Evidence for a two gyre circulation system in the North Atlantic

Deep-Sea Research, 1962, Vol. 9, pp. 51 to 67. Petllamon Press Ltd. Printed in Great Britain

Evidence for a two gyre circulation system in the North Atlantic* L. V. WORTHINGTON (Received 2 January 1962) Abstract--The Gulf Stream, as it passes the southern extremity of the G r a n d Banks, transports water whose oxygen--density relationship closely resembles the Sargasso Sea average established by RtCFtARDS and RED~IELD (1955). The currents which flow to the north, off the Flemish Cap, transport water which has a similar temperature-salinity correlation to that of the Sargasso Sea but which is richer in oxygen by about 1 mi/I at every sigma-t surface in the pycnocline. For this reason it is suggested that the Gulf Stream does not turn to the northward after passing the G r a n d Banks but continues to flow in a southeasterly direction, and that the currents which pass the Flemish Cap are part of a separate, northerly gyre. Evidence is produced for the existence of a nearly permanent trough of low pressure separating the two gyros. A water budget consistent with the distribution of oxygen is presented. INTRODUC'[

ION

RICHARDS and REDFIELD (1955) examined the relationship of dissolved oxygen to sigrna-t in the Florida Current, the Gulf Stream and the Sargasso Sea. They were able to construct an oxygen-sigma-t curve for the Sargasso Sea from which less than half the oxygen observations departed by more than about 0.3 ml/l. By use of this curve they identified water entering the Western North Atlantic from the Caribbean Sea through the Straits of Florida by its low oxygen content. The oxygen-sigma-t correlation in this area proved to be a far more sensitive means of detecting water of Caribbean origin than the temperature-salinity correlation which has been in general use for identifying different water masses since HELLAND-HANSEN(1916) introduced it. In the central and western North Atlantic, ISEUN (1939) showed that the temperature-salinity correlation both at the sea-surface (in late winter) and throughout the main thermocline/halocline (at all seasons) was remarkably constant. This led to the concept that the North Atlantic central water mass, and in fact, water masses in all oceans were formed by the winter convergence of surface water at high and middle latitudes (el'. SVERDRUP,et al. 1942, pp. 144--145). A chart (Fro. 1) has been prepared of the salinity anomaly at the 10°C isotherm throughout the North Atlantict. The salinity anomaly method has been used principally by HELLAND-HANSENand NANSEN (1926) and ISELIN (1936) to delineate the different water masses in the North Atlantic. *Contribution No. 1244 from the Woods Hole Oceanographic Institution. tI.G.Y, data from the following ships have been used in this chart. V.F.S. Gauss (W. Germany), F.R.S. Anton Dohrn (W. Germany), R.R.S. Discovery//(Great Britain), S.S.M. Lomonosov (U.S.S.R.), R.V. Atlantis (U.S.A.), R.V. Crawford (U.S.A.) and Rt.V. Chain (U.S.A.). As far as possible, use was made only of cruises where the salinity was measured by the conductivity method of SCHtttCHER and BRADSHAW 0956) or Cox (1958) but it was necessary to include data from £omonosov because of the large scope of her work in the North Atlantic, and because her determinations of salinity by titration appear to be of high quality a n d should not affect the accuracy of the chart. In addition to I.G.Y. data, stations made in Atlantis 0954, 1955, 1956 and 1960), Caryn 0956) and Chain (1959-1960) have been used. These data have been or will be published in the Bulletin Hydrographique. Data from above 200 m have not been included in this chart to avoid including water that has been seasonally heated to 10°C.

