Eddy-Train Encounters with a Continental Boundary: A South Atlantic Case Study

José L. L. Azevedo Laboratório de Estudos dos Oceanos e Clima, Instituto de Oceanografia, Universidade Federal do Rio Grande-FURG, Rio Grande, Brazil

Search for other papers by José L. L. Azevedo in
Current site
Google Scholar
PubMed
Close
,
Doron Nof Department of Earth, Ocean and Atmospheric Science, and Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, Florida

Search for other papers by Doron Nof in
Current site
Google Scholar
PubMed
Close
, and
Mauricio M. Mata Laboratório de Estudos dos Oceanos e Clima, Instituto de Oceanografia, Universidade Federal do Rio Grande-FURG, Rio Grande, Brazil

Search for other papers by Mauricio M. Mata in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

Satellite altimetry suggests that large anticyclonic eddies (rings) originating from the Agulhas Current retroflection occasionally make their way across the entire South Atlantic Ocean. What happens when these rings encounter a western boundary current? In this work, interactions between a “train” of nonlinear lens-like eddies and a Southern Hemisphere continental boundary are investigated analytically and numerically on a β plane. The train of eddies is modeled as a steady double-frontal zonal current with the same vorticity and transport as the eddies themselves. The continental boundary is represented by a vertical wall, which is purely meridional in one case and is tilted with respect to the north in another case. It is demonstrated analytically that the eddy–wall encounter produces an equatorward flow parallel to the continental wall, thus suggesting a weakening of the transport of the associated (poleward flowing) western boundary current upstream of the encounter zone and unchanged transport downstream. A large stationary eddy is established in the contact zone because its β-induced force is necessary to balance the other forces along the wall. The size of this eddy is directly proportional to the transport of the eddy train and the meridional tilt of the wall. These scenarios are in good agreement with results obtained numerically using an isopycnal Bleck and Boudra model.

Corresponding author address: Doron Nof, The Florida State University, Oceanography, 117 N. Woodward Ave., Tallahassee, FL 32306. E-mail: nof@ocean.fsu.edu

Abstract

Satellite altimetry suggests that large anticyclonic eddies (rings) originating from the Agulhas Current retroflection occasionally make their way across the entire South Atlantic Ocean. What happens when these rings encounter a western boundary current? In this work, interactions between a “train” of nonlinear lens-like eddies and a Southern Hemisphere continental boundary are investigated analytically and numerically on a β plane. The train of eddies is modeled as a steady double-frontal zonal current with the same vorticity and transport as the eddies themselves. The continental boundary is represented by a vertical wall, which is purely meridional in one case and is tilted with respect to the north in another case. It is demonstrated analytically that the eddy–wall encounter produces an equatorward flow parallel to the continental wall, thus suggesting a weakening of the transport of the associated (poleward flowing) western boundary current upstream of the encounter zone and unchanged transport downstream. A large stationary eddy is established in the contact zone because its β-induced force is necessary to balance the other forces along the wall. The size of this eddy is directly proportional to the transport of the eddy train and the meridional tilt of the wall. These scenarios are in good agreement with results obtained numerically using an isopycnal Bleck and Boudra model.

Corresponding author address: Doron Nof, The Florida State University, Oceanography, 117 N. Woodward Ave., Tallahassee, FL 32306. E-mail: nof@ocean.fsu.edu
Save
  • Agra, C., and D. Nof, 1993: Collision and separation of boundary currents. Deep-Sea Res., 40 (11–12), 22592282.

  • Arruda, W. Z., 2002: Eddies along western boundaries. Ph.D. dissertation, The Florida State University, 90 pp.

  • Arruda, W. Z., D. Nof, and J. J. O'Brien, 2004: Does the Ulleung eddy owe its existence to β and nonlinearities? Deep-Sea Res. I, 51, 20732090.

    • Search Google Scholar
    • Export Citation
  • Beal, L. M., and Coauthors, 2011: On the role of the Agulhas system in ocean circulation and climate. Nature, 472, 429436, doi:10.1038/nature09983.

