Geo-Marine Letters

, Volume 31, Issue 5–6, pp 301–314 | Cite as

Topographic control on the nascent Mediterranean outflow

  • M. Gasser
  • J. L. Pelegrí
  • J. D. Nash
  • H. Peters
  • J. García-Lafuente
Original

Abstract

Data collected during a 12-day cruise in July 2009 served to examine the structure of the nascent Mediterranean Outflow Water (MOW) immediately west of the Espartel Sill, the westernmost sill in the Strait of Gibraltar. The MOW is characterized by high salinities (>37.0 and reaching 38.3) and high velocities (exceeding 1 m s−1 at 100 m above the seafloor), and follows a submerged valley along a 30 km stretch, the natural western extension of the strait. It is approx. 150 m thick and 10 km wide, and experiences a substantial drop from 420 to 530 m over a distance of some 3 km between two relatively flat regions. Measurements indicate that the nascent MOW behaves as a gravity current with nearly maximal traveling speed; if this condition is maintained, then the maximum MOW velocity would decrease slowly with distance from the Espartel Sill, remaining significantly high until the gravity current excess density is only a small fraction of its original value. The sharp pycnocline between the Mediterranean and the overlying North Atlantic Central waters is dynamically unstable, particularly where the flow interacts with the 100 m decrease in bottom depth. Here, subcritical gradient Richardson numbers coincide with the development of large interfacial undulations and billows. The very energetic downslope flow is likely responsible for the development of a narrow V-shaped channel downstream of the seafloor drop along the axis of the submerged valley, this probably being the very first erosional scour produced by the nascent MOW. The coincidence of subcritical gradient Richardson numbers with relatively high turbidity values above the channel flanks suggests it may be undergoing upstream erosion.

Notes

Acknowledgements

The authors are deeply grateful to all aboard the R/V García del Cid, in particular Captain Eduardo Otal and technicians José Antonio Pozo, Maribel Lloret and Joaquín Salvador, as well as Mikhail Emelianov and Sam Kelly for their help in gathering data during the cruise. The authors are also very grateful to Gemma Ercilla and two anonymous reviewers for many useful comments. Funding for this work comes from the Ministerio de Ciencia e Innovación, Spain, through project MOC2 (CTM2008-06438-C02-01), project INGRES (CTM2006-02326), and two complementary actions (CTM2008-03422-E/MAR and CTM2009-05810-E/MAR); funding also comes from the US National Science Foundation (grants OCE-0825287 and OCE-0825297).

