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Transport of water through the Drake Passage
Deep-Sea ~
1971, Vol. 18, pp. 51 to 64. PergamonPrea. Printed in Great Britain.
T r a n s p o r t of water t h r o u g h t h e D r a k e P a s s a g e JOSEPH L. Rmr~* and WORTH D. NOWL~, JR.1" (Received 9 February 1970) Abstraet~t meters placed near the bottom in the Drake Passage have been used with calculations of relative geostrophic speed to estimate transport of water from the Pacific to the Atlantic Ocean. The concurrent temperature and salinity measurements reveal a pressure field that is very much like those from earlier expeditions. Relative to the greatest depths sampled, a 8eostrophic transport of 113 × 10ema/sec was obtained. Adjusting this to the current meter restflts yields an • absolute' eastward transport of 237 x 10e mS/see, which is considerably higher than earlier estimates. In earlier results the major transport appeared to be confined to the northern part; the present results show the transport to be distributed more uniformly across the Passage.
INTRODUCTION
IN Om~ERto investigate the transport of the Antarctic Circumpolar Current, measurements of temperature, salinity and near-bottom currents were made across the Drake Passage during the period from 18 to 27 January 1969 (Fig. 1) on the third leg of the Scripps Institution of Oceanography PIQUERO expedition aboard the Thomas
Washington. The temperature and salj,ity measurements were made at discrete depths from the surface to the bottom at nine positions on a line between Cape Horn and Snow Island (near the Antarctic Peninsula) (Fig. 2). Current meters were placed 300 m above the bottom between the hydrographic station positions. Six of the current meters were deployed on the southward crossing: they were recovered, and the temperature and salinity measurements made, on the return path. Meters were not placed between the first pair or the last pair of stations, since these spanned the continental slopes at the northern and southern ends of the line. The southernmost meter (No. 6) was set to operate for 24 hr and the others for successively longer periods, up to 125 hr, to allow for the travel and working time before the ship would be in the proper area to recover them. No. 1 (the northernmost) gave direction only and No. 3 gave no record at all. A seventh current meter was then deployed at about the position of Sta. 15; it operated successfully for 25 hr, returned to the surface and was recovered. Only celestial navigation was available, and the sky was overcast much of the time. Fixes were thus infrequent and of limited accuracy, and the return track was offset somewhat from the southward track, though not so much as to hinder recovery of the current meters. The offset was only a few miles, but the irregular topography produced rather different bottom profiles on the two tracks (Fig. 2). The current meters are similar to those used at about 4000 m depth off California (ISAACS, REID, SCHICK and SCHWARTZLOSE, 1966) and in the deep passage near Samoa (REID, 1969). Each has a rotor, a vane, a recorder, a float, and an anchor. *Scri_'pps Institution of Oceanography. J~3flke of Naval Research (now with Texas A & M University).
51
52
JOSEPH L. REID and WORTHD. Nowta~, JR.
50'
80 °
70*
60 °
50° 50°
60'
60*
70' 80 °
70*
60*
70 ° 50 °
Fig. 1. Positions of hydrographic stations (circles) and current measurements (squares) in the Drake Passage. Bottom topography is taken from the Carte G~n~rale J~tthym~trique des Oedans (MONACO,1968) with some alterations taken from KROENKEand WOOLLA~(1968)and from the soundings taken on the PIQUERO expedition. Dark shading indicates depth 0-3000 m; light shading 3000--4000m; white areas > 4000 m.
After falling freely to the bottom, each instrument operates for a preset period, releases the anchor, and returns to the surface, where it transmits a radio signal to aid in its recovery. Although the field accuracy of the meters could not be evaluated as well as we would like, it is not likely to be better than 0.2 cm/sec, and on any single lowering it might be worse. The earlier results off California and near Samoa (ISAAC,S e t al., 1966; REID, 1969) have been encouraging as to the precision and the comparability of the various instruments. Since some of the series were quite long, we have calculated the means to hundredths of cm/sec, and they are so tabulated and applied as the best estimate in the following calculations of transport. We note, however, that an error of 1 era/see applied to the transport between two stations 100 k m apart in 4 k m of water would be 4 × 10~ m3/sec. Realistically we must acknowledge that errors this large are not impossible. The spacing of the hydrographic casts and the current meters was arranged so
Transport of v~tcr through the Drake Passage
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54
Josm~ L. REID and WORTH D. NOWLIN, JR.
that the relative geostrophic flow (calculated from the density field) could be adjusted to the direct observations made by the current meters. Thus an' absolute' geostrophic transport through the Drake Passage could be calculated. Whether the adjustment to the relatively few and short series of current measurements really yields ' absolute' transport is subject to some doubt, and could be answered only by more detailed and longer current measurements. RESULTS
The current measurements For each half hour the speed and direction were determined from the total number of revolutions of the rotor and the mean direction of the vane. The daily means and grand means for each meter are given in Table 1. Oscillations of approximately diurnal and semidiurnal periods were apparent, with the semidiurnal amplitude larger than the diurnal and, in the northern part of the Passage, sometimes greater than the mean. At the two southernmost current meters the flow was essentially rectilinear. At the other locations a counterclockwise elliptical rotation was indicated with the major axes of the semidiurnals approximately parallel to the direction of the mean flow except in the third and fourth days of current meter No. 2, when the mean flow was perhaps too weak, relative to the fluctuations, for a good estimate of direction. The daily mean velocities varied with both space and time but in most cases were surprisingly high and consistently eastward. Calculations of speed Velocity fields normal to the section have been calculated in three ways. (a) The geostrophic flow relative to the flow at the deepest common depth of each pair of adjacent stations was calculated (Fig. 2a). Co) The flow was adjusted to the measured currents for the station pairs 10-11, 11-12, 12-13, and 14-15; for the others the flow was calculated relative to the deepest common depth for each pair. (These results are seen in Table 2 but are not illustrated.) (c) The flow was adjusted to current meters where available, otherwise as follows: pair 13-14 was adjusted to a value of 2.88 cm/sec at the bottom (interpolated from the horizontally adjacent adjusted values); pair 15-17 was adjusted to the 3.90 cm/sec measurement at current meter 7, assumed to hold also at the deepest sampling depth of station 15; pair 9-10 was adjusted to 15.71 cm/sec at the deepest sample depth of station 9 (horizontally extrapolated from pair 10-11); and pair 17-16 was adjusted to 26.49 cm/sec at the deepest sample depth of station 16 (horizontally extrapolated from pair 15-17). Speeds in the areas of the Passage outside the sampling grid (north, south, and below) have been calculated by extending the adjacent grid values to the boundary. These results are seen in Fig. 2b. The speed and transport Calculations of geostrophic speed relative to the deepest common sampling depth of the various pairs of stations show surface speeds from 36.1 cm/sec (eastward) to 4.5 cm/sec (westward) (Fig. 2a). An assumption of zero flow at the deepest common sampling depth leads to a weak westward flow above 3000 m between Stas. 11 and 12; this becomes eastward when adjusted to the current meter results. On the
Transport of water through the Drake Passage
55
Table 2. Transport (10e mS/see).