    • Search Google Scholar
    • Export Citation
  • Biastoch, A., C. W. Böning, F. U. Schwarzkopf, and J. R. E. Lutjeharms, 2009: Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies. Nature, 462, 495498, doi:10.1038/nature08519.

    • Search Google Scholar
    • Export Citation
  • Bleck, R., and D. Boudra, 1981: Initial testing of a numerical ocean circulation model using a hybrid (quasi-isopycnic) vertical coordinate. J. Phys. Oceanogr., 11, 755770.

    • Search Google Scholar
    • Export Citation
  • Bowen, M., J. L. Wilkin, and W. J. Emery, 2005: Variability and forcing of the East Australian Current. J. Geophys. Res., 110, C03019, doi:10.1029/2004JC002533.

    • Search Google Scholar
    • Export Citation
  • Byrne, D. A., A. L. Gordon, and W. F. Haxby, 1995: Agulhas eddies: A synoptic view using Geosat ERM data. J. Phys. Oceanogr., 25, 902917.

    • Search Google Scholar
    • Export Citation
  • Campos, E. J. D., 2006: Equatorward translation of the Vitoria Eddy in a numerical simulation. Geophys. Res. Lett., 33, L22607, doi:10.1029/2006GL026997.

    • Search Google Scholar
    • Export Citation
  • Chelton, D. B., M. G. Schlax, R. M. Samelson, and R. A. de Szoeke, 2007: Global observations of large oceanic eddies. Geophys. Res. Lett., 34, L15606, doi:10.1029/2007GL030812.

    • Search Google Scholar
    • Export Citation
  • Chelton, D. B., M. G. Schlax, and R. M. Samelson, 2011: Global observations of nonlinear mesoscale eddies. Prog. Oceanogr., 91, 167216, doi:10.1016/j.pocean.2011.01.002.

    • Search Google Scholar
    • Export Citation
  • Cushman-Roisin, B., E. P. Chassignet, and B. Tang, 1990: Westward motion of mesoscale eddies. J. Phys. Oceanogr., 20, 758768.

  • Duncombe Rae, C. M., 1991: Agulhas retroflection rings in the South Atlantic Ocean: An overview. S. Afr. J. Mar. Sci., 11, 327344.

  • Flierl, G. R., 1979: A simple model for a structure of warm and cold core rings. J. Geophys. Res., 84 (C2), 781785.

  • Garzoli, S. L., P. L. Richardson, C. M. Duncombe Rae, D. M. Fratantoni, G. J. Goni, and A. J. Roubicek, 1999: Three Agulhas rings observed during the Benguela Current Experiment. J. Geophys. Res., 104 (C9), 20 97120 985.

    • Search Google Scholar
    • Export Citation
  • Goni, G. J., S. L. Garzoli, A. J. Roubicek, D. B. Olson, and O. B. Brown, 1997: Agulhas ring dynamics from TOPEX/POSEIDON satellite altimeter data. J. Mar. Res., 55, 861883.

    • Search Google Scholar
    • Export Citation
  • Killworth, P. D., 1983: On the motion of isolated lenses on a beta-plane. J. Phys. Oceanogr., 13, 368376.

  • Kundu, P. K., and I. M. Cohen, 2008: Fluid Mechanics. 4th ed. Academic Press, 878 pp.

  • Lamb, H., 1932: Hydrodynamics. Cambridge University Press, 738 pp.

  • Lebedev, I., and D. Nof, 1996: The drifting confluence zone. J. Phys. Oceanogr., 26, 24292448.

  • Lebedev, I., and D. Nof, 1997: Collision of boundary currents: Beyond a steady state. Deep-Sea Res., 44, 771791.

  • Lentini, C. A. D., D. B. Olson, and G. P. Podestá, 2002: Statistics of Brazil Current rings observed from AVHRR: 1993 to 1998. Geophys. Res. Lett., 29, 1811, doi:10.1029/2002GL015221.

