108 



OBSERVATIONS AND RESULTS IN PHYSICAL OCEANOGRAPHY 



The greater part of the water masses which are 

 carried to the east does not turn toward the equator until 

 reaching the South American coast, then it follows this 

 coast toward the north as the Peruvian Current. This 

 current can be traced to a depth below 500 meters. We 

 have previously pointed out that water of low tempera- 

 ture is found at a short distance from the surface off the 

 coast of Peru, and that the surface temperatures are 

 very low in this region. It cannot be doubted that these 

 low surface temperatures are owing to a vertical move- 

 ment which carries water of low temperature to the sur- 

 face, but the accumulation of cold water off the coast can 

 be explained without taking a possible vertical movement 

 into consideration. We must bear in mind that water is 

 transported toward the coast of South America by the 

 predominating current toward the east. This water is 

 forced to change its course and to continue toward the 

 equator. The Peruvian Current is, thus, a "forced" 

 current which must exist because of the land boundaries 

 of the ocean. In such a current we must find the normal 

 distribution of density, which means that in the Southern 

 Hemisphere we must find water of high density on the 

 right-hand side of the current and water of low density 

 on the left-hand side. Consequently the density must in- 

 crease toward the coast or, if the salinity is nearly con- 

 stant, water of low temperature must accumulate along 

 the coast. The accumulation of cold water along the 

 coast of South American gives, therefore, no evidence of 

 an upwelling motion which reaches to great depths, but 

 indicates only that a current follows the coast toward the 

 equator (cf. Helland-Hansen [1912]). On the other hand 

 it is evident because of the conspicuously low surface 

 temperatures, that water from moderate depths is drawn 

 to the surface at the coast. This upwelling from moder- 

 ate depths is probably maintained by prevailing winds 

 and is a secondary effect as compared with the large ac- 

 cumulation of cold water at greater depths. The actual 

 surface current, which represents the combined effect of 

 the distribution of density and of the wind, probably is 

 directed away from the coast, for which reason the con- 

 tinuity would necessitate a supply of water from below, 

 that is, an upwelling. 



As to the direction of the winds which maintain the 

 offshore currents, it should be borne in mind that on ac- 

 count of the effect of the rotation of the earth, the trans- 

 port of water by wind takes place at right angles to the 

 direction of the wind, to the right in the Northern Hemi- 

 sphere and to the left in the Southern Hemisphere. The 

 winds, therefore, which approximately parallel the coast 

 toward the equator, give rise to a transport of water 

 having an offshore component. 



It is well known that the regions of upwelling are all 

 found at the west coasts of the continents where the wind 

 currents transport water away from the land. The up- 

 welling has, therefore, generally been attributed to the 

 effect of the winds. The present interpretation of the ob- 

 served conditions does not differ from the accepted ex- 

 planation of the upwelling, but here it is emphasized that 

 the upwelling water comes from small depths and that the 

 the accumulation of cold water in greater depths is not a 

 result of the upwelling but is associated with the pres- 

 ence of a "forced" current along the coast. 



To the north of latitude 20° south we find, on the 

 whole, currents which are directed toward the west. 

 Here we have the region of the westerly tropical current 

 in the South Pacific. This westerly current appears to 

 be very irregular. In the region from the coast of Peru 



to longitude 120° west it appearsas if divergent currents 

 are found to a depth of about 100 meters. These diverg- 

 ing currents may be an effect of the prevailing east 

 winds which carry the surface water to the north on the 

 northern side of the equator and to the south on the 

 southern side. If this is correct, we must assume that 

 ascending motion takes place in the region to the south 

 and the west of the Galapagos Islands, and such currents 

 would account for the low surface temperature of this 

 region. The high phosphate content of the surface water 

 in this region supports such a conception. Below a 

 depth of 200 meters water from the northwest appears to 

 flow toward this region, perhaps compensating for the 

 water which is drawn to the surface. Farther west, be- 

 tween longitudes 120° and 170° west, we find that the 

 currents have considerable components from the south 

 down to a depth of about 200 meters. In this region, at 

 a depth of between 100 and 200 meters, the southern part 

 of Section V indicates a considerable northward flow of 

 water of high salinity. The dynamic charts confirm that 

 such a flow takes place above the 200-meter level be- 

 cause the northerly component of the current is greatest 

 down to this level. The irregularities in the topography 

 of the isobaric surfaces perhaps indicate that the flow 

 toward the equator of water of high salinity does not take 

 place continuously but has an intermittent character. 

 The fact that the irregularities are especially present 

 above the 200-meter level points in this direction. An 

 intermittent transport cf water toward the equator means 

 that whirls develop within which subsurface water may 

 be transported to the surface. At several stations in this 

 this region water of high phosphate content and low oxy- 

 gen content is found, indicating that such transport takes 

 place. 



Before concluding the discussion of the westerly 

 tropical current, we shall emphasize that this current, 

 aside from the wind current at the surface, dynamically 

 is partly of the same character as the Peruvian Current. 

 The water, which is transported toward the South Amer- 

 ican coast by the easterly current in the South Pacific 

 and is forced toward the equator along the continent, 

 cannot sink because of its low density and must return 

 toward the west as a surface current within which the 

 easterly winds carry the light water to the south. The 

 westerly tropical current is thus in part maintained by 

 the same forces which maintain the easterly current in 

 the southern part of the ocean and in part by the prevail- 

 ing winds. In the Southern Hemisphere we find water of 

 low density on the left-hand side of the current and water 

 of high density on the right-hand side; that is, an accumu- 

 lation of heavy water under the equator and an accumula- 

 tion of light water to the south of the westerly current. 

 This distribution of density must be regarded as a 

 "forced" distribution owing to the limitation of the ocean 

 in an east and west direction and to the effect of the pre- 

 vailing winds. 



To the north of the equator a corresponding current 

 is found, the westerly tropical current of the North Pa- 

 cific. Observations are lacking for a great part of the 

 North Pacific between the coast of Central America and 

 longitude 130° west and our picture, therefore, is incom- 

 plete. It appears, nevertheless, as if the form of the 

 North American Continent is of considerable importance 

 to the development of the currents. We shall deal fur- 

 ther with this subject when discussing the California 

 Current. 



The westerly tropical current in the North Pacific 