Side to9 Ta Cz c, ca
Station pairs
9-10 10-11 11-12 12-13 13-14 14-15 15-17 17-16 16toside 2~ 1
2
(6) 6
Total 2
13
17 37
--6 38
11 20
23
0
0
2
(11) 0
54
32
33
34
34 -9 -1 24
32
1
(8) 1
(13) 10
8
113 86 38 28
41
24
8
265
2~g
199 237 265
Ta ~- gemtrophic transport calculated relative to deepest common depth of each pair of stations. Cx ----correction to relate the transport to current meters where available. C~ ----correction to remaining pairs based on horizontal interpolations between current meters. Cs ----correction for areas outside the sampling grid (sides and bottom). other hand, the current meter between Stas. 14 and 15 showed a mean eastward flow o f 2.47 cm/sec, but the vertical shear in the geostrophic flow was so great that referring the flow to this current meter results in a weak westward flow below 2500 m (maximum 3.27 cm/sec). I.~D~r~v (1961) reports the results o f G E K measurements made aboard the Ob including a northeastward leg crossing the present section at about 57 ° 30'S, between Stas. 15 and 17; the two nearest G E K measurements showed 051 °, 68 cm/sec and 061 °, 44 cm/sec. These directions are approximately normal to the section, which showed 40 cm/sec between Stas. 15 and 17 when adjusted to the current meter. CAPURRO'S (1967, 1968) measurements in the Drake Passage included two at depths greater than 2000 m, made with Swallow floats. These two were launched at the same position (56 ° M'S, 65 ° 56'W) and showed about 5.5 cm/sec toward 332 ° at some 2250 m and about 8.3 cm/sec toward 318 ° at some 2500 m. This location is about 140 k m southeast of Cape Horn and 60 km northeast of one of the ridges which rise above the 3000 m contour and extend well into the Passage; the measured flow may reflect the topographic effects. The calculated transports are given in Table 2. The transport relative to the common depths is about 113 × 10emS/sec, and the transport adjusted to the current measurements is about 237 × 10e n~/sec. The major differences between the calculations based on a level of assumed zero motion and those referrred to direct measurements are found in the southern segments. There the surface speeds relative to the bottom are weak or westward, but adding the very high (over 8 cm/sec) speeds measured near the bottom in that area makes them all eastward. Between Stas. 11 and 12 the bottom speed is about the same as the measurements just north and south but the vertical shear is much reduced, and the speed at shallower depths is much less than that between the adjacent station pairs. The effect o f adjusting the field to the current-meter results can be illustrated by comparing the observed field o f geopotential anomaly relative to 3000 dbars (Fig. 3a) with a field adjusted to correspond to the results of the current meters (Fig. 3b). In constructing Fig. 3b the geopotential anomaly relative to the deepest sampled depth at Sta. 10 was taken as the origin, and increments o f geopotential anomaly were added to each o f the other stations so that the speeds normal to the section at the
56
.Ios~n L. REID and WORT~ D. NOWLIN, JR.
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Fig. 2 (a). Geostrophic speed (cm/sec) relative to depth of deepest samples on a vertical section across the Drake Passage. Positive values indicate eastward flow. Dashed contour is bottom depth on southward leg; solid contour is bottom depth on return leg. Dots represent temperature and salinity observations.
Fig. 2 (b). Geostrophic speed (cm/sec) adjusted to current meters and various interpolations, as described in the text. Crosses indicate positions of current meters, with serial number below and measured speed normal to section above.
depths of the current meters match the component measured by the current meters. In the field relative to 3000 dbars the upward slope to the north between 61 ° and 60 ° requires a westward component of flow everywhere above 3000m; this feature has also been present in all previously reported sections through this area. In the adjusted field (Fig. 3b) the slopes are all downward to the north, indicating eastward flow, except between Stas. 14 and 15, where a weak westward flow is seen below 2500 m. The adjustment for this segment was based on measurements from current meter No. 2; at 2111 m depth (above a ridge or peak) these measurements showed an eastward component decreasing with time (Table 1). In the deep water (4139 m) at Sta. 15 the potential temperature was 0.73°C and the salinity 34.716~oo; these values are not consonant with a Weddell Sea influence, and a major westward flow of Weddell Sea origin between Stas. 14 and 15 does not seem likely. Alternatively we might have used the results of current meter No. 7 to estimate the speed between Stas. 14 and 15. Applying its component of 3.90 cm/sec toward the east at the deepest sample of Sta. 14 (3180 m), instead of the value of 3.27 toward the west at the depth derived from current meter No. 2 at 2111 m, the flow would be everywhere eastward. The surface speed would become 36-3 cm/sec and the transport would increase by about 20 × 106 mS/sec.
Transport of water through the Drake Passage LAT.
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Fig. 3 (a). Geopotential anomaly (dynamic meters) relative to 3000 dbars.
Fig. 3 (b). Geopotentialanomaly(dynamic meters) adjusted to current meter results, beginning the integration from Sta. 10.
DISCUSSION
The low relative speeds between 61 ° and 60°S may be an effect of the bottom topography upon the pressure field. The ridge crossing 60°S at about 58½°W (U.S.H.O. Chart 15254) rises to 1700 m in various places, and extends at least to 59½°S, 59½°W (KROnSr~ and WOOLLARD,1968, Fig. 21) at the 3000 m contour, and possibly farther (Fig. 4). This ridge may be a major obstacle to the deep flow, accounting for the reversal in relative geostrophic flow contoured by CLOWr_S(1933) and GOltOON (1966, 1967b) in the area just west of the ridge. It undoubtedly separates the warm Pacific waters from those more directly affected by the Weddell Sea and accounts for the large temperature differences WOsT (1933) and GORDON (1967b) found in the bottom temperatures in the vicinity of the ridge. In order to investigate this further, we have examined data from a wider area, including the approaches to the Passage from the west and the extension of the flow eastward beyond the ridges near 60°W, into the western Scotia Sea. We show in Figs. 4--7 the relative geostrophic flow and (at depths greater than 3000 m) the near-bottom temperature, salinity and a density parameter. The station data used in the figures were collected aboard the Discovery II, William Scoresby, Meteor, Capitan Canepa, Ob, Eltanin, and Thomas Washington. Figure 4 includes data from nearly all available stations extending to (or nearly to) 3500 m; those in Figs. 5, 6 and 7 include all stations reaching to within 400 m of the bottom in depths below 3000 m. Data were obtained from the National Oceanographic Data Center. The data are from collections made over a period of more than 40 years, and in addition to any real temporal changes they reflect the effects of varying data quality. In Figs. 5-7, which show values
58
Jos~PrI L. REID and W o R ~ D. NowL~, .IR.