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Evidence for a two gyre circulation system in the North Atlantic

53

The 10°C isotherm has been chosen because it is representative of the main thermocline in the North Atlantic. This chart (FIG. 1) outlines the major water masses of the Atlantic; to the south is found the Antarctic Intermediate Water which is relatively fresh. A small volume of this water passes through the Florida Straits with anomalies of -- 10 or more. The effect of the Antarctic Intermediate Water is soon lost through the addition of Sargasso Sea water from the east. The influence of the Mediterranean water appears as a vast tongue extending from the Straits of Gibraltar across the Atlantic as far west as Bermuda and northward to the British Isles (FIG. 1). Water with anomalies of less than 5 occupies the western portions of the Atlantic with minor exceptions. (IsEuN'S T-S curve does not truly describe the water to be found in the geographic center of the North Atlantic, but it has been generally accepted as defining a water mass known as North Atlantic Central Water). The Gulf Stream constitutes the western boundary of North Atlantic Central Water. It transports almost wholly water described by ISEUN'S T-S curve with only minor anomalies except in the upper 100 m. In this chart (FIG. 1) North Atlantic Central Water extends aorthward to the area where water as warm as 10°C is no longer found except during the summer, as part of the seasonal thermocline. From a study of the T-S relationship at other isotherms, both above and below 10°C, there is no reason to suspect any discontinuity in the circulation. It appears, however, that the oxygen-sigma-t relationship in the Gulf Stream system is by no means as consistent, and a major discontinuity appears to exist to the southeast of the Grand Banks. Before and during the International Geophysical Year, four oceanographic sections (FIG. 2) were made to the south and east of the Grand Banks by the V.F.S. Gauss, the R.R.S. Discovery H and the R.V. Atlantis. A fifth (made by Atlantis in 1949) will be discussed later. Within these sections attention has been focused on the stations in each, where the density distribution indicates strong geostrophic currents. For each (that is, the portion where strong horizontal density gradients are found) a density profile has been drawn and a plot of oxygen versus sigma-t made. The first section dealt with here was made in Atlantis in November 1956. From the sigma-t profile (FIG. 3) it can be seen that three distinct currents have been crossed. According to dynamic computations the northernmost current (station 5422-5427) transports 22 million mS/see to the eastward, (relative to the 2000 m surface which has been used throughout). To the south of this, a countercurrent (stations 5427-5432) transports 15 million m3/sec toward the west. Farther south (stations 5432-5439) another easterly current transports 58 million m3/sec. A problem in nomenclature arises here. SOULE (1951) and SOULE et al. (1961) referred to all these currents as the Atlantic Current. MCLELLAN (1957) identified the northernmost current, consistently found by the International Ice Patrol as the Atlantic Current and considered that it consisted of Slope Water. He preserved the name Gulf Stream for the larger, southerly current. In separating these currents he agreed with FUGLISTER (1951) and FUGLISTERand WORTHINGTON(1951). As far as this section is concerned MCLELLAN'S nomenclature appears to be preferable but in 1960 (SOULE et al. 1961) the Contribution of the northernmost current was only 4 million mS/see (compared with 51 million mS/see for the southernmost current) and it is perhaps dangerous to regard it as a permanent and separate feature of the circulation.

54

L . V . WOR'mZNO'rON

In the sigma-t profile and in the oxygen-sigma-t diagram different symbols have been used to distinguish the three currents : Atlantic Current water is represented by circles, countercurrent water by triangles and Gulf Stream water by squares. A further distinction (FIG. 3) is made (in this figure and subsequent ones) between water of the North Atlantic Central Water mass and the relatively fresh water found on the lefthand side of the currents and, particularly, in the surface layer*.

