THE PACIFIC OCEAN 



111 



profiles of the isobaric surfaces down to the surface of 

 300 decibars. Furthermore, we have represented the 

 distribution of density, salinity, and temperature by 

 means of vertical sections which are extended to a depth 

 of 300 meters, and in the case of the central section we 

 have also represented the amount of oxygen, but from 

 the western section no observations of oxygen are avail- 

 able. 



The profiles of the isobaric surfaces of the central 

 section are represented in figure 25. As in the other 

 vertical sections, north is to the right and south is to the 

 left. These profiles are of the type shown schematically 

 in figure 24c. When drawing them, one has a certain 

 freedom because of the considerable distances between 

 the stations, but a minimum must be placed somewhere 

 near the equator, and then it is permissible to place it at 

 the equator where it theoretically should be. 



It is seen on the figures that currents toward the 

 west are dominating. At the surface they are found to 

 the north of latitude 10° north and to the south of latitude 

 03° 40' north. Currents in the opposite direction, to- 

 ward the east, are at the surface between the two lati- 

 tudes 10° and 7° 30' north. Within these latitudes the 

 Equatorial Countercurrent is fully developed. The most 

 interesting feature shown by the profiles is that the ve- 

 locity of the countercurrent decreases very rapidly with 

 increasing depth. At a level of 100 meters it is already 

 much weaker than at the surface, and at a level of 200 

 meters it has practically disappeared. The westerly 

 current also decreases with increasing depth, especially 

 near the equator, but in latitudes 20° north and 10° south 

 a considerable current toward the west still exists at 

 300 meters. These observations show that the Equatori- 

 al Countercurrent does not widen with depth but^ on the 

 contrary, becomes narrower and narrower, and disap- 

 pears above a level of 200 meters. 



Turning next to the western section, figure 30, we 

 find essentially the same features but here the profiles 

 of the isobaric surfaces are of the type shown in figure 

 24d. In this case, a maximum must be placed some- 

 where near the equator, and it is permissible to place it 

 at the equator, in agreement with the theoretical condi- 

 tions. 



The currents toward the west are also dominating 

 here, extending to the north of latitude 6° 20' north and 

 to the south of latitude 3° 20' south. Between latitudes 

 6° 20' north and 3° 20' south the current runs in the 

 opposite direction, toward the east. In this case we find 

 a maximum elevation of the isobaric surfaces at the 

 equator. The Equatorial Countercurrent is thus extend- 

 ed over a broad area on both sides of the equator. It 

 has its maximum velocity somewhat below the surface 

 at a level of about 100 meters but from this level the 

 velocity decreases rapidly with increasing depth until 

 the current practically disappears at 300 meters. The 

 currents toward the west also decrease with increasing 

 depth, especially at the shortest distance from the equa- 

 tor as was the case in the preceding section. The two 

 sections give us essentially the same results. The dif- 

 ferent position and development of the countercurrent 

 may perhaps be explained by the fact that the western 

 section was taken in April, at the beginning of the north- 

 ern summer, whereas the central section was taken in 

 November, at the beginning of the southern summer, 

 but it also may be related to the different geographic lo- 

 cations. 



The different development of the countercurrent at 



the two crossings of the equator makes it impossible to 

 combine the observations to form a consistent picture of 

 the topography of the isobaric surfaces in the vicinity of 

 the equator. There the lines of equal elevation in the 

 charts, therefore, have no physical significance. 



The density sections, figures 26 and 31, give the 

 same picture in both cases. We find accumulations of 

 heavy water under the northern and southern borders of 

 the countercurrent, whereas lighter water extends to 

 greater depths within the countercurrent itself. In the 

 central section the upheaval of the cold water is espe- 

 cially characteristic at the northern border of the 

 countercurrent. 



Turning to the salinity, temperature, and oxygen 

 sections, figures 27, 28, and 29 from the central region, 

 we obtain some information as to the character of the 

 vertical motion. From the salinity sections it is evident 

 that we find pscending motion along the borders of the 

 westerly currents. The ascending motion is especially 

 strong on the northern side of the countercurrent, where 

 water of low salinity is brought practically to the sur- 

 face. The course of the isohalines indicates that the 

 surface water is driven away from the countercurrent 

 both on the northern and the southern sides, and the sa- 

 linity section, therefore, supports the opinion that di- 

 verging surface currents are present and are of impor- 

 tance to the development of the system. The tempera- 

 ture section discloses the same features as the salinity 

 sections. It shows especially the upward movement on 

 both sides of the countercurrent and, in addition, a 

 downward movement at the southern boundary. 



The oxygen section, figure 29, shows some very in- 

 teresting features. The axis of the lowest salinity 

 values in the salinity section follows exactly the line of 

 4 ml/L. The ascending water on the northern side has 

 thus, on the whole, an oxygen content above 4 ml/L. On 

 the southern side we find that the ascending water has a 

 somewhat lower oxygen content, namely, 3 ml/L. The 

 descending movement at the southern boundary of the 

 countercurrent can hardly reach to any considerable 

 depth because even in the central part we find a rapid 

 decrease of the oxygen content below a level of 150 me- 

 ters. 



A very rapid change of density with depth is found at 

 a short distance below the surface at the stations where 

 the heavy deep water reaches almost to the surface, and 

 where the stable stratification prevents mixing between 

 surface water and the deep water. The deep water, 

 which rises as a wedge at the northern border of the 

 countercurrent, is without any communication with the 

 surface water and consequently we find that this deep 

 water is practically without oxygen. Values as low as 

 0.03 ml/L were observed in this region and values below 

 0.25 ml/L occur within an extensive mass of water. The 

 contrasts are smaller on the southern side of the counter- 

 current where the density changes more gradually with 

 depth, and where a slow mixing between the surface 

 waters and the deep waters may take place. 



The western temperature and salinity sections show 

 several features which are similar to those of the cen- 

 tral regions, but the contrasts are less conspicuous and 

 the indications of vertical movement are less definite. 

 From the salinity section it is evident that ascending 

 motion takes place along the borders of the westerly 

 currents, especially on the northern side of the counter- 

 current. 



The conditions which are revealed by the observations 