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Transport of water through the Drake Passage
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JOSEPHL. ReID and WORTHD. NOWLIN,JR.
at the deepest observations of each station, the varying height above the bottom (up to 400 m) may cause some additional uncertainty. The reversal or weakening in relative geostrophic flow which appears between 60 ° and 61°S on Figs. 2a and 3a, and which has been shown in vertical sections by EsKn~ (1959), OSXAPO~ (1960) and GORDON (1967a), appears clearly in the geostrophic flow at 2000 dbars relative to 3500 dbars. The incoming water approaches slowly from the west and accelerates as the Passage narrows and shoals; after wending its way through the array of ridges, the Pacific water moves eastward as a narrow, rapid stream on the northern side of the Passage. Though the northward shiftmay be the result of the topography alone, the other parameters suggest that it may be associated with an influx of water from the Weddell Sea. If this influx is substantial, it constitutes a major thermohaline effect, which could seriously limit interpretations based solely upon a wind-driven model of the circulation. The potential temperature values at depths below 3 k m and within 400 m of the bottom (Fig. 5) seem to define the field clearly in spite of the limitations. The water coming in from the west is coldest (about 0°C) in the south increasing to a littleover 0.6°C in the north. The gradient is strongest in the north, but this may reflectthe northward shoaling of the bottom. Potential temperatures less than --0.2°C arc found in the Scotia Sea east of 60°W; W O S T (1933) and GORDON (1967b) point out that these waters have come northward from the Wcddell Sea, crossing the Scotia Ridge (along 61°S) at about 39°W. The potential temperature changes abruptly from -- 0.2 to + 0.2°C across the line of ridges. Apparently these ridges separate the warmer water coming in from the west from the colder waters that have entered over the Scotia Ridge, and confine further eastward movement of the warmer waters to the northern part of the Passage. The salinity values at the bottom (Fig. 6) show a similar pattern, with kighcst values in the northwest, a sharp boundary along the ridges, and the lowest salinityin the West Scotia Basin. The density parameter shown in Fig. 7 is the density that the bottom water would have if moved adiabatically to the depth where the pressure is 4000 dbars (LYNN and REID, 1968). The bottom-density of the incoming water, like the temperature and salinity, varies principally north and south. South of 58°S near the 78°W meridian the density parameter is almost constant (about 46.11, or 1.04611 g/cmS), although the depth decreases southward from more than 4000 m to 3000 m; northward from 58°S the density decreases to lessthan 46.04. The lowest value (lessthan 46.0) appears at the tip of South America, and is consonant with a faster geostrophic flow through the constricted area. In the West Scotia Basin values up to 46.16 are found, in the less saline,colder water. The characteristicsof the water from the Weddcll Sea are sccn in all of the parameters (Figs. 5-7), and their effectupon the relativegeostrophic flow is clear in Fig. 4. The resulting loop in the relativeflow has been present whenever measurements wcrc taken, since the first Discovery data used by CLOWES(1933). We assume it is a permanent feature associated with the ridge. Perhaps the cold dense water can accumulate in the southwestern part of the Scotia Basin, despite the strong eastward flow of the Antarctic Circumpolar Current, because of the shielding effect of the ridge. Some east-west exchange of characteristics appears to take place through the gap in the ridge near 59°S, 60°W.
Transport of water through the Drake Passage
61
The effect of this exchange upon the density is apparent in all of the maps of geopotential anomaly. One would be inclined to accept the indicated westward flow near 59°S, 62°W as a real reversal except for the evidence of the fourth and fifth current meters (at 59° 43.5'S, 64° 18'W; 60° 41.5'S, 63° 26'W). Although not located at positions where strong eastward flow is indicated by the water characteristics, both these current meters recorded flow toward the northeast at speeds above 8 cm/sec (Table 1). The meters were recording while hydrographic Stas. 10 and 11 were taken; Sta. 12 was taken a few hours after recovering the fourth current meter. One cannot then assume that the measurements are not simultaneous and represent different situations. Stations 11 and 12 show the whorl in the geopotential anomaly, which is confirmed by the various other data (Figs. 5-7) and has apparently always been found when measurements were made there.
Comparison with other results The relative transport of 113 × 100 mS/sec in Table 2 agrees well with CLOWF.S' (1933) value of 110 × 10° relative to 3500 dbars and with SVERDRUPet al.'s (1942) value of 90 X 106 relative to 3000 dbars but not so well with KORT'S (1959) values at 134 × 106 relative to 3000 dbars and 165 × 10e relative to the bottom, or to EsKltq's (1959) value of 141 × 106 for total transport. OSTAPOFF(1961) pointed out that he could not obtain KORT'S (1959) value from the data KORT (1959) used, and neither can we; apparently KORT (1959) used additional assumptions about the flow. Likewise F_.SKIN(1959) used additional assumptions which are not specific enough for us to duplicate. In order to compare the various sets of data we have recalculated the transport for the Ob data used by KORT(1959) and ESrdS (1959) and for two Discovery II sections. The results are given in Table 3, where they may be compared with the present results. Table 3. Transport through the Drake Passage (106 mS/sec). ,Assuming z e r o Ship Discovery II stations 378, 382-8 Discovery II stations 1312, 14, 16, 18, 20 Ob stations 460, 459, 461-8 Thomas Washington
Year Month
velocity at and below 3000 db* 3500 db* Bottom
1930 April
84
92
92
1934
88
105
117
87
108
113
103
113
113
March
1958 June 1969 January
*Where the bottom was shallower than this, or the observations did not reach this level, the deepest common depth was used.
The various sets of data yield remarkably similar results when treated in the same manner. Indeed, considering the nature of the geostrophic assumptions, it is surprising that the calculations from various sets of data have not shown variations by at least a factor of two. The pressure field appears to be rather stable; the various vertical sections of temperature and salinity across the Drake Passage (CLOWn_S,1933; DEACOlq, 1937; OST~OFF, 1961) are very much like those resulting from the present data. Other investigators have reported calculations based on the assumption of an
62
JOSEPH L. R£m and WORTH D. NOWL1N, JR.
equivalent-barotropic system, introduced by NEUMANN (1956). OSTAPOFF (1960) found, using Discovery II Stas. 382-387, an eastward flow of 40 × 100 ma/sec above a varying reference level and an equal westward flow beneath. In 1961 he reported (for the area north of the polar front which he located at about 59.5° to 60.5°S) eastward transports of 40 and 50 x 100 ma/sec from the Ob data, with more than half of the transport in an 80-km section between Ob Stas. 466 and 467 (at 56° 34-4'S, 63° 07.7'W and 55° 38-7'S, 64° 21.9'W) in the northern part of the section. The corresponding part of the Washington section accounts for less than a quarter of the total transport, which is much more evenly distributed across the section than in the cases discussed by either OST~a,OFF (1960) or GORDON(1967a). The Ob section (Fig. 8) lies immediately east of the ridge, which appears to reach northward to about 60°S, and the section passes through the set of rises centered about 58°S 61°W. All of the calculations made by OSTPa'OFF(1961) and GORDON(1967b), whether relative to isobaric surfaces or to a surface derived from an assumption of an equivalent barotropic system, indicate that the major transport lies north of 59°S; GOgDON (1967) finds more than half of the transport north of 57°S. Considering the presence of the ridges to be a barrier in the south, this seems a plausible result. The 80*
7(3*
60 °
50° 50 °
o°
80*
70*
60*.
70* 50*
Fig. 8. Positions of the hydrographic stations used in the calculations in Table 3. Diamonds indicate Discovery H Stas. 1312-1320. Squares indicate Discovery H Stas. 378-388. Triangles indicate Ob Stas. 459--468. Circles indicate Thomas Washington stations. Depths as in Fig. 1.