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In the Atlantis section (FIG. 3) the oxygen-sigma-t relationship in all three currents adheres closely to the Sargasso Sea curve established by R]CHARDS and REDFIELD (which is reproduced here). Exceptionally high oxygen values were found in the relatively fresh water only, as the open symbols indicate. North Atlantic Central Water carried by these currents is slightly oxygen deficient. This deficiency is probably due to the continued presence of water of Caribbean origin. In August 1958 a section was made in Gauss to the east-southeast of the Flemish Cap (FIG. 4). The density distribution indicates two northerly currents separated by a southerly countercurrent. The first northerly current (stations 181-179) transports 16 million mZ/sec, according to dynamic computations and is slightly richer in oxygen *Above the 189C isotherm an unpublished T - S curve of FUGLISTER'Sfor the Bermuda area was used. T - S values for this curve are as follows: 28°C-36-47~, 26°C-36-52~, 24°C-36.55~,

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Evidence for a two gyre circulation system in the North Atlantic

57

than the other two. The countercurrent (stations 179-177) transports 9 million mS/see and the third current (stations 177-171) 23 million mS/sec. One cannot say whether these three currents are distinct or are formed by the meanderings of a single current, but since the countercurrent appears in all three sections to the east of the banks they are treated separately here. There is a strong pycnocline, the result of seasonal warming in the top 100 m. This layer was anomalously fresh; its salinity was lower than that of the upper 100 m layer in the two late winter sections discussed below although the seasonal warming also contributed to the anomalies. (If pure North Atlantic Central Water has a winter temperature of 15°C its salinity will be 36-0%0. If this water is heated to 20°C during the spring and summer it will incur a salinity anomaly of -- 58 parts per hundred thousand since North Atlantic Central Water has a normal salinity of 36-58%° at 20°C). In the permanent pycnocline the oxygen values in this section are consistently higher than the values at the same sigma-t surfaces in the Atlantic Current and the Gulf Stream. The same relatively high oxygen is found in the two late winter sections made in Discovery H and, again, in Gauss. In the Discovery H section made in April 1957 (FIG. 5), there are again three currents indicated. The first (stations 3518-3520) transports 12 million mS/sec, the countercurrent (stations 3520-3521) only 4 million mS/see, but the third (stations 3521-3523) 34 million mZ/sec. The cluster of solid symbols at sigma-t = 27.0 and 02 = 5.3 ml/1 represent samples taken at the sea-surface and their oxygen values are understandably higher than those of Gauss's summer section. Sigma-t surfaces 27.2 to 27-4 are occupied by a relatively fresh layer which does not occur in either Gauss section. Its oxygen values do not appear to be abnormally high as would be expected if its fieshness were traceable to the water of the Labrador Sea. In the Gauss section made in April 1958 (FIG. 6) the currents appear to be broader and slower than those of the other section. This, however, may be the result of the currents' running more or less parallel to the section. The first current in this section transports 33 million mS/sec, the countercurrent 26 million mS/see and the third 21 million mS/sec, according to dynamic computations. As in the previous sections, east of the Grand Banks, the oxygen values are consistently higher by about 1 ml/l. at all sigma-t surfaces in the main pycnocline. By means of the dynamic computations the ' a n o m a l y transport of oxygen' can be arrived at. This value represents the difference between the calculated transport of the current in question and that of an equivalent volume of Sargasso Sea water, and is expressed in millions of litres per second. The steps involved in arriving at this figure are as follows : The average sigma-t of each standard layer and between each pair of stations is obtained from the sigma-t curves drawn for each station. The standard value of oxygen for this sigrrra-t is read from RICHARDS' and REDFIELD'S curve for the Sargasso Sea. The difference between this value and the average observed oxygen in the layer represents the anomaly of oxygen; this, multiplied by the volume transport, represents the anomaly transport of oxygen. The relatively high oxygen concentration to the east of the Grand Banks can be explained in two ways. The first, and less satisfactory explanation is that the Gulf Stream and the North Atlantic Current pick up oxygen between the Tail of the Banks and the Flemish Cap. There are two sources available : the atmosphere and the

