The Deep Water Regime in the Equatorial Atlantic

  • M. Rhein
  • F. Schott
  • J. Fischer
  • U. Send
  • L. Stramma

Abstract

The importance of the Deep Western Boundary Current (DWBC) in the Atlantic for the interhemispheric exchange of water masses and of heat is well known, but data to estimate transports and to follow its pathways are sparse, especially in the equatorial Atlantic. New insight into the distribution of water masses in the DWBC, their transports and their variability off Brazil were gained in a contribution to the WOCE (World Ocean Circulation Experiment) program. In these studies, moored current meter measurements were combined with shipboard data from three cruises in the years 1990, 1991 and 1992. Besides tracer (Chlorofluoromethanes CFMs, components F11 and F12) and hydrographic data, direct velocity measurements were carried out using a lowered ADCP attached to the CTD, and the Pegasus profiling system.

The estimated transports of deep water in the equatorial Atlantic, net eastward transport of 19–22 Sv at 44°W, and 26.8 ± 7.0 Sv at 35°W, net southward transport of 19.5 ± 5.3 Sv at 5°S, are in the range of previously published estimates farther west and south. The data show significant spatial and temporal variability of the flow field and of the estimated transports as well as variability in the tracer distributions. This can lead to large uncertainties in the interpretation of single cruise observations.

The transient tracer distributions (CFMs and tritium) along the DWBC from the northern North Atlantic to 10°S have been used within a box model to estimate the mean spreading velocities of the tracer bearing water masses of the DWBC. Together with assumptions about the mean spatial extent of the DWBC and the vertical velocity structure a mean transport of the DWBC of 9–12 Sv is obtained. These numbers represent the spatially and temporally averaged net DWBC transport, i.e. the deep part of the thermohaline circulation. Thus, the tracer derived estimates are comparable to the estimates from inverse calculations. The different results between the transports obtained from direct observations on one side and from inverse calculations and tracer distributions on the other side are thought to be caused by various recirculation cells along the path of the DWBC. Indications for various recirculation paths of part of the NADW have also been found off Brazil.

Keywords

Convection Tritium Verse Rhein Chlorofluoromethane 

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References

  1. Bryden HL, Hall MM (1980) Heat transport by currents across 25°N in the Atlantic Ocean. Science 207:884–886CrossRefGoogle Scholar
  2. Fine RA, Molinari RL (1988) A continuous deep western boundary current between Abaco (26.5°N) and Barbados (13°N). Deep-Sea Res. 35:1441–1450CrossRefGoogle Scholar
  3. Fischer J, Visbeck M (1993) Deep velocity profiling with self-contained ADCPs. J Atm Ocean Techn. 10:764–773CrossRefGoogle Scholar
  4. Fischer J, Rhein M, Schott F, Stramma L (1995) Deep Water masses and transports in the Vema Fracture Zone. Deep-Sea Res (submitted)Google Scholar
  5. Friedrichs MAM, McCartney MS, Hall MM (1994) Hemispheric asymmetry of deep water transport modes in the Atlantic. J Geophys Res 99:25196–25179Google Scholar
  6. Hall MM, Bryden HL (1982) Direct estimates and mechanisms of ocean heat transport. Deep-Sea Res 29:339–359CrossRefGoogle Scholar
  7. McCartney MS (1993) Crossing of the equator by the Deep Western Boundary Current in the Western Atlantic Ocean. J Phys Oceanogr 23:1953–1974CrossRefGoogle Scholar
  8. Molinari RL, Fine RA, Johns E (1992) The Deep Western Boundary Current in the western tropical North Atlantic Ocean. Deep-Sea Res 39:1967–1984CrossRefGoogle Scholar
  9. Pickart RS (1992) Water mass components of the North Atlantic Deep Western Boundary Current. Deep-Sea Res 39:1553–1572CrossRefGoogle Scholar
  10. Pickart RS, Hogg NG, Smethie WM Jr (1989) Determining the strength of the Deep Western Boundary Current using the chlorofluoromethane ratio. J Phys Oceanogr 19:940–951CrossRefGoogle Scholar
  11. Read JF, Gould WJ (1992) Cooling and freshening of the subpolar North Atlantic since the 1960s. Nature 360:55–57CrossRefGoogle Scholar
  12. Rhein M (1994) The Deep Western Boundary Current: Tracers and Velocities. Deep-Sea Res 41:263–281CrossRefGoogle Scholar
  13. Rhein M, Stramma L, Send U (1995) The Atlantic Deep Western Boundary Current: Water masses and transports near the equator. J Geophys Res 100:2441–2457CrossRefGoogle Scholar
  14. Rhein M (1995) The Shallow Component of the Atlantic Deep Western Boundary Current: tracers, velocities and transports. Deep-Sea Res (submitted)Google Scholar
  15. Richardson PL, Schmitz WJ (1993) Deep cross-equa- torial flow in the Atlantic measured with SOFAR floats. J Geophys Res 98:8371–8387CrossRefGoogle Scholar
  16. Roemmich D (1980) Estimation of meridional heat flux in the North Atlantic by inverse methods. J Phys Oceanogr 10:1972–1983CrossRefGoogle Scholar
  17. Schmitz WJ Jr, McCartney MS (1993) On the North Atlantic Circulation. Rev of Geophys 31:29–49CrossRefGoogle Scholar
  18. Schott F, Fischer J, Reppin J, Send U (1993) On mean and seasonal currents and transports at the western boundary of the equatorial Atlantic. J Geophys Res 98:14353–14368CrossRefGoogle Scholar
  19. Spain PF, Dorson DL, Rossby HT (1981) PEGASUS: A simple acoustically tracked velocity profiler. Deep-Sea Res 28:1553–1567CrossRefGoogle Scholar
  20. Speer KG, McCartney MS (1991) Tracing lower North Atlantic Deep Water across the equator. J Geophys Res 96:20,443–20,448Google Scholar
  21. Watts DR (1991) Equatorwards currents in the temperature range 2°-6°C on the continental slope, Mid Atlantic Bight. In: Chu H, Gascard JG (eds) Deep water convection and deep water formation. Elsevier, New York, pp 183–196CrossRefGoogle Scholar
  22. Wunsch C, Grant B (1982) Towards the general circulation of the North Atlantic Ocean. Prog Oeeanogr 11:1–59.CrossRefGoogle Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 1996

Authors and Affiliations

  • M. Rhein
    • 1
  • F. Schott
    • 1
  • J. Fischer
    • 1
  • U. Send
    • 1
  • L. Stramma
    • 1
  1. 1.Institut für Meereskunde an der Universität KielKielGermany

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