TEMPERATURE OF THE SURFACE WATERS OF THE ATLANTIC OCEAN 



11 



clouds (Bewolkung) in both these latitudes might in- 

 crease the effect of radiation. It is questionable 

 whether these factors in themselves suffice to form such 

 a relatively narrowly limited belt of rapid temperature 

 increase in a northerly direction. It appears rather 

 that the activity processes at the surface of the sea have 

 an equally large share in the formation of this zone 

 of large temperature gradients B'. In figures 9, 10, 

 and 11, which show the currents south of Africa accord- 

 ing to A. Merz " and M. Willimzik in the months of 



Figure 10. — Surface currents south of Africa in May, drawn on 

 the basis of the Dutch current displacements by A. Merz. 

 Designation of the fronts is the same as in figure 4. 



January, May, and October, the doubtful boundaries 

 are entered, and the chief directions of activity em- 

 phasized by the heavy arrows. It is clear that the 

 subtropical boundary coincides with a marked change 

 of direction in the current. South of the line B', 

 north-south components predominate, and north of it, 

 east-west components. In the latter case, local con- 

 vergences also occur in part. These phenomena obvi- 

 ously cause a certain accumulation of the water masses 

 transported here from the south and therewith form 

 the strip of especially high temperature gradients, 

 which previously has been called the subtropical 

 boundary. 



A current system can be drawn approximately cor- 

 responding to a convergence-divergence line given by 



Sandstrom and Bjerknes,^^ without doing violence to 

 the observed current displacement. Figure 12 pic- 

 tures a mirror-image modification of Bjerknes' con- 

 vergence-divergence line and probably explains suffi- 

 ciently in what way the origin of the temperature 

 gradients at this line B' can be considered. 



The name "subtropical boundary," which marks 

 the limit of the oceanic subtropical area toward the 

 south and toward the pole-side, has been chosen to 

 distinguish it from the subtropical convergence, which 

 runs within the subtropics. The chart on the color 

 of water by Schott (see Plate XXV) '' shows divergence 

 phenomena which are connected with the upwelling 

 of water from deeper strata. Along 40°, areas of green 

 water are indicated, which, according to E. Hentschel, 

 are connected with an abundance of plankton. Wattcn- 

 berg discovered that the food material necessary for 



" A. Merz, Die Deutsche Atlantische Expedition auf dem Vermess. — u. Forschunss- 

 tchiff M«(eor.— Sitzgs.-Ber. Preuss. Akad. d. Wiss. XXXI. Beriin, 1925. 



Figure 11. — Surface currents south of .4frica in October, drawn 

 on the basis of the Dutch current displacements by A. Merz. 

 Designation of the fronts is the same as in figure 4. 



the growth of the plankton comes from the deeper 

 strata at these points. 



The real subtropical convergence, which is clearly 

 revealed in the current displacements, is indicated 

 very slightly in the temperature distribution, since 

 masses of water of approximately equal warmth are 

 conducted to it from the north and south. The tem- 

 perature differences north and south of the line C are 



" V. Bjerknes, Dynamische Meteorolooie und Hydrographie. Braunschweig, 1910 u. 

 1913. 

 " Tafel Vn from G. Schott, Oeographie det Atlanl. Ozeans. Hamburg, 1920. 



485460— 43— Vol. VI, No. 1- 