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high oxygen water of Labrador Sea origin which is found on the left-hand side of these currents. The atmosphere has undoubtedly added oxygen to the surface water in the late winter sections made from G a u s s and D i s c o v e r y H but the~mixed surface layer was only deeper than 100 m on one station in these sections, the r:maining water being cut off from the atmosphere by thermal stability. The high oxygen water to the left of these currents is between 1.5%o and 0-5%0 less saline at isopycnals throughout the thermocline and it would be difficult for the Gulf Stream to acquire oxygen from this source without an accompanying reduction in the salinity; (a possible exception is the D i s c o v e r y H section in the sigma-t range 27-2-27.4). Further, the oxygen values on the right-hand sides of these currents are almost equally as high as on the left-hand sides which are adjacent to the Labrador Sea water. The more reasonable hypothesis is that the North Atlantic Current and the Gulf Stream do not turn the corner at the Tail of the Banks, but that instead the currents which pass Flemish Cap arc composed of water of different origin.

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Evidence for a two gyre circulation system in the North Atlantic

61

occupied by the currents themselves. In North Atlantic Central Water the 10°C isotherm falls at the isopycnal of 27.2 ( T - 10°C, S --:- 35"29~oo) close to the oxygen minimum. The general level of oxygen at the minimum is higher in all the waters to the east of the Grand Banks. It is suggested that these waters contribute to a second gyre in the North Atlantic Circulation and that this gyre is essentially independent of the Gulf Stream. There is a sharp boundary between the high oxygen water of the northern gyre and the low oxygen water of the southern gyre. In this boundary region exceptionally high values of oxygen are found. These are associated with what is clearly water of Labrador Sea origin which has higher oxygen and lower temperature and salinity at any sigma-t surface than the waters of either gyre. Dynamically, this water represents a trough of low pressure extending from the Tail of the Banks in a southeasterly direction and constitutes the boundary between the northern and southern gyres. Many observers, the author among them, have encountered Labrador Sea water far to the southeast of the Grand Banks. There are bathythermograms on file at the Woods Hole Oceanographic Institution by which it can be traced as far as 38 ° North 42 ° West. Hydrographic stations are rather scarce in this area and if a station happens to fall in the trough the observer is naturally prone to believe that he has encountered an isolated bubble of cold water (which may be interpreted as the centre of a small local cyclonic eddy). On the basis of existing data it is not possible to establish beyond doubt the existence of a continuous trough separating the northern and southern gyres. An Atlantis section made in September 1949 crossed the trough (or an eddy) in latitude 38 ° North, longitude 45 ° West (FIG. 8). The extreme narrowness of the band of cold water (FIG. 8) suggests that ships not equipped with bathythermographs could easily cross it without being aware of its presence. In this section the 10°C isotherm rose to the 300 m level on station 4821 but to within 90 m of the surface between stations 4821 and 4822, and the coldest water would have been missed if bathythermograms had not been taken. Volume transports have been computed for the two opposed currents in this section. In order to include the information from the bathythermograph observations it was assumed that the isopycnals below the 200 m level all rose by 140 m which was the amount by which the 9°C isotherm rose between station 4821 and the coldest bathythermogram to the southwest. The computed volume transport (northwestward) to the north of the trough was 37 million m3/sec, and the southeasterly transport south of the trough was 58 million ma/sec. If the cold water in this case (FIG. 8) is construed as an isolated eddy it becomes difficult to explain why the northeastern quadrant of such an eddy should have higher oxygen values in the pycnocline than the southwestern quadrant. If, however, the cold water is interpreted as part of a continuous trough the oxygen values become more reasonable. The existence of the trough as a permanent feature extending southeastwards from the Tail of the Banks is essential to the two gyre hypothesis since two opposed currents as envisioned here must be separated by such a trough. Assuming that the trough exists as a nearly permanent feature of the circulation in this region, it has been shown that the general level of oxygen in the water to the north of the trough is higher than that of the water to the south of the trough. It can be further seen (TABLE 1, column 3) that the average oxygen in the Discovery

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4820

.