Transport of water through the Drake Passage
63
same sort of pressure field is found on the Discovery Hand Washington sections, which are 300 km west (upstream) from these particular ridges, and just east of another series of ridges along 66°W which extend nearly across the Passage and appear to be somewhat more open in the south. OSTAPO~ (1961) finds roughly the same sort of distribution of transport on both the Discovery II and Ob sections, using either isobaric surfaces or a varying depth surface as a reference. On the other hand, the mass distribution observed from the Washington, when combined with the current meter data, shows a much more nearly equal spread of the transport, 49 Yo of the transport through the southern half of the section. Relative to the deepest common depth only 18 % of the transport is through the southern half of the section. One would be inclined to accept the results of the calculations from the Ob stations as representing the real distribution of the flow, with the major part passing north of the ridge, except for the similarity to the relative calculations from the Discovery II and Washington stations: these show relative transports like those from the Ob, but the current measurements show a strong flow in the south, and the ridges in the north do seem a more effective barrier. On the other hand, it does not seem likely that the current measurements made to the west of the ridge could be applied to the southern part of the Ob section, since it lies immediately east of the shallower ridge (Fig. 8), and with colder, less saline water, plainly of eastern origin, at the bottom. It appears plausible that the flow might enter all across the Passage and leave in the northern half, since the southern part is blocked by ridges with water from a different source lying behind them. The Ob transports relative to the deepest common depthwere ll3 × lOe ma/sec, and the part of the section north of the ridge was about 434 km long with a mean depth of about 3500 m. If one assumes that the deficit between the 237 × l0 s mS/see of • absolute' transport from the Washington observations and the Ob relative transport of 113 × lOe mS/see is made up by a greater transport through the northern part of the Ob section, then the calculated increase in speed (the bottom current) would be about 8 cm/sec, remarkably like the deep measurements in the southern part of the Washington section. Since the relative transports were almost identical (about ll3 × 10e mS/see), this is not a surprising result. However, this sort of forced continuity in the flow calculations would not be very convincing. It seems unlikely that the ridges could affect the flow so severely while leaving the pressure field undisturbed. CONCLUSION
Current measurements made near the bottom in the Drake Passage showed mean daily speeds ranging from 0.5 to 14.7 cm/sec; within the southern half of the Passage speeds were about 8 cm/sec toward the northeast. Measurements of temperature and salinity revealed a pressure fieldwhich is remarkably consistent with earliermeasurements. Referring the pressure gradients to those consonant with the measured currents gives an ' absolute' transport of about 237 × I0e mS/see. Since two of the current meters failed and a third was beneath the sampling depth, there is some ambiguity as to how the calculations should be made. Also, the variation shown by the longer series of current measurements limits the confidence that can be placed in such calculations. A n d of course, the assumptions involved in all such calculations of geostrophic flow place additional and even lessprecise limits.
64
JOSEPHL. ItEm and WORTHD. NOWLIN,JR.
The large loop in the relative geostrophic flow may be accounted for by exchange across 60°W at about 59°S of water characteristics between the warm, saline Pacific waters and the colder, less saline and denser waters accumulating on the eastern side of a ridge extending part way across the Passage. The current measurements are not consistent with a westward flow of Scotia Sea bottom water, but rather with a weakening of the eastward flow in that area that may allow, with the strong gradients of temperature and salinity, exchange by mixing. Acknowledgements--This paper represents one of the results of research conducted under the Marine Life Research Program of Scripps Institution of Oceanography, and research conducted under contract with the Office of Naval Research. REFERENCES CAPURRO L. R. A. (1967) Medicion de corrientes supet~iales y profundas en el Pasaje Drake. Publnes. Serv. Hidrogr. Nav., B. Aires, H. 641, 57 pp. CAPURRO L. R. A. (1968) Velocity measurements in the Southern Ocean. Main Review Paper, Section 1: Surface and upper layers. Symposium on Antarctic Oceanography, Santiago, Chile, 13-16 Sept. 1966, Scott Polar Res. Inst., 58-66. CLOWES A. J. (1933) Influence of the Pacific on the circulation in the southwest Atlantic Ocean. Nature, Lend., 131, 189-191. DFACON G. E. R. (1937) The hydrology of the Southern Ocean. Discovery Rep., 15, 1-124. ESrdN L. I. (1959) Contribution to the study of the water and ~ balance of Drake Passage (In Russian). In: Inform. BiulL sov. antarkt. Eksped., No. 12, 29-32. (Eng. transl., Sov. Antarct. Exped. Inf. Bull., 2, 52-55, Elsevier, Amsterdam, 1964). GORDON A. L. (1966) Potential temperature, oxygen and circulation ofbottom water in the Southern Ocean. Deep-Sea Res., 13 (6), 1125-1138. GORDON A. L. (1967a) Geostrophic transport through the Drake Passage. Science, 156 0783), 1732-1734. GORDON A. L. (1967b) Structure of Antarctic waters between 20°W and 170°W. Antarctic Map Folio Sex., Am. Geogr. See., Folio 6, 9 pp., 14 plates. ISAACS J. D., J. L. R I ~ , JR., G. B. SCHICKand R. A. SCHWARTZLOSE(1966) Near-bottom currents measured in 4 kilometers depth off the Baja California coast. J. geophys. Res., 71 (18), 4297--4304. KORT V. G. (1959) New data on the transport of Antarctic waters (In Russian). In: Inform. Biull. soy. antarkt. Eksped., No. 9, 31-34. (Eng. transl., Soy. Anturet. Exped. Inf. Bull., 1, 358-361, Elsevier, Amsterdam, 1964). KROeNKe L. W. and G. P. WOOLLARV(1968) Magnetic Investigations in the Labrador and Scotia Seas, U.S.N.S. Eltanin Cruises 1-10, 1962-1963. Hawaii Inst. Geophysics, Univ. Hawaii, HIG--68-4, 59 pp. (Unpublished manuscript). LED~q~VV. G. (1961) Contribution to the study of surface currents in the seas of the Pacific sector of the Antarctic (In Russian). In: Inform. Biull. sov. antarkt. F_,ksped., No. 27, 18-24. (Eng. transl., Sov. Antaret. Exped. Inf. Bull., 3, 257-262, Elsevier, Amsterdam, 1965). LYNN R. J. and J. L. REID (1968) Characteristics and circulation of deep and abyssal waters. Deep-Sea Res., 15, 577-598. MONACO, BUREAU HYVROORAPmQu-e INTI~ATIONAL (1968) Carte G ~ a l e Bathym6trique des ~ , FeuilleB'l,la 4• blition,publi~e~tParispar l'InstitutGb3graphique National. N ~ G. (1956) Z u m Problem der 'Dynamischen Bezugsflache' imbesondere im Golfstromgebeit. Dt. Hydrogr. Z., 9 (2), 66-78. OSTAPO1,FF. (1960) On the mass transport through the Drake Passase. J. geophys. Res., 65 (9), 2861-2868. OSTAVO~ F. (1961) A contribution to the problem of the Drake Passage Circulation. DeepSea Res., 8, 111-120. REID J. L. (1969) Preliminary results of measurements of deep currents in the Pacific Ocean. Nature, Lend., 221 (5183), 848. SVeRDRUP H. U., M. W. JOHNSONand R. H. ~ G (1942) The oceans; their physics, chemistry, and general b/o/ogy. Prentice Hall, New York, 1087 pp., 7 charts. WOsr G. (1933) Das Bodenwasser und die Gliederung der a t l a n ~ Tiefsee. Wiss. Ergebn. dt. atlant. Exped. "Meteor" 1925-27, 6 (1), (1), 107 pp.
1971, Vol. 18, pp. 51 to 64. PergamonPrea. Printed in Great Britain.
