110 



OBSERVATIONS AND RESULTS IN PHYSICAL OCEANOGRAPHY 



which is known to be a narrow current on the surface, 

 widens with depth. The latter conception, as to the in- 

 creasing width of the Equatorial Countercurrent, cannot 

 be upheld according to the Carnegie results; on the con- 

 trary, the current is typical of the upper layers only, 

 and since it has a very limited extension it must be 

 doubted that this flow of water represents a compensa- 

 tion action. It will be shown on the basis of the Carne - 

 gie data that the countercurrent probably is owing to the 

 asymmetric development of the westerly tropical cur- 

 rents of the two hemispheres and to the effect of diverg- 

 ing surface currents in the vicinity of the equator. 



Before turning to the observations of the Carnegie 

 a few general considerations are necessary. Attention 

 should be drawn to the fact that the inclination of the 

 isobaric surfaces must change when passing the equator 

 because of the change of the direction of the deflecting 

 force of the earth's rotation. In other words, the iso- 

 baric surfaces must have a maximum or a minimum at 

 the equator. If the isobaric surfaces had a definite in- 

 clination at the equator, the direction of the current 

 would change by 180° when passing the equator and such 

 a condition cannot be stable. 



If the isobaric surfaces show a maximum at the 

 equator, the surfaces are inclined to the north in the 

 Northern Hemisphere and to the south in the Southern 

 Hemisphere, and the current is directed toward the east 

 on both sides. If, on the other hand, the isobaric sur- 

 faces have a minimum at the equator the current is di- 

 rected toward the west. 



We have seen previously that the westerly tropical 

 currents in both hemispheres must be regarded as 

 forced currents, which are maintained partly by the pre- 

 vailing winds and partly by the density currents in the 

 northern and southern parts of the ocean. Within these 

 westerly tropical currents in the Northern Hemisphere 

 we must have the heavy water to the left, which means 

 near the equator, and in the Southern Hemisphere the 

 heavy water must lie to the right, which also means near 

 the equator. Therefore, since these forced currents to- 

 ward the west exist, we must find an accumulation of 

 heavy water in the vicinity of the equator. Assuming, for 

 for the sake of simplicity, that we have two layers only, 

 one light on top and one heavy below, the conditions have 

 been represented schematically in figure 24a, in which 

 the boundary surface between the two water masses 

 shows an upheaval under the equator. Assuming the iso- 

 baric surfaces in the heavy water to be horizontal, the 

 isobaric surfaces must have the courses which are in- 

 dicated by means of the thin lines. In this case the to- 

 pography of the isobaric surfaces shows a minimum at 

 the equator, and within the light water, we find a current 

 toward the west on both sides of the equator, whereas 

 the heavy water is at rest. No countercurrent exists. 



If, however, for some reason the accumulation of 

 heavy water is asymmetric when referred to the equator, 

 a different system is developed. The conditions which 

 are shown in figure 24b cannot exist. We cannot find a 

 single upheaval of the heavy water on one side of the 

 equator because this would give the isobaric surfaces an 

 inclination at the equator. Considering that the isobaric 

 surfaces must have a maximum or a minimum at the 

 equator, two types of asymmetric development are pos- 

 sible, as shown in figures 24c and 24d. In figure 24c we 

 have a small upheaval of the heavy water under the equa- 

 tor and a big upheaval to the north. When such a distri- 

 bution of density is present, the isobaric surfaces show 



two minima, one at the equator and one to the north of 

 the equator. These two minima are separated by a 

 maximum and the water in the region between this max- 

 imum and the northern deep minimum must flow to the 

 east. That means that here we find a countercurrent 

 which, however, is present in the upper light water only. 

 The light water reaches to greater depths to the north 

 and to the south of the minima and the westerly current 

 is, therefore, deeper than the countercurrent. In the 

 second case, figure 24d, we find accumulations of heavy 

 water on both sides of the equator but the accumulation 

 on the northern side is the greater. The isobaric sur- 

 faces show a maximum at the equator and minima on 

 both sides, and between the two minima a countercurrent 

 flows toward the east. The case in which the two up- 

 heavals of the heavy water are equally developed is 

 probably of minor interest because then symmetry 

 exists as to the equator and the simpler system in fig- 

 gure 24a seems more probable. The greatest upheaval 

 may, of course, be found in the Southern Hemisphere, 

 but this cannot lead to any principal differences. 



From these considerations it seems probable that an 

 asymmetric development of the westerly tropical cur- 

 rents may give rise to an asymmetric accumulation of 

 heavy water near the equator, and that the dynamic sys- 

 tem which then is established leads to the countercur- 

 rent toward the east between the two westerly currents. 

 The width of the countercurrent and the one-sided devel- 

 opment in reference to the equator depends on the char- 

 acter of the asymmetry, but the countercurrent must in 

 all cases be regarded as a dynamically conditioned cur- 

 rent.^ 



We have the possibility of discussing the equatorial 

 currents for two occasions when the Carnegie crossed 

 the equator. The sections were both taken in directions 

 which form angles less than 90° with the equator, but, 

 for the sake of simplicity, we shall plot the values as if 

 they were taken along two meridians; that is, we shall 

 plot the stations at the observed latitudes and disregard 

 the differences in longitude between the stations. The 

 eastern section is taken nearly in the central part of the 

 Pacific along the average meridian of 145° west, where- 

 as the western section is taken in the western half of the 

 Pacific approximately along the meridian of 180°. 



In order to study these sections we have computed 

 the distances in dynamic meters between the isobaric 

 surface of 700 decibars and the isobaric surfaces 0, 50, 

 100, 150, 200, 250, and 300 decibars. We have selected 

 the isobaric surface of 700 decibars as the reference 

 surface because this surface is practically parallel to 

 the surface of 2000 meters. Also, accidental errors of 

 observation exercise a greater influence at depths be- 

 low 700 meters since the intervals between the observa- 

 tions there are greater. Assuming the isobaric surface 

 of 700 decibars to be horizontal, we have constructed 



1 Later on the author (1939) has pointed out that the 

 observed distribution of mass does not given any clue to 

 the understanding of the dynamics of the countercurrent. 

 The dynamics have recently been discussed by Mont- 

 gomery (1940) and by Montgomery and Palmfen (1940). 

 They state that the trade winds by continually exerting a 

 westward stress on the sea surface produce a westward 

 ascent of the sea level in the equatorial region. The 

 equatorial countercurrents are found in the doldrums 

 and apparently result as a down slope flowing in this 

 zone where the winds maintaining the slope are absent. 