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4824

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4823

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4819

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4820 4821 4822

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SEPTEMBER

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1949

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Fig. 8. Atlantis section, September 1949 (temperature and oxygen-signm-t). In this section bathythermograph data have been use~_ for the upper 200 m and reversing thermometer data for the deeper water. In the oxygen-sigma-t plots open symbols indicate stations to the northeast ot the colaeSt water aria closed symbols to the southwest. Positions of these stations are plotted in Fig. 2.

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48124

.<

63

Evidence for a two gyre circulation system in the North Atlantic

section m a d e i n April 1957 was a b o u t 0.45 ml/l. higher t h a n it was d u r i n g the Gauss section m a d e almost exactly a year later*.

Table 1. Volume transport, relative to the 2000 m surface, anomaly transport of oxygen, and average oxygen anomaly in each section, weighted with respect to transport. (This value is obtained by dividing the anomaly transport by the volume transport). Volume transport (m*/sec x 1~)

~lt/antis November 1956 North Atlantic current Countercurrent Gulf stream

Anomaly transport of 0s (l./sec

22E 15W 58 E

Total

+

x

10e)

Averaae anomaly of 0s in current (re.l/l)

2.2 0.8

+ 0"10

10.1

--0.17

+ 14.0 + 5.8 + 15-0

+ 0.88

-

-- 0~)5

65 E

Gauss August 1958 1st current Countercurrent 3rd current

16N 9S 23 N

Total

30 N

+ 0.64 + 0.65

Discovery April 1957 lst current Countercurrent 3rd current

12N 4S 34 N

+

+ 16.1 6"4 + 47.9

+ 1"34 + 1"60 -t- 1.41

1st current Coun~nt 3rd current

33 N 26S 22N

+ 32.1 + 25.4 + 23-6

+ 0.98

Total

29N

Total

Gauss April 1958 + 0.97 + 1-07

T h e reason for this appears to be that the winter of 1956-57 was m o r e severe i n this p a r t o f the N o r t h A t l a n t i c t h a n the winter o f 1957-58. I n TABLE 2 are listed average m o n t h l y air a n d water temperatures at W e a t h e r Station Delta, (44°N 41°W; from N o r t h e r n Hemisphere D a t a T a b u l a t i o n s , published by the U.S. W e a t h e r Bureau).

Table 2. Average monthly sea and air temperature at weather station Delta (Fahrenheit) February

1956-1957 Air Water Water-Air

December 55"1 60.5 5.4

49.0 59"5 10.5

47"5 56-6 9"1

49"8 56"7 6"9

1957-1958 Air Water Water-Air

December

January

February

March

55-0 58.5 3"5

58"2 61 "6 3-4

55"2 60"9 5"7

53.5 59.0 5-5

January

March

*According to WORTm~3TON(1959) a discrepancy was found between the oxygen values obtained by the National Institute of Oceanography and by the Woods Hole Oceanographic Institution. The Woods Hole method (used in the Discovery 11 section of 1957) relied on a potassium dichromate standard and gave values 3 % lower than the N.I.O. method using a biniodate standard. Since testa' made at Woods Hole by DAYTONE. CARmTr indicated that the Woods Hole method gave resulta as much as 4.8% too low, the N.I.O. method was judged to be more nearly correct. The D/.~oeery H section and the Atlantis sections reported on here probably give oxygen value~ that are slillhtly low.

