Northward transport of antarctic bottom water in the western Atlantic Ocean

Decp-Soa Research, 1970, Vol. 17, pp. 367 to 371. Pergamon Pross. Printed in Great Britain.

Northward transport of Antarctic Bottom Water in the Western Atlantic Ocean* W. R. WmGH~

(Received 30 May 1969) Abstract--The volume transport of Antarctic Bottom Water in the western basin of the Atlantic Ocean has been determined by dynamic calculations for seven oceanographic sections made during the International Geophysical Year between 32°S and 16°N. The reference level was based on the sharp bend, seen in both temperature--depth and salinity--depth traces, that marks the transition from North Atlantic Deep Water to Antarctic Bottom Water. The northward transport decreases from 5-6 × 10° mZ/sec in the southern sections to about 10e mS/'sec in the northern sections. The results are consistent with those obtained by solving a set of conservation equations for a simple box model of the deep circulation in the western Atlantic.:~ SINCE the time of the Challenger Expedition nearly a century ago it has been known (BucHhN, 1895, quoted in WAR,N, 1969) that the Antarctic Bottom Water is an important factor in the circulation of the deep Atlantic Ocean, but it is not at all well known how big its contribution is. In the Atlantic the Antarctic Bottom Water can be recognized as a very cold and somewhat less saline water mass underlying the warmer and more saline North Atlantic Deep Water, and extending in the western basin as far north as 35°N (Fig. 1). Except for a very small quantity in the vicinity of the Romanche Trench, this water is not found in the eastern basin north of about 25°S because it is deep enough to be blocked by the Walvis and Mid-Atlantic ridges. This distribution was shown by the Meteor survey of 1925-1927 (WOsT 1935) and was confirmed in the International Geophysical Year survey of 1957-1958 (FuGuSTER 1960). Figure 1 (McGILL 1963) was based on IGY data, but it closely resembles Wrist's profile for the western basin. As the distribution of Antarctic Bottom Water appears constant in time, at least during the 30 years separating the two surveys, it is clear that if any of it is being transported northward in the western basin, it must eventually mix into the southward-moving North Atlantic Deep Water. This would imply a reduction in the northern transport of the bottom water as it moves north and an increase in the southern transport of the deep water as it moves south. Such a picture has been borne out to some degree by the few transport estimates which have been made, although none of them has been reinforced by direct current measurements, and the results have not been entirely consistent. Transport figures in SVEg.DRtW, et al. (1942), based partly on Meteor data, show the Antarctic Bottom Water carrying 3 × 106 m3/sec northward at 30°S and 10e mZ/sec at the equator, and the North Atlantic Deep Water carrying 9 × 10e m3/sec southward at the equator, increasing to 18 × 106 at 30°S. The discrepancy is balanced in the upper layers. Comprehensive dynamic calculations were made by Wiast for the Meteor data, including transport computations for six sections between 33°S and 19°N. He used an intermediate reference level based on the method of DEvAr,rr (1941), which sloped from less than 1000 m in the north to about 2000 m in the south and roughly approximated the boundary between the North Atlantic Deep Water and the Antarctic Intermediate Water. The results (Wf2sT, 1955 ; WOST, 1957) indicated that the strongest transport of both Antarctic Bottom Water and North Atlantic Deep Water was concentrated along the South American coast, thus anticipating the abyssal theory of STOMMELand ARoNs (1960), but in some respects they were not satisfactory. The transport in the western basin varied widely from section to section: although the average Antarctic Bottom Water transport for the six sections was 4 × l0 Bm3/sec to the north, the individual values ranged from 12 × 10e northbound at the section from 6-19°N to 2 × 106 southbound at 28°S, with no apparent pattern. The North Atlantic Deep *Contribution No. 2328 from the Woods Hole Oceanographic Institution. tWoods Hole Oceanographic Institution, Woods Hole, Mass. U.S.A. :~A version of this paper was given at the annual meeting of the American Geophysical Union in Washington, D.C., in April 1969. The abstract was published in Trans. Am. Geophys. Un., 50, (4), 193. 367

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Fig. 1. N o r t h - s o u t h profiles of temperature (top) and salinity in the western basin of the Atlantic Ocean, using I.G.Y. data between 45°N and 35°S, extended by earlier observations of Discovery H in the south, Marion and General Greene in the north (from McGILL, 1963).

