Ocean Dynamics

, Volume 67, Issue 10, pp 1351–1365 | Cite as

Wind-induced subduction at the South Atlantic subtropical front

Article
Part of the following topical collections:
  1. Topical Collection on the 48th International Liège Colloquium on Ocean Dynamics, Liège, Belgium, 23-27 May 2016

Abstract

The South Atlantic Subtropical Front, associated with the eastward-flowing South Atlantic Current, separates the colder, nutrient-rich waters of the subpolar gyre from the warmer, nutrient-poor waters of the subtropical gyre. Perturbations to the quasi-geostrophic, eastward flow generate meanders and filaments which induce cross-frontal exchange of water properties. Down-front winds transport denser waters from the South over warm waters from the North, inducing convective instability and subduction. Such processes occur over spatial scales of the order of 1 km and thus require high horizontal spatial resolution. In this modeling study, a high-resolution (4 km) regional grid is embedded in a basin-wide configuration (12 km) of the South Atlantic Ocean in order to test the importance of submesoscale processes in water mass subduction along the subtropical front. Stronger and more numerous eddies obtained in the high-resolution run yield more intense zonal jets along the frontal zone. Such stronger jets are more susceptible to instabilities, frontogenesis, and the generation of submesoscale meanders and filaments with \(\mathcal {O}(1)\) Rossby number. As a consequence, vertical velocities larger than 100 md 1 are obtained in the high-resolution run, one order of magnitude larger than in the low-resolution run. Wind-driven subduction occurs along the frontal region, associated with negative Ertel potential vorticity in the surface layer. Such processes are not observed in the low-resolution run. A passive tracer experiment shows that waters with density characteristics similar to subtropical mode waters are preferentially subducted along the frontal region. The wind-driven buoyancy flux is shown to be much larger than thermal or haline fluxes during the wintertime, which highlights the importance of the frictional component in extracting PV from the surface ocean and inducing subduction, a process that has been overlooked in subtropical mode water formation in the region.

Keywords

Subtropical front Submesoscale Wind-driven subduction 

Notes

Acknowledgements

This research was funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Bolsa de Produtividade em Pesquisa (Process: 306971/2016-0) and Project 457118/2012-1. Funding from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) - Projeto REMARSUL (Processo CAPES 23038.004299/2014-53) is also acknowledged. The author would like to thank one anonymous reviewer for comments that significantly improved the manuscript.