64

L . V . WORTHINOTON

It is possible that the gyre to the north of the trough responds rapidly to changes in the weather regime, and that some renewal of water takes place at all sigma-t surfaces in the pycnocline during a severe winter. The figures for the winter of 1957-58 show very little contrast between air and water temperatures and the process of convergence in this area must certainly have been much reduced in comparison with the previous winter. If one assumes that no renewal of water in the pycnocline took place during the winter of 1957-58 the oxygen consumption must have been of the order of 0-5 ml/I per yr throughout the pycnocline, a not unreasonable figure according to RILEY (1951). The greater number of observations from the northern gyre (F1G. 7) were made during 1958 and it seems certain that the contrast in oxygen between the two gyres would have appeared greater if the Gauss and Lomonosov had performed their cruises in 1957. A simplified scheme of circulation consistent with the oxygen distribution in the pycnocline has been drawn in FIG. 9. It is based on a reference level at 2000 m except in the Florida Straits. This level is used because observations are made to that depth in nearly all sections, whereas the choice of a deeper level would exclude much valuable data. The Florida Current according to WiJST (1924), MONTGOMERY (1941) and WERTHEIM (1954) transports about 25 million ma/sec. The flow past the Montauk Pt. - Bermuda section is 82 million m~/sec, the average of 15 sections of ISELIN'S (1940). The net eastward flow past the 50th meridian is between 50 and 60 million m3/sec. This figure is far in excess of SVERDRtJP'S (1942) estimate o f 38 million m3/sec, also relative to 2000 m, but no section was available to him which crossed both the North Atlantic Current and the Gulf Stream. At the trough, in the neighbourhood of 38°N 45°W, the Gulf Stream is estimated at 58 million m3/sec, and the northern gyre at 37 million m3/sec, according to the Atlantis section of September 1949. The transport of the northern gyre as it passes Flemish Cap is given as 40 million m3/sec. This figure favours the larger transport of Discovery's 1957 section (42 million m3/sec) since it is tentatively assumed that the northern gyre was relatively inactive during 1958 when Gauss made her two sections. No attempt has been made to divide the northern gyre into multiple currents such as the sections seem to indicate; a more detailed survey is needed. The major circulation has been confined to the western side of the ocean in both gyres consistent with both the T - S relationship (Fie3. 1) and the oxygen distribution. It has been estimated that only 10 million m'~/sec pass from the southern gyre into the northern gyre. This has been done because a band of slightly lower oxygen is found in the northern gyre immediately to the north of the trough, (FIG. 7). Of this 10 million m3/sec, approximately 3 million m3/sec pass into the Arctic mediterranean sea (SVERDRUP, 1942) and a slightly larger amount re-enters the North Atlantic as the East Greenland Current at a lower salinity, the result of precipitation and run off within the Arctic mediterranean sea. This allows only about 7 million m3/sec of high oxygen water to return to the southern gyre from the northern gyre to the east of the low pressure trough separating the two gyres. This amount seems very much too small to maintain the oxygen-sigma-t relationship in the southern gyre unless biological consumption in the Sargasso Sea is very much lower than that in the northern gyre, particularly since, as RICHARDS

....

0

I

60"

f,w

1

40"

40"

/

/

/

/

/

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Fig. 9. Water budget, relative to.the 2000 m surface for the North Atlantic. Each streamline represents 10 million mS/sec,

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os ~t

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66

L . V . WORTHINGTON

and REDFIELD (1955) have shown it must also balance a considerable flow of oxygendeficient water entering the Sargasso Sea through the Florida Straits. The role of lateral mixing (as opposed to direct transport of oxygen-rich or oxygen-deficient water by major ocean currents) has been neglected here and this can possibly assist in maintaining the oxygen-sigma-t relationship in the Sargasso Sea. It is noteworthy that the trough as pictured here lies directly over the Southeast Newfoundland Rise which, according to WORTHINGTONand METCALF(1961), separates the deepest waters of the l a b r a d o r Basin from those of the main Atlantic Basin. The bottom topography at the southeastern extremity of the Grand Banks is encouraging to this interpretation of the current system. More encouraging still is the study of North Atlantic copepods by GRtCE (1962, in press) which shows that many northern species do not penetrate south of the 40th parallel. If there were direct flow of Sargasso Sea water across the 40th parallel in the amount of 40 million ma/sec, a like amount would have to return to the Sargasso Sea from the north. Such a return flow would inevitably transport copepods of the northern species into the more southerly regions. Acknowledgements--This research was sponsored by the Office of Naval Research under Contract Nonr-2196. REFERENCES Cox, R. A. (1958) The thermostat salinity meter. Nat. Inst. Oceanogr. Internal Report No. C-2, 1-23. (Unpublished manuscript). FUGLISTER,F. C. (1951) Multiple currents in the Gulf Stream System. Tellus, 3 (4), 230-233. FUGLISTER,F. C. and WORTHINGTON,L. V. (1951) Some results of a multiple ship survey of the Gulf Stream. Tellus, 3 (1), 1-14. GRICE, G. D. (1962) Copepods collected by the Nuclear Submarine Seadragon on a cruise to and from the North Pole with remarks on their geographical distribution. J. Mar. Res., 7,0, (I). HELLAND-HANSEN, B. (1916) Nogen Hydrografiske metoder. Skand. Naturforsker m6te, Kristiania (Oslo). HELLAND-HANSEN, B. and NANSEN, F. (1926) The eastern North Atlantic. Geofys. Pub., 4, (2), 76 pp.