References

  1. Ambar I, Howe MR (1979) Observations of the Mediterranean outflow I. Mixing in the Mediterranean outflow. Deep Sea Res A 26:535–554CrossRefGoogle Scholar
  2. Ambar I, Armi L, Bower A, Ferreira T (1999) Some aspects of time variability of the Mediterranean water off south Portugal. Deep Sea Res I 46:1109–1136CrossRefGoogle Scholar
  3. Baraza J, Ercilla G, Nelson CH (1999) Potential geologic hazards on the eastern Gulf of Cadiz slope (SW Spain). Mar Geol 155:191–215CrossRefGoogle Scholar
  4. Baringer MO, Price JF (1997a) Mixing and spreading of the Mediterranean outflow. J Phys Oceanogr 27:1654–1677CrossRefGoogle Scholar
  5. Baringer MO, Price JF (1997b) Momentum and energy balance of Mediterranean outflow. J Phys Oceanogr 27:1678–1692CrossRefGoogle Scholar
  6. Baringer MO, Price JF (1999) A review of the physical oceanography of the Mediterranean outflow. Mar Geol 155:63–82CrossRefGoogle Scholar
  7. Benjamin TB (1968) Gravity current and related phenomena. J Fluid Mech 31:209–248CrossRefGoogle Scholar
  8. Borenäs KM, Wahlin AK, Ambar I, Serra N (2002) The Mediterranean outflow splitting—a comparison between theoretical models and CANIGO data. Deep Sea Res II 49:4195–4205CrossRefGoogle Scholar
  9. Bower AS, Serra N, Ambar I (2002) Structure of the Mediterranean Undercurrent and Mediterranean Water spreading around the southwestern Iberian Peninsula. J Geophys Res 107:3161–3179CrossRefGoogle Scholar
  10. Bryden HL, Stommel HM (1984) Limiting processes that determine basic features of the circulation in the Mediterranean Sea. Oceanol Acta 7:289–296Google Scholar
  11. Candela J, Beardsley RC, Limeburner R (1990) Removing tides from ship-mounted ADCP data, with application to the Yellow Sea. In: Appell GF, Curtin TB (eds) Proc IEEE 4th Working Conf Current Measurements. Institute of Electrical and Electronics Engineers, New York, pp 258–266CrossRefGoogle Scholar
  12. García M, Hernández-Molina FJ, Llave E, Stow DAV, León R, Fernández-Puga MC, Díaz del Río V, Somoza L (2009) Contourite erosive features caused by the Mediterranean Outflow Water in the Gulf of Cadiz: quaternary tectonic and oceanographic implications. Mar Geol 257:24–40CrossRefGoogle Scholar
  13. García Lafuente J, Sánchez Román A, Díaz del Río G, Sannino G, Sánchez Garrido JC (2007) Recent observations of seasonal variability of the Mediterranean outflow in the Strait of Gibraltar. J Geophys Res 112:C10005. doi:10.1029/2006JC003992 CrossRefGoogle Scholar
  14. García-Lafuente J, Delgado J, Sánchez Román A, Soto J, Carracedo L, Díaz del Río G (2009) Interannual variability of the Mediterranean outflow observed in Espartel Sill, western Strait of Gibraltar. J Geophys Res 114:C10018. doi:10.1029/2009JC005496 CrossRefGoogle Scholar
  15. Habgood EL, Kenyon NH, Masson DG, Akhmetzhanov A, Weaver PPE, Gardner J, Mulder T (2003) Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic. Sedimentology 50:483–510CrossRefGoogle Scholar
  16. Hernández-Molina FJ, Llave E, Stow DAV, García M, Somoza L, Vázquez JT, Lobo FJ, Maestro A, Díaz del Río V, León R, Medialdea T, Gardner J (2006) The contourite depositional system of the Gulf of Cadiz: a sedimentary model related to the bottom current activity of the Mediterranean outflow water and its interaction with the continental margin. Deep Sea Res II 53:1420–1463CrossRefGoogle Scholar
  17. Holst SE, Koseff JR, Ferziger JH (1992) A numerical study of the evolution and structure of homogeneous stable stratified sheared turbulence. J Fluid Mech 237:499–539CrossRefGoogle Scholar
  18. Johnson GC, Lueck RG, Sanford TB (1994) Stress on the Mediterranean outflow plume: Part II. Turbulent dissipation and shear measurements. J Phys Oceanogr 24:2084–2092CrossRefGoogle Scholar
  19. Kennett JP (1982) Marine geology. Prentice-Hall, Englewood CliffsGoogle Scholar
  20. Kenyon NH, Belderson RH (1973) Bed forms of the Mediterranean undercurrent observed with side-scan sonar. Sediment Geol 9:77–99CrossRefGoogle Scholar
  21. León R, Somoza L, Medialdea T, Hernández-Molina FJ, Vázquez JT, Díaz-del-Río V, González FJ (2010) Pockmarks, collapses and blind valleys in the Gulf of Cádiz. Geo Mar Lett 30(3/4):231–247. doi:10.1007/s00367-009-0169-z CrossRefGoogle Scholar