T r a n s p o r t of water t h r o u g h t h e D r a k e P a s s a g e JOSEPH L. Rmr~* and WORTH D. NOWL~, JR.1" (Received 9 February 1970) Abstraet~t meters placed near the bottom in the Drake Passage have been used with calculations of relative geostrophic speed to estimate transport of water from the Pacific to the Atlantic Ocean. The concurrent temperature and salinity measurements reveal a pressure field that is very much like those from earlier expeditions. Relative to the greatest depths sampled, a 8eostrophic transport of 113 × 10ema/sec was obtained. Adjusting this to the current meter restflts yields an • absolute' eastward transport of 237 x 10e mS/see, which is considerably higher than earlier estimates. In earlier results the major transport appeared to be confined to the northern part; the present results show the transport to be distributed more uniformly across the Passage.
INTRODUCTION
IN Om~ERto investigate the transport of the Antarctic Circumpolar Current, measurements of temperature, salinity and near-bottom currents were made across the Drake Passage during the period from 18 to 27 January 1969 (Fig. 1) on the third leg of the Scripps Institution of Oceanography PIQUERO expedition aboard the Thomas
Washington. The temperature and salj,ity measurements were made at discrete depths from the surface to the bottom at nine positions on a line between Cape Horn and Snow Island (near the Antarctic Peninsula) (Fig. 2). Current meters were placed 300 m above the bottom between the hydrographic station positions. Six of the current meters were deployed on the southward crossing: they were recovered, and the temperature and salinity measurements made, on the return path. Meters were not placed between the first pair or the last pair of stations, since these spanned the continental slopes at the northern and southern ends of the line. The southernmost meter (No. 6) was set to operate for 24 hr and the others for successively longer periods, up to 125 hr, to allow for the travel and working time before the ship would be in the proper area to recover them. No. 1 (the northernmost) gave direction only and No. 3 gave no record at all. A seventh current meter was then deployed at about the position of Sta. 15; it operated successfully for 25 hr, returned to the surface and was recovered. Only celestial navigation was available, and the sky was overcast much of the time. Fixes were thus infrequent and of limited accuracy, and the return track was offset somewhat from the southward track, though not so much as to hinder recovery of the current meters. The offset was only a few miles, but the irregular topography produced rather different bottom profiles on the two tracks (Fig. 2). The current meters are similar to those used at about 4000 m depth off California (ISAACS, REID, SCHICK and SCHWARTZLOSE, 1966) and in the deep passage near Samoa (REID, 1969). Each has a rotor, a vane, a recorder, a float, and an anchor. *Scri_'pps Institution of Oceanography. J~3flke of Naval Research (now with Texas A & M University).
51
52
JOSEPH L. REID and WORTHD. Nowta~, JR.
50'
80 °
70*
60 °
50° 50°
60'
60*
70' 80 °
70*
60*
70 ° 50 °
Fig. 1. Positions of hydrographic stations (circles) and current measurements (squares) in the Drake Passage. Bottom topography is taken from the Carte G~n~rale J~tthym~trique des Oedans (MONACO,1968) with some alterations taken from KROENKEand WOOLLA~(1968)and from the soundings taken on the PIQUERO expedition. Dark shading indicates depth 0-3000 m; light shading 3000--4000m; white areas > 4000 m.
After falling freely to the bottom, each instrument operates for a preset period, releases the anchor, and returns to the surface, where it transmits a radio signal to aid in its recovery. Although the field accuracy of the meters could not be evaluated as well as we would like, it is not likely to be better than 0.2 cm/sec, and on any single lowering it might be worse. The earlier results off California and near Samoa (ISAAC,S e t al., 1966; REID, 1969) have been encouraging as to the precision and the comparability of the various instruments. Since some of the series were quite long, we have calculated the means to hundredths of cm/sec, and they are so tabulated and applied as the best estimate in the following calculations of transport. We note, however, that an error of 1 era/see applied to the transport between two stations 100 k m apart in 4 k m of water would be 4 × 10~ m3/sec. Realistically we must acknowledge that errors this large are not impossible. The spacing of the hydrographic casts and the current meters was arranged so
Transport of v~tcr through the Drake Passage
~,~ ~
~
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o o o o
~
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/
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53
54
Josm~ L. REID and WORTH D. NOWLIN, JR.
that the relative geostrophic flow (calculated from the density field) could be adjusted to the direct observations made by the current meters. Thus an' absolute' geostrophic transport through the Drake Passage could be calculated. Whether the adjustment to the relatively few and short series of current measurements really yields ' absolute' transport is subject to some doubt, and could be answered only by more detailed and longer current measurements. RESULTS
The current measurements For each half hour the speed and direction were determined from the total number of revolutions of the rotor and the mean direction of the vane. The daily means and grand means for each meter are given in Table 1. Oscillations of approximately diurnal and semidiurnal periods were apparent, with the semidiurnal amplitude larger than the diurnal and, in the northern part of the Passage, sometimes greater than the mean. At the two southernmost current meters the flow was essentially rectilinear. At the other locations a counterclockwise elliptical rotation was indicated with the major axes of the semidiurnals approximately parallel to the direction of the mean flow except in the third and fourth days of current meter No. 2, when the mean flow was perhaps too weak, relative to the fluctuations, for a good estimate of direction. The daily mean velocities varied with both space and time but in most cases were surprisingly high and consistently eastward. Calculations of speed Velocity fields normal to the section have been calculated in three ways. (a) The geostrophic flow relative to the flow at the deepest common depth of each pair of adjacent stations was calculated (Fig. 2a). Co) The flow was adjusted to the measured currents for the station pairs 10-11, 11-12, 12-13, and 14-15; for the others the flow was calculated relative to the deepest common depth for each pair. (These results are seen in Table 2 but are not illustrated.) (c) The flow was adjusted to current meters where available, otherwise as follows: pair 13-14 was adjusted to a value of 2.88 cm/sec at the bottom (interpolated from the horizontally adjacent adjusted values); pair 15-17 was adjusted to the 3.90 cm/sec measurement at current meter 7, assumed to hold also at the deepest sampling depth of station 15; pair 9-10 was adjusted to 15.71 cm/sec at the deepest sample depth of station 9 (horizontally extrapolated from pair 10-11); and pair 17-16 was adjusted to 26.49 cm/sec at the deepest sample depth of station 16 (horizontally extrapolated from pair 15-17). Speeds in the areas of the Passage outside the sampling grid (north, south, and below) have been calculated by extending the adjacent grid values to the boundary. These results are seen in Fig. 2b. The speed and transport Calculations of geostrophic speed relative to the deepest common sampling depth of the various pairs of stations show surface speeds from 36.1 cm/sec (eastward) to 4.5 cm/sec (westward) (Fig. 2a). An assumption of zero flow at the deepest common sampling depth leads to a weak westward flow above 3000 m between Stas. 11 and 12; this becomes eastward when adjusted to the current meter results. On the
Transport of water through the Drake Passage
55
Table 2. Transport (10e mS/see).