ISELtN, C. O'D. (1936) A study of the circulation of the western North Atlantic. Pap. Phys. Oceanogr. Meteor., 4 (4), 1-101. ISEUN, C. O'D. (1939) The influence of vertical and lateral turbulence on the characteristics of waters at mid-depths. Trans. Amer. Geophys. Union, 414-417. ISELIN, C. O'D. (1940) Preliminary report on long-period variations in the transport of the Gulf Stream System. Pap. Phys. Oceanogr. Meteor., 8 (1), 1--40. McLELLAN, H. J. (1957) On the distinctness and origin of the Slope Water off the Seotian Shelf and its easterly flow south of the Grand Banks. J. Fish. Res. Bd., Canad., 14 (2), 213-239. MONTGOMERY, R. B. (1941) Transport of the Florida Current off Havana. J. Mar. Res., 4, (1), 32-37. RICHARDS, F. A. and REDFIELO, A. C. (1955) Oxygen-density relationships in the western North Atlantic. Deep-Sea Res., 2 (3), 182-199. RILEY, G. A. (1951) Oxygen, phosphate, and nitrate in the Atlantic Ocean. Bull. Bingham Oceanogr. Coll., 13 (!), 1-126. SCHLEICHER,K.. E. and BRADSHAW,A. L. (1956) A conductivity bridge for measurements of the salinity of sea water. J. Cons., 22 (I), 9-20. SOULE, F. M. (1951) Physical oceanography of the grand Banks region and the Labrador Sea in 1950. U.S. Coast Guard Bull., 36, 61-127. SOULE, F. M., MORRILL, P. A. and FRANCESCHETTI,A. P. (1961) Physical oceanography of the Grand Banks region and the Labrador Sea in 1960. U.S. Coast Guard Bull., 46, 31-114. SVERDRUP, H. U., JOHNSON, M. W. and FLEMING, R. H. (1942) The Oceans. Prentice Hall, New York, 1087 pp. WERTHEIM, G. K. (1954) Studies of the electric potential between Key West, Florida, and Havana, Cuba. Trans. Amer. Geophys. Un., 35 (6) 872-882.

Evidence for a two gyre circulation system in the North Atlantic

67

WORTHINGTON,L. V. (1959) Oceanographic data from the R.R.S. Discovery H International Geophysical Year, Cruise Three, 1958. W,H.O.I. Ref No. 59-54 (Unpublished manuscript). WORTHINGTON, L. V. and METCALF, W. G. (1961) The relationship between potential temperature and salinity in deep Atlantic water. Rapp. Proc. Verb. Reun. Cons. Perm. Int. Expl. Met., 149, 122-128. Wi)ST, G. (1924) Florida- und Antillenstrom. Inst. f Meereskunde an. d. Univ. Berlin, N.F., Reihe A: Geogr.-naturwiss., Heft 12. 1-48.