see Schumacher, 1933). Here also a sharp density transition layer can be 

 found extending along the edge of the shelf until just off the coast. 



Particularly well-developed upwelling phenomena occur in the region off the 

 western coast of North America between about 46° N and 25° N., especially off Cali- 

 fornia with extreme conditions at Cape Mendocina (north of San Francisco). An 