Side to9 Ta Cz c, ca
Station pairs
9-10 10-11 11-12 12-13 13-14 14-15 15-17 17-16 16toside 2~ 1
2
(6) 6
Total 2
13
17 37
--6 38
11 20
23
0
0
2
(11) 0
54
32
33
34
34 -9 -1 24
32
1
(8) 1
(13) 10
8
113 86 38 28
41
24
8
265
2~g
199 237 265
Ta ~- gemtrophic transport calculated relative to deepest common depth of each pair of stations. Cx ----correction to relate the transport to current meters where available. C~ ----correction to remaining pairs based on horizontal interpolations between current meters. Cs ----correction for areas outside the sampling grid (sides and bottom). other hand, the current meter between Stas. 14 and 15 showed a mean eastward flow o f 2.47 cm/sec, but the vertical shear in the geostrophic flow was so great that referring the flow to this current meter results in a weak westward flow below 2500 m (maximum 3.27 cm/sec). I.~D~r~v (1961) reports the results o f G E K measurements made aboard the Ob including a northeastward leg crossing the present section at about 57 ° 30'S, between Stas. 15 and 17; the two nearest G E K measurements showed 051 °, 68 cm/sec and 061 °, 44 cm/sec. These directions are approximately normal to the section, which showed 40 cm/sec between Stas. 15 and 17 when adjusted to the current meter. CAPURRO'S (1967, 1968) measurements in the Drake Passage included two at depths greater than 2000 m, made with Swallow floats. These two were launched at the same position (56 ° M'S, 65 ° 56'W) and showed about 5.5 cm/sec toward 332 ° at some 2250 m and about 8.3 cm/sec toward 318 ° at some 2500 m. This location is about 140 k m southeast of Cape Horn and 60 km northeast of one of the ridges which rise above the 3000 m contour and extend well into the Passage; the measured flow may reflect the topographic effects. The calculated transports are given in Table 2. The transport relative to the common depths is about 113 × 10emS/sec, and the transport adjusted to the current measurements is about 237 × 10e n~/sec. The major differences between the calculations based on a level of assumed zero motion and those referrred to direct measurements are found in the southern segments. There the surface speeds relative to the bottom are weak or westward, but adding the very high (over 8 cm/sec) speeds measured near the bottom in that area makes them all eastward. Between Stas. 11 and 12 the bottom speed is about the same as the measurements just north and south but the vertical shear is much reduced, and the speed at shallower depths is much less than that between the adjacent station pairs. The effect o f adjusting the field to the current-meter results can be illustrated by comparing the observed field o f geopotential anomaly relative to 3000 dbars (Fig. 3a) with a field adjusted to correspond to the results of the current meters (Fig. 3b). In constructing Fig. 3b the geopotential anomaly relative to the deepest sampled depth at Sta. 10 was taken as the origin, and increments o f geopotential anomaly were added to each o f the other stations so that the speeds normal to the section at the
56
.Ios~n L. REID and WORT~ D. NOWLIN, JR.
LAT.
62*
STA. .9 /0 Om ~62-
61"
60*
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12 13 14 /5 17 16 15.4 17.6 52.5 36.1 Z2
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1500
--6------8"--15----~20~,
__4 ~
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3500
4000
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4500
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S
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5000
Fig. 2 (a). Geostrophic speed (cm/sec) relative to depth of deepest samples on a vertical section across the Drake Passage. Positive values indicate eastward flow. Dashed contour is bottom depth on southward leg; solid contour is bottom depth on return leg. Dots represent temperature and salinity observations.
Fig. 2 (b). Geostrophic speed (cm/sec) adjusted to current meters and various interpolations, as described in the text. Crosses indicate positions of current meters, with serial number below and measured speed normal to section above.
depths of the current meters match the component measured by the current meters. In the field relative to 3000 dbars the upward slope to the north between 61 ° and 60 ° requires a westward component of flow everywhere above 3000m; this feature has also been present in all previously reported sections through this area. In the adjusted field (Fig. 3b) the slopes are all downward to the north, indicating eastward flow, except between Stas. 14 and 15, where a weak westward flow is seen below 2500 m. The adjustment for this segment was based on measurements from current meter No. 2; at 2111 m depth (above a ridge or peak) these measurements showed an eastward component decreasing with time (Table 1). In the deep water (4139 m) at Sta. 15 the potential temperature was 0.73°C and the salinity 34.716~oo; these values are not consonant with a Weddell Sea influence, and a major westward flow of Weddell Sea origin between Stas. 14 and 15 does not seem likely. Alternatively we might have used the results of current meter No. 7 to estimate the speed between Stas. 14 and 15. Applying its component of 3.90 cm/sec toward the east at the deepest sample of Sta. 14 (3180 m), instead of the value of 3.27 toward the west at the depth derived from current meter No. 2 at 2111 m, the flow would be everywhere eastward. The surface speed would become 36-3 cm/sec and the transport would increase by about 20 × 106 mS/sec.
Transport of water through the Drake Passage LAT.
62 °
STA. 9 I~ .98 Orn
6l o
60 °
/I
/2
t.16
L09
59 °
/3 1.22
58 °
/4 IA7
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62 °
/7 /6
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50
1000
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2000
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57 59 °
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1.85 2.05
~8 o
57os
/4
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236
2G8 2.99 320
50C.
5000
Fig. 3 (a). Geopotential anomaly (dynamic meters) relative to 3000 dbars.
Fig. 3 (b). Geopotentialanomaly(dynamic meters) adjusted to current meter results, beginning the integration from Sta. 10.
DISCUSSION
The low relative speeds between 61 ° and 60°S may be an effect of the bottom topography upon the pressure field. The ridge crossing 60°S at about 58½°W (U.S.H.O. Chart 15254) rises to 1700 m in various places, and extends at least to 59½°S, 59½°W (KROnSr~ and WOOLLARD,1968, Fig. 21) at the 3000 m contour, and possibly farther (Fig. 4). This ridge may be a major obstacle to the deep flow, accounting for the reversal in relative geostrophic flow contoured by CLOWr_S(1933) and GOltOON (1966, 1967b) in the area just west of the ridge. It undoubtedly separates the warm Pacific waters from those more directly affected by the Weddell Sea and accounts for the large temperature differences WOsT (1933) and GORDON (1967b) found in the bottom temperatures in the vicinity of the ridge. In order to investigate this further, we have examined data from a wider area, including the approaches to the Passage from the west and the extension of the flow eastward beyond the ridges near 60°W, into the western Scotia Sea. We show in Figs. 4--7 the relative geostrophic flow and (at depths greater than 3000 m) the near-bottom temperature, salinity and a density parameter. The station data used in the figures were collected aboard the Discovery II, William Scoresby, Meteor, Capitan Canepa, Ob, Eltanin, and Thomas Washington. Figure 4 includes data from nearly all available stations extending to (or nearly to) 3500 m; those in Figs. 5, 6 and 7 include all stations reaching to within 400 m of the bottom in depths below 3000 m. Data were obtained from the National Oceanographic Data Center. The data are from collections made over a period of more than 40 years, and in addition to any real temporal changes they reflect the effects of varying data quality. In Figs. 5-7, which show values
58
Jos~PrI L. REID and W o R ~ D. NowL~, .IR.
0,..~
o
"
~o
0 ,aw~ .