References

  1. Bachman S, Taylor J (2014) Modelling of partially-resolved oceanic symmetric instability. Ocean Model 82:15–27CrossRefGoogle Scholar
  2. Belkin IM, Gordon AL (1996) Southern ocean fronts from the greenwich meridian to tasmania. J Geophys Res Oceans 101(C2):3675–3696CrossRefGoogle Scholar
  3. Carton JA, Giese BS (2008) A reanalysis of ocean climate using simple ocean data assimilation (soda). Mon Weather Rev 136(8):2999–3017CrossRefGoogle Scholar
  4. D’Asaro E, Lee C, Rainville L, Harcourt R, Thomas L (2011) Enhanced turbulence and energy dissipation at ocean fronts. Science 332(6027):318–322CrossRefGoogle Scholar
  5. Debreu L, Vouland C, Blayo E (2008) Agrif: adaptive grid refinement in fortran. Comput Geosci 34 (1):8–13CrossRefGoogle Scholar
  6. Eady ET (1949) Long waves and cyclone waves. Tellus 1(3):33–52CrossRefGoogle Scholar
  7. Egbert GD, Erofeeva SY (2002) Efficient inverse modeling of barotropic ocean tides. J Atmos Ocean Technol 19(2):183–204CrossRefGoogle Scholar
  8. Fox-Kemper B, Ferrari R, Hallberg R (2008) Parameterization of mixed layer eddies. Part i: Theory and diagnosis. J Phys Oceanogr 38(6):1145–1165CrossRefGoogle Scholar
  9. Gordon AL, Weiss RF, Smethie WM, Warner MJ (1992) Thermocline and intermediate water communication between the south atlantic and indian oceans. J Geophys Res Oceans 97(C5):7223–7240CrossRefGoogle Scholar
  10. Guinehut S, Dhomps A, Larnicol G, Le Traon PY (2012) High resolution 3-d temperature and salinity fields derived from in situ and satellite observations. Ocean Sci 8(5):845–857CrossRefGoogle Scholar
  11. Hart J (1996) On nonlinear ekman surface-layer pumping. J Phys Oceanogr 26(7):1370–1374CrossRefGoogle Scholar
  12. Hosegood P, Gregg M, Alford M (2013) Wind-driven submesoscale subduction at the north pacific subtropical front. J Geophys Res Oceans 118(10):5333–5352CrossRefGoogle Scholar
  13. Joyce TM, Thomas LN, Bahr F (2009) Wintertime observations of subtropical mode water formation within the gulf stream. Geophys Res Lett 36:L02607. doi: 10.1029/2008GL035918 CrossRefGoogle Scholar
  14. Karleskind P, Lévy M, Mémery L (2011) Modifications of mode water properties by sub-mesoscales in a bio-physical model of the northeast atlantic. Ocean Model 39(1):47–60CrossRefGoogle Scholar
  15. Lévy M, Klein P, Tréguier A M, Iovino D, Madec G, Masson S, Takahashi K (2010) Modifications of gyre circulation by sub-mesoscale physics. Ocean Model 34(1):1–15CrossRefGoogle Scholar
  16. Mahadevan A, Tandon A (2006) An analysis of mechanisms for submesoscale vertical motion at ocean fronts. Ocean Model 14:241–256CrossRefGoogle Scholar
  17. Mahadevan A, Tandon A, Ferrari R (2010) Rapid changes in mixed layer stratification driven by submesoscale instabilities and winds. J Geophys Res 115(C3):C03,017CrossRefGoogle Scholar
  18. Marshall JC, Nurser AG (1992) Fluid dynamics of oceanic thermocline ventilation. J Phys Oceanogr 22 (6):583–595CrossRefGoogle Scholar
  19. Mulet S, Rio MH, Mignot A, Guinehut S, Morrow R (2012) A new estimate of the global 3d geostrophic ocean circulation based on satellite data and in-situ measurements. Deep-Sea Res II Top Stud Oceanogr 77:70–81CrossRefGoogle Scholar
  20. Provost C, Escoffier C, Maamaatuaiahutapu K, Kartavtseff A, Garċon V (1999) Subtropical mode waters in the south atlantic ocean. J Geophys Res 104(C9):21–033CrossRefGoogle Scholar
  21. Shcherbina A, Gregg M, Alford M, Harcourt R (2010) Three-dimensional structure and temporal evolution of submesoscale thermohaline intrusions in the North Pacific subtropical frontal zone. J Phys Oceanogr 40:1669–1689CrossRefGoogle Scholar
  22. Smythe-Wright D, Chapman P, Rae CD, Shannon L, Boswell S (1998) Characteristics of the south atlantic subtropical frontal zone between 15 w and 5 e. Deep-Sea Res I Oceanogr Res Pap 45(1):167–192CrossRefGoogle Scholar
  23. Stern M (1965) Interaction of a uniform wind stress with a geostrophic vortex. Deep-Sea Res 12:355–367Google Scholar
  24. Stone PH (1970) On non-geostrophic baroclinic stability: Part ii. J Atmos Sci 27(5):721–726CrossRefGoogle Scholar
  25. Stramma L, Peterson RG (1990) The south atlantic current. J Phys Oceanogr 20(6):846–859CrossRefGoogle Scholar
  26. Thomas L, Lee C (2005) Intensification of ocean fronts by down-front winds. J Phys Oceanogr 35(6):1086–1102CrossRefGoogle Scholar
  27. Thomas LN (2005) Destruction of potential vorticity by winds. J Phys Oceanogr 35(12):2457–2466CrossRefGoogle Scholar
  28. Thomas LN, Taylor JR, Ferrari R, Joyce TM (2013) Symmetric instability in the gulf stream. Deep-Sea Res II Top Stud Oceanogr 91:96–110CrossRefGoogle Scholar
  29. Williams R, Follows M (1998) The ekman transfer of nutrients and maintenance of new production over the north atlantic. Deep-Sea Res I Oceanogr Res Pap 45(2):461–490CrossRefGoogle Scholar

Copyright information

© Springer-Verlag GmbH Germany 2017

Authors and Affiliations

  1. 1.Laboratório de Dinâmica e Modelagem Oceânica (DinaMO)Instituto de Oceanografia - Universidade Federal do Rio GrandeRio GrandeBrazil

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