THE NORTH ATLANTIC OCEAN 



87 



Results of Dynamic Calculations 



Helland-Hansen and Nansen (1926) have published 

 maps of the eastern North Atlantic showing the topogra- 

 phy of a number of isobaric surfaces relative to the to- 

 pography of the 2000-decibar surface, and Jacobsen 

 (1929) has published corresponding maps of the greater 

 part of the North Atlantic using the 1000-decibar sur- 

 face as a basis. Most of the Carnegie stations reach to 

 greater depths than 2000 meters and the results, there- 

 fore, can be used for amplifying the maps by Helland- 

 Hansen and Nansen. 



Figure 5 shows the topography of the 100-decibar 

 surface relative to the topography of the 2000-decibar 

 surface, based on the anomalies in dynamic meters of 

 the distances between the surfaces. The lines repre- 

 senting the relative topography of the 100-decibar sur- 

 face are drawn for intervals of 10 dynamic centimeters. 

 The relative flow of the water at a pressure of 100 deci- 

 bars is parallel to the lines and in the direction which is 

 indicated by the arrows. The flow of the water at a 

 pressure of 2000 decibars is undoubtedly very slow and 

 the lines, therefore, represent very nearly the direction 

 of the absolute currents at the depth where the pressure 

 is 100 decibars, or at a depth of about 100 meters below 

 the surface. 



The continuous lines in the figure have been copied, 

 with a few simplifications, from Helland-Hansen and Nan- 

 sen's map. These lines are based on numerous obser- 

 vations over a period of many years and must, there- 

 fore, be expected to represent nearly a true picture of 

 the average topography of the 100-decibar surface in the 

 stated units. The corresponding values at the Carnegie 

 stations are entered in the same units. Some of the 

 Carnegie stations fall within the area which has been 

 examined by Helland-Hansen and Nansen, and the values 

 at these stations are in excellent agreement with their 

 map, except the value 1.30 to the northwest of the Azores. 

 It is very probable, however, that this station is situated 

 in a region where minor whirls occur, such as those 

 which are indicated at other localities in Helland-Hansen 

 and Nansen's map. The broken lines in the figure rep- 

 resent the relative topography of the 100-decibar sur- 

 face according to the Carnegie data outside the region 

 which previously had been studied. It is seen that these 

 lines can be readily united with Helland-Hansen and 

 Nansen's lines. As we proceed from north to south, 

 along the route of the Carnegie, we recognize the Lab- 

 rador Current, the Atlantic Drift, the anticyclonic cir- 

 culation around the Sargasso Sea, and the north equato- 

 rial trade-wind drift, which partly continues into the 

 Caribbean Sea. 



Figure 6 shows the profile of the isobaric surfaces 

 0, 100, 200, 300, 400, 500, 700, 1000, and 1500 decibars 

 along Section I, referred to the 2000-decibar surface. It 

 is evident that the currents are strongest in the upper 

 layers because the slopes decrease with increasing 

 depth. It especially should be noted that the 1500-deci- 

 bar surface is almost level when referred to the 2000- 

 decibar surface, meaning that the currents at 1500 me- 

 ters vary but little from the cxirrents at 2000 meters. 



There are strong reasons for assuming the currents to 

 be very weak at a depth of 2000 meters and we can, 

 therefore, regard the relative slopes in the figure as 

 representing very nearly the actual slopes of the sur- 

 faces. The section runs approximately north and south, 

 and a slope to the north represents a current to the east, 

 and vice versa. 



To the right in the figure the steep slope between 

 stations 15 and 16 indicates the Gulf Stream. The maxi- 

 mum velocity of this current is reached, according to 

 the slope of the isobaric surfaces, at a level of about 200 

 meters. Examining the difference between the elevations 

 of the isobaric surface above the 2000-decibar surface, 

 we find: 



Isobaric 

 surface 



Decibars 

 100 200 300 400 500 700 1000 1500 

 Dynamic meters 



Difference in 

 elevation 

 (15-16) .32 .41 .44 .41 .38 .34 .25 .12 .03 



The slope of the 200-decibar surface is thus the 

 greatest and remains considerable down to a depth of 

 more than 1000 meters. 



Aside from the maximum elevation of the isobaric 

 surfaces at station 15 which must be associated with 

 the presence of whirl, we find the maximum elevation of 

 the 0-decibar surface at station 20, or \n latitude about 

 20°, but the maximum shifts toward the north with in- 

 creasing depth and at the 200-decibar surface is found at 

 station 18, or latitude 30°-. Between stations 18 and 20 

 we find, thus, a current which is directed toward the 

 east at the surface but toward the west at a depth of 200 

 meters; below 500 meters it is again directed toward 

 the east. 



Disregarding what are probably local conditions be- 

 tween stations 15 and 16, the strongest westerly current 

 is found between stations 20 and 21. This westerly cur- 

 rent, in contrast with the Gulf Stream, has the greatest 

 velocity at the surface, but it decreases so rapidly with 

 depth that the current is weak below 500 meters. Exam- 

 ining the differences between the elevations of the iso- 

 baric surfaces we find: 



Isobaric 

 surface 



Decibars 

 100 200 300 400 500 700 1000 1500 

 Dynamic meters 



Difference in 

 elevation 

 (20-21) .24 .23 .17 .13 .10 .08 .07 .05 



.03 



South of station 20, that is, south of latitude 20°, the 

 current is, on the whole, directed toward the west, but 

 irregularities appear to be present. 



A corresponding examination of the profiles of the 

 isobaric surface along the parallel of 12° north. Section 

 II, does not produce definite results, which indicates that 

 in this region the east and west currents are much 

 stronger than the north and south currents. 