0 ~0
59
Transport of water through the Drake Passage
~o
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¢D
60
JOSEPHL. ReID and WORTHD. NOWLIN,JR.
at the deepest observations of each station, the varying height above the bottom (up to 400 m) may cause some additional uncertainty. The reversal or weakening in relative geostrophic flow which appears between 60 ° and 61°S on Figs. 2a and 3a, and which has been shown in vertical sections by EsKn~ (1959), OSXAPO~ (1960) and GORDON (1967a), appears clearly in the geostrophic flow at 2000 dbars relative to 3500 dbars. The incoming water approaches slowly from the west and accelerates as the Passage narrows and shoals; after wending its way through the array of ridges, the Pacific water moves eastward as a narrow, rapid stream on the northern side of the Passage. Though the northward shiftmay be the result of the topography alone, the other parameters suggest that it may be associated with an influx of water from the Weddell Sea. If this influx is substantial, it constitutes a major thermohaline effect, which could seriously limit interpretations based solely upon a wind-driven model of the circulation. The potential temperature values at depths below 3 k m and within 400 m of the bottom (Fig. 5) seem to define the field clearly in spite of the limitations. The water coming in from the west is coldest (about 0°C) in the south increasing to a littleover 0.6°C in the north. The gradient is strongest in the north, but this may reflectthe northward shoaling of the bottom. Potential temperatures less than --0.2°C arc found in the Scotia Sea east of 60°W; W O S T (1933) and GORDON (1967b) point out that these waters have come northward from the Wcddell Sea, crossing the Scotia Ridge (along 61°S) at about 39°W. The potential temperature changes abruptly from -- 0.2 to + 0.2°C across the line of ridges. Apparently these ridges separate the warmer water coming in from the west from the colder waters that have entered over the Scotia Ridge, and confine further eastward movement of the warmer waters to the northern part of the Passage. The salinity values at the bottom (Fig. 6) show a similar pattern, with kighcst values in the northwest, a sharp boundary along the ridges, and the lowest salinityin the West Scotia Basin. The density parameter shown in Fig. 7 is the density that the bottom water would have if moved adiabatically to the depth where the pressure is 4000 dbars (LYNN and REID, 1968). The bottom-density of the incoming water, like the temperature and salinity, varies principally north and south. South of 58°S near the 78°W meridian the density parameter is almost constant (about 46.11, or 1.04611 g/cmS), although the depth decreases southward from more than 4000 m to 3000 m; northward from 58°S the density decreases to lessthan 46.04. The lowest value (lessthan 46.0) appears at the tip of South America, and is consonant with a faster geostrophic flow through the constricted area. In the West Scotia Basin values up to 46.16 are found, in the less saline,colder water. The characteristicsof the water from the Weddcll Sea are sccn in all of the parameters (Figs. 5-7), and their effectupon the relativegeostrophic flow is clear in Fig. 4. The resulting loop in the relativeflow has been present whenever measurements wcrc taken, since the first Discovery data used by CLOWES(1933). We assume it is a permanent feature associated with the ridge. Perhaps the cold dense water can accumulate in the southwestern part of the Scotia Basin, despite the strong eastward flow of the Antarctic Circumpolar Current, because of the shielding effect of the ridge. Some east-west exchange of characteristics appears to take place through the gap in the ridge near 59°S, 60°W.
Transport of water through the Drake Passage
61
The effect of this exchange upon the density is apparent in all of the maps of geopotential anomaly. One would be inclined to accept the indicated westward flow near 59°S, 62°W as a real reversal except for the evidence of the fourth and fifth current meters (at 59° 43.5'S, 64° 18'W; 60° 41.5'S, 63° 26'W). Although not located at positions where strong eastward flow is indicated by the water characteristics, both these current meters recorded flow toward the northeast at speeds above 8 cm/sec (Table 1). The meters were recording while hydrographic Stas. 10 and 11 were taken; Sta. 12 was taken a few hours after recovering the fourth current meter. One cannot then assume that the measurements are not simultaneous and represent different situations. Stations 11 and 12 show the whorl in the geopotential anomaly, which is confirmed by the various other data (Figs. 5-7) and has apparently always been found when measurements were made there.
Comparison with other results The relative transport of 113 × 100 mS/sec in Table 2 agrees well with CLOWF.S' (1933) value of 110 × 10° relative to 3500 dbars and with SVERDRUPet al.'s (1942) value of 90 X 106 relative to 3000 dbars but not so well with KORT'S (1959) values at 134 × 106 relative to 3000 dbars and 165 × 10e relative to the bottom, or to EsKltq's (1959) value of 141 × 106 for total transport. OSTAPOFF(1961) pointed out that he could not obtain KORT'S (1959) value from the data KORT (1959) used, and neither can we; apparently KORT (1959) used additional assumptions about the flow. Likewise F_.SKIN(1959) used additional assumptions which are not specific enough for us to duplicate. In order to compare the various sets of data we have recalculated the transport for the Ob data used by KORT(1959) and ESrdS (1959) and for two Discovery II sections. The results are given in Table 3, where they may be compared with the present results. Table 3. Transport through the Drake Passage (106 mS/sec). ,Assuming z e r o Ship Discovery II stations 378, 382-8 Discovery II stations 1312, 14, 16, 18, 20 Ob stations 460, 459, 461-8 Thomas Washington
Year Month
velocity at and below 3000 db* 3500 db* Bottom
1930 April
84
92
92
1934
88
105
117
87
108
113
103
113
113
March
1958 June 1969 January
*Where the bottom was shallower than this, or the observations did not reach this level, the deepest common depth was used.
The various sets of data yield remarkably similar results when treated in the same manner. Indeed, considering the nature of the geostrophic assumptions, it is surprising that the calculations from various sets of data have not shown variations by at least a factor of two. The pressure field appears to be rather stable; the various vertical sections of temperature and salinity across the Drake Passage (CLOWn_S,1933; DEACOlq, 1937; OST~OFF, 1961) are very much like those resulting from the present data. Other investigators have reported calculations based on the assumption of an
62
JOSEPH L. R£m and WORTH D. NOWL1N, JR.
equivalent-barotropic system, introduced by NEUMANN (1956). OSTAPOFF (1960) found, using Discovery II Stas. 382-387, an eastward flow of 40 × 100 ma/sec above a varying reference level and an equal westward flow beneath. In 1961 he reported (for the area north of the polar front which he located at about 59.5° to 60.5°S) eastward transports of 40 and 50 x 100 ma/sec from the Ob data, with more than half of the transport in an 80-km section between Ob Stas. 466 and 467 (at 56° 34-4'S, 63° 07.7'W and 55° 38-7'S, 64° 21.9'W) in the northern part of the section. The corresponding part of the Washington section accounts for less than a quarter of the total transport, which is much more evenly distributed across the section than in the cases discussed by either OST~a,OFF (1960) or GORDON(1967a). The Ob section (Fig. 8) lies immediately east of the ridge, which appears to reach northward to about 60°S, and the section passes through the set of rises centered about 58°S 61°W. All of the calculations made by OSTPa'OFF(1961) and GORDON(1967b), whether relative to isobaric surfaces or to a surface derived from an assumption of an equivalent barotropic system, indicate that the major transport lies north of 59°S; GOgDON (1967) finds more than half of the transport north of 57°S. Considering the presence of the ridges to be a barrier in the south, this seems a plausible result. The 80*
7(3*
60 °
50° 50 °
o°
80*
70*
60*.
70* 50*
Fig. 8. Positions of the hydrographic stations used in the calculations in Table 3. Diamonds indicate Discovery H Stas. 1312-1320. Squares indicate Discovery H Stas. 378-388. Triangles indicate Ob Stas. 459--468. Circles indicate Thomas Washington stations. Depths as in Fig. 1.
