642 The Tropospheric Circulation 



Since the sub-Antarctic water forms an almost zonal boundary to the lighter water 

 mass in the south, the trough of lighter water is narrowed towards west by the African 

 continent, until it finally takes almost a wedge-form at the southern peak of the 

 Agulhas Bank. In the further course this wedge then splits into three separate branches 

 with simultaneously occurring vortex formations; the southernmost of these intrude 

 into the heavier sub-Antarctic water and the northernmost intrude into the sub- 

 tropical water of the South Atlantic. The lighter water thereby decreases considerably 

 in thickness. 



A dynamic interpretation of the above-mentioned section running south-west of 

 Capetown has been attempted in Fig. 206 ; similar scientific evaluation of the other 

 sections gave results in agreement with this. The nature of the current is shown more 

 clearly by the dynamic topography of the isobaric surfaces. Figure 300 shows the 

 dynamic depth anomaly for the 200 decibar-surface relative to that of the 1000 decibar- 

 surface; the first one can be taken as an approximation to the absolute topography 

 of the 200 decibar-surface. According to this the Agulhas Current at the 200 m depth 

 flows with intense velocities along the continental coast as far as the southern tip of 

 Africa. However, it thereby diminishes rapidly its mass and velocity and finally loses its 

 current character forming three large quasi-stationary vortices, the cores of which are 

 identical with the three branches of lighter water mentioned before. According to 

 Dietrich about three-quarters of the water masses of the Agulhas Current, transported 

 at the southern tip of Africa into the South Atlantic, is drawn into these vortices and 

 after mixing with the current of the higher latitudes returns to the Indian Ocean. 



Analysis of the pressure distribution in the current interior shows it to be the resul- 

 tant of two components. The first is an effect of the internal pressure determined by 

 the mass distribution, and corresponds to the normal pressure distribution in a system 

 in which a lighter motionless water mass is embedded between two denser moving 

 water bodies. The second component corresponds to a ridge of high pressure occurring 

 in the boundary region between the two currents flowing in opposite direction and is 

 due to the piling up of water. Since the Agulhas Current in the northern part of the 

 current system as well as the broad oceanic West Wind Drift in its south both give 

 a total water transport towards left. In the boundary region between them water 

 accumulates giving rise to the second pressure component. In combination with the 

 first a total pressure distribution is generated which is characteristic for that found in 

 the Agulhas Current. Especially typical is the circumstance that the two adjacent 

 currents of opposite direction face each other with their faster moving parts. The large 

 lateral shearing forces thus formed give rise to large vortical movements (p. 570) in 

 which most of the flow energy is dissipated. 



Dietrich, 1936 has given a comparative discussion about the structure and move- 

 ment of the Gulf Stream and of the Agulhas Current and reference is made to this 

 investigation here. 



4. Upwelling Phenomena 



A characteristic phenomenon occurring in the narrow oceanic strips off" the western 

 coast of the continents in middle latitudes is the observed cold coastal water, wliich due 

 to its influence on the atmosphere is of considerable climatological importance. Until 

 recently the investigation of these phenomena had to be based on surface data only. 



