RESEARCH LETTERS

 

Surface drift at the western edge of the Agulhas Bank

 

 

J.R.E. Lutjeharms; J.V. Durgadoo; I.J. Ansorge

Department of Oceanography, University of Cape Town, Private Bag, Rondebosch 7701,South Africa

 

 

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ABSTRACT

The Agulhas Bank is a wide continental shelf that forms the southern tip of the African continent. On the eastern side of this shelf the flow of water is dominated by the adjacent Agulhas Current. On its western border, the movement is more complex. It is influenced by the Benguela Current, by the occasional presence of products from the Agulhas Current such as Agulhas rings, Agulhas filaments and by lee eddies. Understanding the flow on this western side of the Agulhas Bank is of considerable ecological importance because it has been assumed that a shelf edge jet carries immotile or weakly motile fish larvae and eggs from the spawning region on the bank to the biologically productive regions of the Benguela upwelling regime. We have used the tracks of a set of surface drifters to study the movement at the western edge of the bank, and show that on average the movement is indeed equatorward along this shelf edge, but that this movement is not persistent in direction or magnitude. Instead, this movement appears to be driven entirely by mesoscale turbulence created at the termination of the Agulhas Current.

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References

1. Lutjeharms J.R.E., Catzel R. and Valentine H.R. (1989). Eddies and other border phenomena of the Agulhas Current. Cont. Shelf Res. 9, 597-616.         [ Links ]

2. Beckley L.E. (1998). The Agulhas Current ecosystem with particular reference to dispersal of fish larvae. In Large Marine Ecosystems of the Indian Ocean, eds K. Sherman, E.N. Okemwa and M.J. Ntiba, pp. 255-276. Blackwell Science, Oxford.         [ Links ]

3. Luschi P., Sale A., Mencacci R., Hughes G.R., Lutjeharms J.R.E. and Papi F. (2003). Current transport of leatherback sea turtles (Dermochelys coriacea) in the ocean. Proc. R. Soc. Lond. (Suppl.), 270, S129-S132; doi 10.1098/rsbl.203.0036.         [ Links ]

4. Luschi P., Lutjeharms J.R.E., Lambardi P., Mencacci R., Hughes G.R. and Hayes G.C. (2006). A review of migratory behaviour of sea turtles off southeastern Africa. S. Afr. J. Sci. 102, 51-58.         [ Links ]

5. Lutjeharms J.R.E. (2006). The Agulhas Current. Springer, Berlin.         [ Links ]

6. Lutjeharms J.R.E. and van Ballegooyen R.C. (1988). The retroflection of the Agulhas Current. J. Phys. Oceanogr. 18, 1570-1583.         [ Links ]

7. Lutjeharms J.R.E. and Valentine H.R. (1988). Evidence for persistent Agulhas rings south-west of Cape Town. S. Afr. J. Sci. 84, 781-783.         [ Links ]

8. Boebel O., Lutjeharms J.R.E., Schmid C., Zenk W., Rossby T. and Barron C. (2003). The Cape Cauldron: a regime of turbulent inter-ocean exchange. Deep-Sea Res. II 50, 57-86.         [ Links ]

9. Byrne D.A., Gordon A.L. and Haxby W.F. (1995). Agulhas eddies: a synoptic view using Geosat ERM data. J. Phys. Oceanogr. 25, 902-917.         [ Links ]

10. Penven P., Lutjeharms J.R.E., Marchesiello P., Weeks S.J. and Roy C. (2001). Generation of cyclonic eddies by the Agulhas Current in the lee of the Agulhas Bank. Geophys. Res. Lett. 28, 1055-1058.         [ Links ]

11. Lutjeharms J.R.E., Penven P. and Roy C. (2003). Modelling the shear edge eddies of the southern Agulhas Current. Cont. Shelf Res. 23, 1099-1115.         [ Links ]

12. Lutjeharms J.R.E. and Cooper J. (1996). Interbasin leakage through Agulhas Current filaments. Deep-Sea Res. I 43, 213-238.         [ Links ]

13. Fowler J.L. and Boyd A.J. (1998). Transport of anchovy and sardine eggs and larvae from the western Agulhas Bank to the west coast during the 1993/94 and 1994/95 spawning seasons. S. Afr. J. Mar. Sci. 19, 181-195.         [ Links ]

14. Van der Lingen C.D., Hutchings L., Merkle D., van der Westhuizen J.J. and Nelson J. (2001). Comparative spawning habitats of anchovy (Engraulis capensis) and sardine (Sardinops sagax) in the southern Benguela upwelling ecosystem. In Spatial Processes and Management of Marine Populations, eds G.H. Kruse, N. Bez, A. Booth, M.W. Dorn, S. Hills, R.N. Lipcius, D. Pelletier, C. Roy, S.J. Smith and D. Witherell, pp. 185-209. University of Alaska Sea Grant, Fairbanks.         [ Links ]

15. Barange M., Hampton I. and Roel B.A. (1999). Trends in the abundance and distribution of anchovy and sardine on the South African continental shelf in the 1990s, deduced from acoustic surveys. S. Afr. J. Mar. Sci. 21, 367-391.         [ Links ]

