The Tropospheric Circulation 



647 



Divergence 



Horizonol temp, distribution, °C 

 ■^15° 14° 13° 12° 11° 

 -3° -4°-5°-6°-7^jemperature anomaly, 







-100 



--200 Q 



-300 



500 



400 300 200 



Distance from coast in Sm 



100 



Fig. 302. Schematic cross-section normal to the coast of south-west Africa. Full lines, 

 isopycnals ; arrows, zonal and vertical velocity components (the length of the arrows can be 

 taken approximately as a measure of the speed) ; letters, meridional velocity components and 

 in special ; A^, parallel to the coast towards north ; S, parallel to the coast towards south (the 

 size of the letters can be taken approximately as a measure of the speed) ; wavy lines, axis of 

 the vertical current vortex (vertical exaggeration 1 :2300). 



nearest the coast, is a consequence of internal tidal waves which gradually become 

 unstable as is definitely shown by the series of observations. This is thus a precondition 

 for the upwelling of deep water (see vol. ii, p. 581). 



SvERDRUP (1938a) in the evaluation of the almost synoptic surveys made by the 

 Scripps Institution of Oceanography, La Jolla, from March to June 1937 along a 

 transverse section off and at right angles to the Califomian coast from Port San 

 Luis (35-2° N., 120-7° W.) has obtained good insight into the dynamics of the up- 

 welling processes. Figure 303 presents two topographies of the physical sea level as well 

 as the 100 and 200 decibar-surfaces relative to that of the 500 decibar-surface. In the 

 time between the two surveys typical mass displacements have occurred. The changes 

 in the profile occurring down to the 200 decibar-surface can only be interpreted by a 

 water transport away from the coast and by the piling up of the lighter surface water 

 near Sts. 4 and 5. These movements can be looked upon as a consequence of the winds 

 which blow with little variation for long periods, on the average from N. 23° W. at 

 about 6-7 m/sec, almost parallel to the coast. According to the Ekman-theory under 

 these conditions a transport directed away from the coast can be expected. This trans- 

 port can be derived from the change in the course of the density lines between the two 

 surveys. These surface waters are carried outwards and piled up about 100 km off 

 the coast. 



From the analysis of all the fields Sverdrup has derived the mean current field 

 shown in Fig. 304 during the period between the surveys. The calculated maximum 

 transverse velocity seawards thereby amounts to 1 1 cm/sec, in good agreement with 

 the velocity of the wind drift. Between the coast and the water piled up further out 