    • Search Google Scholar
    • Export Citation
  • Lutjeharms, J. R. E., 2010: The Agulhas Current. Springer-Verlag, 342 pp.

  • Masuda, A., 1988: A skewed eddy of Batchelor-modon type. J. Oceanogr. Soc. Japan, 43, 383394.

  • Mata, M. M., S. E. Wijffels, J. A. Church, and M. Tomczak, 2006: Eddy shedding and energy conversions in the East Australian Current. J. Geophys. Res., 111, C09034, doi:10.1029/2006JC003592.

    • Search Google Scholar
    • Export Citation
  • Matano, R. P., 1993: On the separation of the Brazil Current from the coast. J. Phys. Oceanogr., 23, 7990.

  • McDonagh, E. L., K. J. Heywood, and M. P. Meredith, 1999: On the structure, paths, and fluxes associated with Agulhas rings. J. Geophys. Res., 104 (C9), 21 00721 020.

    • Search Google Scholar
    • Export Citation
  • Minato, S., 1982: Geostrophic adjustment near the coast. J. Oceanogr. Soc. Japan, 38, 225235.

  • Minato, S., 1983: Geostrophic response near the coast. J. Oceanogr. Soc. Japan, 39, 141149.

  • Nof, D., 1981: On the β-induced movement of isolated baroclinic eddies. J. Phys. Oceanogr., 11, 16621672.

  • Nof, D., 1988a: Draining vortices. Geophys. Astrophys. Fluid Dyn., 42, 187208.

  • Nof, D., 1988b: Eddy-wall interactions. J. Mar. Res., 46, 527555.

  • Nof, D., 1999: Strange encounters of eddies with walls. J. Mar. Res., 57, 739761.

  • Orlanski, I., 1976: A simple boundary condition for unbounded hyperbolic flows. J. Comput. Phys., 21, 251269.

  • Palma, E. D., R. P. Matano, and A. R. Piola, 2008: A numerical study of the Southwestern Atlantic Shelf circulation: Stratified ocean response to local and offshore forcing. J. Geophys. Res., 113, C11010, doi:10.1029/2007JC004720.

    • Search Google Scholar
    • Export Citation
  • Pichevin, T., and D. Nof, 1996: The eddy cannon. Deep-Sea Res. I, 43, 14751507.

  • Pichevin, T., D. Nof, and J. Lutjeharms, 1999: Why are there Agulhas rings? J. Phys. Oceanogr., 29, 693707.

  • Saffman, P., 1979: The approach of a vortex pair to a plane surface in inviscid fluid. J. Fluid Mech., 92, 497503.

  • Shi, C., and D. Nof, 1993: The splitting of eddies along boundaries. J. Mar. Res., 51, 771795.

  • Shi, C., and D. Nof, 1994: The destruction of lenses and generation of wodons. J. Phys. Oceanogr., 24, 11201136.

  • Umatani, S., and T. Yamagata, 1987: Evolution of an isolated eddy near a coast and its relevance to the “Kyucho.” J. Oceanogr. Soc. Japan, 43, 197203.

    • Search Google Scholar
    • Export Citation
  • Wainer, I., P. Gent, and G. Goni, 2000: Annual cycle of the Brazil–Malvinas confluence region in the National Center for Atmospheric Research Climate System Model. J. Geophys. Res., 105 (C11), 26 16726 177.

    • Search Google Scholar
    • Export Citation
  • Witter, D. L., and A. L. Gordon, 1999: Interannual variability of South Atlantic circulation from 4 years of TOPEX/POSEIDON satellite altimeter observations. J. Geophys. Res., 104, 20 92720 948.

    • Search Google Scholar
    • Export Citation
  • Yasuda, I., K. Okuda, and K. Mizuno, 1986: Numerical study on the vortices near boundaries—Considerations on warm core rings in the vicinity of east coast of Japan. Bull. Tohoku Regional Fish. Res. Lab., 48, 6786.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 262 101 8
PDF Downloads 153 58 1