16. BeckleyL.E. and van der Lingen C.D. (1999). Biology, fishery and management of sardines (Sardinops sagax) in southern African waters. Mar. Freshw. Res. 50, 955-978.         [ Links ]

17. Roel B.A., Hewitson J., Kerstan S. and Hampton I. (1994). The role of the Agulhas-Bank in the life-cycle of pelagic fish. S. Afr. J. Sci. 90, 185-196.         [ Links ]

18. Boyd A.J., Taunton-ClarkJ. and Oberholster G.PJ. (1992). Spatial features of the near-surface and midwater circulation patterns off western and southern South Africa and their role in the life histories of various commercially fished species. S. Afr. J. Mar. Sci. 12, 189-206.         [ Links ]

19. Lett C., Roy C., Levasseur A., van der Lingen C.D. and Mullon C. (2006). Simulation and quantification of enrichment and retention processes in the southern Benguela upwelling ecosystem. Fish. Oceanogr. 15, 363-372.         [ Links ]

20. Hutchings L. (1994). The Agulhas Bank: a synthesis of available information and a brief comparison with other east-coast shelf regions. S. Afr. J. Sci., 90, 179-185.         [ Links ]

21. Hutchings L., Beckley L.E., Griffiths M.H., Roberts M.J., Sundby S. and van der Lingen C. (2002). Spawning on the edge: spawning grounds and nursery areas around the southern African coastline. Mar. Freshw. Res. 53, 307-318.         [ Links ]

22. Roy C., Weeks S., Rouault M., Nelson G., Barlow R. and van der Lingen C. (2001). Extreme oceanographic events recorded in the Southern Benguela during the 1999-2000 summer season. S. Afr. J. Sci. 97, 465-471.         [ Links ]

23. Nelson G. and Hutchings L. (1987). Passive transportation of pelagic system components in the Southern Benguela area. S. Afr. J. Mar. Sci. 5, 223-234.         [ Links ]

24. Huggett J., Fréon P., Mullon C. and Penven P. (2003). Modelling the transport success of anchovy Engraulis encrasicolus eggs and larvae in the southern Benguela: the effect of spatio-temporal spawning patterns. Mar. Ecol. Progr. Ser. 250, 247-262.         [ Links ]

25. Van derLingen C.D. and Huggett J.A. (2003). The role of ichthyoplankton surveys in recruitment, research and management of South African anchovy and sardine. In The Big Fish Bang, Proc. 26th Annual Larval Fish Conference, eds H.I. Browman and A.B. Skiftesvik, pp. 303-343. Institute of Marine Research, Bergen, Norway.         [ Links ]

26. Bang N.D. and Andrews W.R.H. (1974). Direct current measurements of a shelf-edge frontal jet in the southern Benguela system. J. Mar. Res. 32, 405-417.         [ Links ]

27. Armstrong D.A., Mitchell-Innes B.A., Verheye-Dua F., Waldron H. and Hutchings L. (1987). Physicaland biological features across an upwelling front in the Southern Benguela. S. Afr. J. Mar. Res. 5, 171-190.         [ Links ]

28. Shelton P.A. and Hutchings L. (1982). Transport of anchovy, Engraulis capensis Gilchrist, eggs and early larvae by a frontal jet current. J. Cons. Int. Explor. Mer 40, 185-198.         [ Links ]

29. Huggett J.A.,BoydA.J., Hutchings L. and Kemp A.D. (1998). Weekly variability of clupeoid eggs and larvae in the Benguela jet current: implications for recruitment. S. Afr. J. Mar. Sci. 19, 197-210.         [ Links ]

30. Armstrong D.A., Mitchell-Innes B.A., Verheye-Dua F., Waldron H. and Hutchings L. (1987). Physical and biological features across an upwelling front in the southern Benguela. S. Afr. J. Mar. Sci. 5, 171-190.         [ Links ]

31. SSALTO/DUACS User Handbook (2006). (M)SLA and (M)ADT Near-real time and delayed time products. Version 1rev5.         [ Links ]

32. Boebel O. and Barron C. (2003). A comparison of in-situ float velocities with altimeter derived geostrophic velocities. Deep-Sea Res. II 50, 119-139.         [ Links ]

33. Garzoli S.L., Richardson P.L., Duncombe Rae C.M., Fratantoni D.M., Goñi G.J. and Roubicek A.J. (1999). Three Agulhas rings observed during the Benguela Currrent Experiment. J. Geophys. Res. 104, 20, 971-20, 985.         [ Links ]

34. Lutjeharms J.R.E. and Matthysen C.P. (1995). A recurrent eddy in the upwelling front off Cape Town. S. Afr. J. Sci. 91, 355-357.         [ Links ]

35. Schouten M.W., de Ruijter W.P.M., van Leeuwen P.J. and Lutjeharms J.R.E. (2000). Translation, decay and splitting of Agulhas rings in the south-eastern Atlantic Ocean. J. Geophys. Res. 105, 21,913-21,925.         [ Links ]

 

 

Received 6 October 2006.
Accepted 13 December 2006.

 

 

† Author for correspondence. E-mail: johann.lutjeharms@uct.ac.za