Transport of water through the Drake Passage
63
same sort of pressure field is found on the Discovery Hand Washington sections, which are 300 km west (upstream) from these particular ridges, and just east of another series of ridges along 66°W which extend nearly across the Passage and appear to be somewhat more open in the south. OSTAPO~ (1961) finds roughly the same sort of distribution of transport on both the Discovery II and Ob sections, using either isobaric surfaces or a varying depth surface as a reference. On the other hand, the mass distribution observed from the Washington, when combined with the current meter data, shows a much more nearly equal spread of the transport, 49 Yo of the transport through the southern half of the section. Relative to the deepest common depth only 18 % of the transport is through the southern half of the section. One would be inclined to accept the results of the calculations from the Ob stations as representing the real distribution of the flow, with the major part passing north of the ridge, except for the similarity to the relative calculations from the Discovery II and Washington stations: these show relative transports like those from the Ob, but the current measurements show a strong flow in the south, and the ridges in the north do seem a more effective barrier. On the other hand, it does not seem likely that the current measurements made to the west of the ridge could be applied to the southern part of the Ob section, since it lies immediately east of the shallower ridge (Fig. 8), and with colder, less saline water, plainly of eastern origin, at the bottom. It appears plausible that the flow might enter all across the Passage and leave in the northern half, since the southern part is blocked by ridges with water from a different source lying behind them. The Ob transports relative to the deepest common depthwere ll3 × lOe ma/sec, and the part of the section north of the ridge was about 434 km long with a mean depth of about 3500 m. If one assumes that the deficit between the 237 × l0 s mS/see of • absolute' transport from the Washington observations and the Ob relative transport of 113 × lOe mS/see is made up by a greater transport through the northern part of the Ob section, then the calculated increase in speed (the bottom current) would be about 8 cm/sec, remarkably like the deep measurements in the southern part of the Washington section. Since the relative transports were almost identical (about ll3 × 10e mS/see), this is not a surprising result. However, this sort of forced continuity in the flow calculations would not be very convincing. It seems unlikely that the ridges could affect the flow so severely while leaving the pressure field undisturbed. CONCLUSION
Current measurements made near the bottom in the Drake Passage showed mean daily speeds ranging from 0.5 to 14.7 cm/sec; within the southern half of the Passage speeds were about 8 cm/sec toward the northeast. Measurements of temperature and salinity revealed a pressure fieldwhich is remarkably consistent with earliermeasurements. Referring the pressure gradients to those consonant with the measured currents gives an ' absolute' transport of about 237 × I0e mS/see. Since two of the current meters failed and a third was beneath the sampling depth, there is some ambiguity as to how the calculations should be made. Also, the variation shown by the longer series of current measurements limits the confidence that can be placed in such calculations. A n d of course, the assumptions involved in all such calculations of geostrophic flow place additional and even lessprecise limits.
64
JOSEPHL. ItEm and WORTHD. NOWLIN,JR.
The large loop in the relative geostrophic flow may be accounted for by exchange across 60°W at about 59°S of water characteristics between the warm, saline Pacific waters and the colder, less saline and denser waters accumulating on the eastern side of a ridge extending part way across the Passage. The current measurements are not consistent with a westward flow of Scotia Sea bottom water, but rather with a weakening of the eastward flow in that area that may allow, with the strong gradients of temperature and salinity, exchange by mixing. Acknowledgements--This paper represents one of the results of research conducted under the Marine Life Research Program of Scripps Institution of Oceanography, and research conducted under contract with the Office of Naval Research. REFERENCES CAPURRO L. R. A. (1967) Medicion de corrientes supet~iales y profundas en el Pasaje Drake. Publnes. Serv. Hidrogr. Nav., B. Aires, H. 641, 57 pp. CAPURRO L. R. A. (1968) Velocity measurements in the Southern Ocean. Main Review Paper, Section 1: Surface and upper layers. Symposium on Antarctic Oceanography, Santiago, Chile, 13-16 Sept. 1966, Scott Polar Res. Inst., 58-66. CLOWES A. J. (1933) Influence of the Pacific on the circulation in the southwest Atlantic Ocean. Nature, Lend., 131, 189-191. DFACON G. E. R. (1937) The hydrology of the Southern Ocean. Discovery Rep., 15, 1-124. ESrdN L. I. (1959) Contribution to the study of the water and ~ balance of Drake Passage (In Russian). In: Inform. BiulL sov. antarkt. Eksped., No. 12, 29-32. (Eng. transl., Sov. Antarct. Exped. Inf. Bull., 2, 52-55, Elsevier, Amsterdam, 1964). GORDON A. L. (1966) Potential temperature, oxygen and circulation ofbottom water in the Southern Ocean. Deep-Sea Res., 13 (6), 1125-1138. GORDON A. L. (1967a) Geostrophic transport through the Drake Passage. Science, 156 0783), 1732-1734. GORDON A. L. (1967b) Structure of Antarctic waters between 20°W and 170°W. Antarctic Map Folio Sex., Am. Geogr. See., Folio 6, 9 pp., 14 plates. ISAACS J. D., J. L. R I ~ , JR., G. B. SCHICKand R. A. SCHWARTZLOSE(1966) Near-bottom currents measured in 4 kilometers depth off the Baja California coast. J. geophys. Res., 71 (18), 4297--4304. KORT V. G. (1959) New data on the transport of Antarctic waters (In Russian). In: Inform. Biull. soy. antarkt. Eksped., No. 9, 31-34. (Eng. transl., Soy. Anturet. Exped. Inf. Bull., 1, 358-361, Elsevier, Amsterdam, 1964). KROeNKe L. W. and G. P. WOOLLARV(1968) Magnetic Investigations in the Labrador and Scotia Seas, U.S.N.S. Eltanin Cruises 1-10, 1962-1963. Hawaii Inst. Geophysics, Univ. Hawaii, HIG--68-4, 59 pp. (Unpublished manuscript). LED~q~VV. G. (1961) Contribution to the study of surface currents in the seas of the Pacific sector of the Antarctic (In Russian). In: Inform. Biull. sov. antarkt. F_,ksped., No. 27, 18-24. (Eng. transl., Sov. Antaret. Exped. Inf. Bull., 3, 257-262, Elsevier, Amsterdam, 1965). LYNN R. J. and J. L. REID (1968) Characteristics and circulation of deep and abyssal waters. Deep-Sea Res., 15, 577-598. MONACO, BUREAU HYVROORAPmQu-e INTI~ATIONAL (1968) Carte G ~ a l e Bathym6trique des ~ , FeuilleB'l,la 4• blition,publi~e~tParispar l'InstitutGb3graphique National. N ~ G. (1956) Z u m Problem der 'Dynamischen Bezugsflache' imbesondere im Golfstromgebeit. Dt. Hydrogr. Z., 9 (2), 66-78. OSTAPO1,FF. (1960) On the mass transport through the Drake Passase. J. geophys. Res., 65 (9), 2861-2868. OSTAVO~ F. (1961) A contribution to the problem of the Drake Passage Circulation. DeepSea Res., 8, 111-120. REID J. L. (1969) Preliminary results of measurements of deep currents in the Pacific Ocean. Nature, Lend., 221 (5183), 848. SVeRDRUP H. U., M. W. JOHNSONand R. H. ~ G (1942) The oceans; their physics, chemistry, and general b/o/ogy. Prentice Hall, New York, 1087 pp., 7 charts. WOsr G. (1933) Das Bodenwasser und die Gliederung der a t l a n ~ Tiefsee. Wiss. Ergebn. dt. atlant. Exped. "Meteor" 1925-27, 6 (1), (1), 107 pp.