  22. Llave E, Schönfeld J, Hernández-Molina FJ, Mulder T, Somoza L, Díaz del Río V, Sánchez-Almazo I (2006) High-resolution stratigraphy of the Mediterranean outflow contourite system in the Gulf of Cadiz during the late Pleistocene: the impact of Heinrich events. Mar Geol 227:241–262CrossRefGoogle Scholar
  23. Llave E, Hernández-Molina SDAV, Fernández-Puga MC, García M, Vázquez JT, Maestro A, Somoza L, Díaz del Río V (2007) Reconstructions of the Mediterranean Outflow Water during the Quaternary based on the study of changes in buried mounded drift stacking pattern in the Gulf of Cadiz. Mar Geophys Res 28:379–394CrossRefGoogle Scholar
  24. Madelain F (1970) Influence de la topographie du fond sur l’écoulement méditerranéen entre le Détroit de Gibraltar et le Cap Saint-Vincent. Cahiers Océanogr 22:43–61Google Scholar
  25. Mulder T, Voisset M, Lecroart P, Le Drezen E, Gonthier E, Hanquiez V, Faugères J-C, Habgood E, Hernandez-Molina FJ, Estrada F, Llave-Barranco E, Poirier D, Gorini C, Fuchey Y, Voelker A, Freitas P, Lobo Sanchez F, Fernandez LM, Kenyon NH, Morel J (2003) The Gulf of Cádiz: an unstable giant contouritic levee. Geo Mar Lett 23(1):7–18. doi:10.1007/s00367-003-0119-0 CrossRefGoogle Scholar
  26. Nelson CH, Baraza J, Maldonado A, Rodero J, Escutia C, Barber JH Jr (1999) Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the Gulf of Cadiz continental margin. Mar Geol 155:99–129CrossRefGoogle Scholar
  27. Ochoa J, Bray NA (1991) Water mass exchange in the Gulf of Cádiz. Deep Sea Res 38:S465–S503CrossRefGoogle Scholar
  28. Pawlovicz R, Beardsley B, Lentz S (2002) Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE. Comput Geosci 28:929–937CrossRefGoogle Scholar
  29. Pelegrí JL (1988) Tidal fronts in estuaries. Estuar Coast Shelf Sci 27:45–60CrossRefGoogle Scholar
  30. Pelegrí JL, Sangrà P (1998) A mechanism for layer formation in stratified geophysical flows. J Geophys Res 103:30679–30693CrossRefGoogle Scholar
  31. Price JF, Baringer MO, Lueck RG, Johnson GC, Ambar I, Parrilla G, Cantos A, Kennelly MA, Sanford TB (1993) Mediterranean outflow mixing and dynamics. Science 259:1277–1282CrossRefGoogle Scholar
  32. Sánchez-Román A, Criado-Aldeanueva F, García-Lafuente J, Sánchez JC (2008) Vertical structure of tidal currents over Espartel and Camarinal sills, Strait of Gibraltar. J Mar Syst 74:120–133CrossRefGoogle Scholar
  33. Serra N, Ambar I, Kase RH (2005) Observations and numerical modelling of the Mediterranean outflow splitting and eddy generation. Deep Sea Res II 52:383–408CrossRefGoogle Scholar
  34. Stanton BR (1983) Low frequency variability in the Mediterranean outflow west of Gibraltar. Deep Sea Res 30:743–761CrossRefGoogle Scholar
  35. Van Gastel P, Pelegrí JL (2004) Estimates of gradient Richardson numbers from vertically smoothed data. Sci Mar 68:459–482Google Scholar
  36. Xu X, Chassignet EP, Price JF, Özgökmen TM, Peters H (2007) A regional modeling study of the entraining Mediterranean outflow. J Geophys Res 112:C12005. doi:10.1029/2007JC004145 CrossRefGoogle Scholar
  37. Zenk W (1975) On the Mediterranean outflow west of Gibraltar. Meteor Forsch Ergebnisse A 16:23–34Google Scholar
  38. Zenk W, Armi L (1990) The complex spreading patterns of Mediterranean water off the Portuguese continental slope. Deep-Sea Res 37:1805–1823CrossRefGoogle Scholar
  39. Zitellini N, Gràcia E, Matias L, Terrinha P, Abreu MA, DeAlteriis G, Henriet JP, Dañobeitia JJ, Masson DG, Mulder T, Ramella R, Somoza L, Diez S (2009) The quest for the Africa-Eurasia plate boundary west of the Strait of Gibraltar. Earth Planet Sci Lett 280:13–50CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • M. Gasser
    • 1
  • J. L. Pelegrí
    • 1
  • J. D. Nash
    • 2
  • H. Peters
    • 3
  • J. García-Lafuente
    • 4
  1. 1.Departament d’Oceanografia FísicaInstitut de Ciències del Mar, CSICBarcelonaSpain
  2. 2.College of Oceanic and Atmospheric SciencesOregon State UniversityCorvallisUSA
  3. 3.Earth and Space ResearchSeattleUSA
  4. 4.Grupo de Oceanografía FísicaUniversidad de MálagaMálagaSpain

Personalised recommendations