Water transport averaged 25 x 106 ma/sec to the south but ranged from 57 x 106 at 10°S to 4 x 106 at 33°S, actually decreasing toward the south. In a different approach, transport figures for the deep water of the western basin were calculated by means of a box model, which involved solution of a system of equations for the conservation of heat, mass, salt, carbon-14 and oxygen. I G Y data were used. The results (We.iGrrr, 1969) indicated a deep ocean somewhat more active than Sverdrup's: The Antarctic Bottom Water transport decreased from 7 x 106 mS/sec at 32°S to 1"4 x 106 mS/sec at 16°N, and the N o r t h Atlantic Deep Water transport increased from 13 x 106 m~/sec at 16°N to 20 x 106 at 32°S. This report deals with dynamic calculations for the deep western basin using the I G Y data published by FUGLISTER(1960). The effort seemed worthwhile, even in the absence of direct measurements, because of the relative abundance of good deep temperature and salinity data. Although section spacing and the number of bottles at each station were about the same as in the Meteor survey, the I G Y data density is nearly twice that of the Meteor because the stations were closer together. Furthermore, the I G Y salinities were done by conductivity salinometer and were reported to 0-001%o, a big advantage over titrated values, particularly in the deep water.

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The sections used (Fig. 2) are at eight-degree intervals of latitude from 32°S to 16°N except at the equator, where the geostrophic equation is not valid. The southern portion of Atlantis Cruise 229 along 50°W, which crossed the basin between the 8°N and 16°N sections, was also included. The observations were all made between November 1956 and May 1959. A reference level was picked between the Antarctic Bottom Water and the N o r t h Atlantic Deep Water, using the temperature-depth and salinity-depth traces, both of which have a sharp change in slope at the transition between the two water masses. The resulting reference level sloped from about 3400 m at 32°S to 4400 m at 16°N, corresponding roughly to the depth of the 2 ° isotherm and the 34'89~o isohaline. The transport calculations were made by 200-meter intervals, using the method of t~I~LAr,'D-HANsEN (1934) for extrapolation of dynamic heights along the bottom. Because of the extrapolation and other uncertainties in the method, and because of occasional sparseness of data, the results are not considered reliable to better than ~ 25 %. The Antarctic Bottom Water transports are shown in Fig, 3. On each section west is to the left, northward transport is up and black, southbound transport is white and down. Two points are

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Fig. 3. Volume transports of Antarctic Bottom Water. Black is northward transport, white is to the south. Net transport, in 106 ma/sec, is printed to the right of each section. Question marks are used in the western end of the 16°N section because thele were no observations colder than 2 ° at Stas. 306-308. noteworthy : (1) although it is little uneven, there is a systematic decrease in tranport from about 6 x 106 mZ/sec in the southern sections to about 106 in the northern sections, corresponding to the box model results ; (2) although in the southern sections the bulk of the northward flow is on the western side of the basin, in the three northern sections it is on the east, which might be expected as a result of the change of sign of the coriolis parameter at the equator. The transport of the North Atlantic Deep Water has been calculated at 16°N, 8°S and 32°S. At all three latitudes the transport is close to 9 × 106 mZ/sec toward the south. Even though the value at 16°N is roughly comparable to Sverdrup and the box model results, there is no evidence of an increase to the south. It is possible that some deep water crosses the Mid-Atlantic Ridge into the eastern basin at depths shallower than 3000 m; the deep eastern basin is, after all, filled with North Atlantic Deep Water north of the Walvis Ridge. However, dynamic computations across the eastern ends of the sections using a 3000-m reference level gave no indication of such a flow. Another possibility is an upward flux of North Atlantic Deep Water into the overlying Antarctic Intermediate Water, but it is generally believed that if there is any net exchange between these two water masses it is in the other direction. The results given here support those of the box model for the Antarctic Bottom Water but arc inconclusive in the North Atlantic Deep Water. What is needed now is a series of direct measurements in combination with oceanographic stations that are even more closely spaced than those of the IGY, particularly along the sides of the basin where boundary currents may be found. Acknowledgement--This work was supported by the Office of Naval Research under Contract NO 0014-66-CO 241-12. REFERENCES

BUCHAN A. (1895) Report on oceanic circulation. Rep. scient. Res. H.M.S. Challenger 1872-76. Summary of Results, Second Part, Appendix (Physics and Chemistry, Part VIII) 38 pp. DEFA~rr A. (1941) Die absolute Topographic des physikalischen Meeresniveaus und der Druckflachen, sowie die Wasserbewegungen im Atlantischen Ozean. Wiss Ergebn. dt. atlant. Exped. Meteor 1925-1927, 6, (2), (5), 191-260. FUGLIS~R F. C. (1960) Atlantic Ocean Atlas of Temperature and Salinity Profiles and Data for the International Geophysical Year of 1957-1958. Atlas Seri~s, Woods Hole Oceanographic Institution, 1.

Shorter Contributions

371

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