646 The Tropospheric Circulation 



under stationary conditions the lower and cold as well as nearly always weakly saline 

 waters are lifted on the right-hand side of the current core in the Northern Hemisphere 

 and on the left-hand side in the Southern Hemisphere. If there is a parallel coast along 

 this special side of the current the water off the coast already for this reason alone will 

 be colder and will have a lower salinity than further out. This state does not represent 

 an upwelling phenomenon, but rather a state of long duration dependent on the nature 

 of the vertical water stratification and on the current strength. Most of the anomalies 

 appearing off the coasts are due to such a simple effect on the mass field produced by 

 the currents. Upwelling of cold deep water occurs only if in a wind-driven current 

 with a flow component parallel to the coast a water transport away from the coast 

 sets in. The continuity condition then requires a rising water movement at the 

 coast. 



In a first attempt in order to explain this phenomenon Thorade, 1909 used this 

 theory, and later on particular interest has been devoted to the determination of the 

 vertical velocity profiles in the rising water (McEwen, 1912) and to the determination 

 of the depths in which the upwelling phenomenon starts out (Sverdrup, 1930). It 

 was soon found out from the thermo-haline structure in the upwelling region, that 

 these depths could not be large and that due to the inclination of the isothermal layers 

 off the coast an upward water movement of only a few hundred metres would be 

 sufficient to explain the observed sea surface anomaly. The formation of a one-sided 

 divergence line running more or less parallel to the coast is the characteristic feature of 

 the current field. The occurrence of rising movements at divergence lines in the case 

 of non-stationary discontinuity surfaces and vortices is, of course, understood 

 theoretically (p. 469) and water movements of this type are shown definitely by 

 numerous observations of the vertical and horizontal distribution of the oceano- 

 graphic factors (for example, equatorial cold tonges in the Atlantic and Pacific 

 (pp. 558 and 569); boundary regions at the oceanic polar fronts, p. 471). 



In the upwelling regions off the west coasts of continents all upwelling phenomena 

 are of a similar type as discussed above. From the analysis of the mean oceanic state 

 off the coast of South West Africa Defant (1936^) has derived the schematic diagram 

 shown in Fig. 302 of the structure and the water movements in a cross-section at 

 right angles to the coast. Essentially the cross-sectional movement consists of an 

 elongated vortical motion around a horizontal axis which is superimposed on a 

 much stronger and uniform current parallel to the coast. The water beneath the 

 axis of the transverse vortical motion flows in the lower part of the top layer, in the 

 density transition layer and beneath it towards the coast and gradually rises just off 

 the coast. The upwelling phenomenon is very largely confined to the narrow strip 

 between the divergence line and the coast. It rises up to the sea surface from a depth of 

 only 100-200 m and as a consequence of the current field the temperature distribution, 

 observed in vertical direction remote from the coast, is twisted around and changes 

 its position into a horizontal one; so to say is projected on the horizontal sea 

 surface. 



A necessary consequence of this circulation is the destruction of the density transi- 

 tion layer in the upwelling region off the coast. This is clearly shown by the "Meteor" 

 cross-section (1937) over the shelf off the north-west African shelf. The gradual break 

 down of the transition layer, which at times is also strongly developed in the area 



