Editorial Type:
Article
Article Type:
Research Article

Influence of the Barotropic Mean Flow on the Width and the Structure of the Atlantic Equatorial Deep Jets

Martin Claus GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany

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Richard J. Greatbatch GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany

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Peter Brandt GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany

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Print Publication:
01 Sep 2014
DOI:
https://doi.org/10.1175/JPO-D-14-0056.1
Page(s):
2485–2497
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Abstract

A representation of an equatorial basin mode excited in a shallow-water model for a single high-order baroclinic vertical normal mode is used as a simple model for the equatorial deep jets. The model is linearized about both a state of rest and a barotropic mean flow corresponding to the observed Atlantic Equatorial Intermediate Current System. It was found that the eastward mean flow associated with the North and South Intermediate Counter Currents (NICC and SICC, respectively) effectively shields the equator from off-equatorial Rossby waves. The westward propagation of these waves is blocked, and focusing on the equator due to beta dispersion is prevented. This leads to less energetic jets along the equator. On the other hand, the westward barotropic mean flow along the equator reduces the gradient of absolute vorticity and hence widens the cross-equatorial structure of the basin mode. Increasing lateral viscosity predominantly affects the width of the basin modes’ Kelvin wave component in the presence of the mean flow, while the Rossby wave is confined by the flanking NICC and SICC. Independent of the presence of the mean flow, the application of sufficient lateral mixing also hinders the focusing of off-equatorial Rossby waves, which is hence an unlikely feature of a low-frequency basin mode in the real ocean.

Corresponding author address: Martin Claus, GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany. E-mail: mclaus@geomar.de

Abstract

A representation of an equatorial basin mode excited in a shallow-water model for a single high-order baroclinic vertical normal mode is used as a simple model for the equatorial deep jets. The model is linearized about both a state of rest and a barotropic mean flow corresponding to the observed Atlantic Equatorial Intermediate Current System. It was found that the eastward mean flow associated with the North and South Intermediate Counter Currents (NICC and SICC, respectively) effectively shields the equator from off-equatorial Rossby waves. The westward propagation of these waves is blocked, and focusing on the equator due to beta dispersion is prevented. This leads to less energetic jets along the equator. On the other hand, the westward barotropic mean flow along the equator reduces the gradient of absolute vorticity and hence widens the cross-equatorial structure of the basin mode. Increasing lateral viscosity predominantly affects the width of the basin modes’ Kelvin wave component in the presence of the mean flow, while the Rossby wave is confined by the flanking NICC and SICC. Independent of the presence of the mean flow, the application of sufficient lateral mixing also hinders the focusing of off-equatorial Rossby waves, which is hence an unlikely feature of a low-frequency basin mode in the real ocean.

Corresponding author address: Martin Claus, GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany. E-mail: mclaus@geomar.de
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  • Ascani, F., E. Firing, P. Dutrieux, J. P. McCreary, and A. Ishida, 2010: Deep equatorial ocean circulation induced by a forced–dissipated Yanai beam. J. Phys. Oceanogr., 40, 1118–1142, doi:10.1175/2010JPO4356.1.

    • Search Google Scholar
    • Export Citation

  • Bourlès, B., and Coauthors, 2003: The deep currents in the eastern equatorial Atlantic Ocean. Geophys. Res. Lett., 30, 8002, doi:10.1029/2002GL015095.

    • Search Google Scholar
    • Export Citation

  • Brandt, P., V. Hormann, B. Bourlès, J. Fischer, F. A. Schott, L. Stramma, and M. Dengler, 2008: Oxygen tongues and zonal currents in the equatorial Atlantic. J. Geophys. Res., 113, C04012, doi:10.1029/2007JC004435.

    • Search Google Scholar
    • Export Citation

  • Brandt, P., V. Hormann, A. Körtzinger, M. Visbeck, G. Krahmann, L. Stramma, R. Lumpkin, and C. Schmid, 2010: Changes in the ventilation of the oxygen minimum zone of the tropical North Atlantic. J. Phys. Oceanogr., 40, 1784–1801, doi:10.1175/2010JPO4301.1.

    • Search Google Scholar
    • Export Citation

  • Brandt, P., A. Funk, V. Hormann, M. Dengler, R. J. Greatbatch, and J. M. Toole, 2011: Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean. Nature, 473, 497–500, doi:10.1038/nature10013.

    • Search Google Scholar
    • Export Citation

  • Brandt, P., and Coauthors, 2012: Ventilation of the equatorial Atlantic by the equatorial deep jets. J. Geophys. Res.,117, C12015, doi:10.1029/2012JC008118.

  • Bunge, L., C. Provost, B. L. Hua, and A. Kartavtseff, 2008: Variability at intermediate depths at the equator in the Atlantic Ocean in 2000–06: Annual cycle, equatorial deep jets, and intraseasonal meridional velocity fluctuations. J. Phys. Oceanogr., 38, 1794–1806, doi:10.1175/2008JPO3781.1.

    • Search Google Scholar
    • Export Citation

  • Cane, M. A., and D. W. Moore, 1981: A note on low-frequency equatorial basin modes. J. Phys. Oceanogr., 11, 1578–1584, doi:10.1175/1520-0485(1981)011<1578:ANOLFE>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Cane, M. A., and P. R. Gent, 1984: Reflection of low-frequency equatorial waves at arbitrary western boundaries. J. Mar. Res., 42, 487–502, doi:10.1357/002224084788505988.

    • Search Google Scholar
    • Export Citation

  • Chang, P., and S. Philander, 1989: Rossby wave packets in baroclinic mean currents. Deep-Sea Res.,36A, 17–37, doi:10.1016/0198-0149(89)90016-2.

    • Search Google Scholar
    • Export Citation

  • d’Orgeville, M., B. L. Hua, and H. Sasaki, 2007: Equatorial deep jets triggered by a large vertical scale variability within the western boundary layer. J. Mar. Res., 65, 1–25, doi:10.1357/002224007780388720.

    • Search Google Scholar
    • Export Citation

  • Eriksen, C. C., 1982: Geostrophic equatorial deep jets. J. Mar. Res., 40, 143–157.

  • Firing, E., 1987: Deep zonal currents in the central equatorial Pacific. J. Mar. Res., 45, 791–812, doi:10.1357/002224087788327163.

  • Gill, A. E., 1982: Atmosphere–Ocean Dynamics.Academic Press, 664 pp.

  • Gouriou, Y., and Coauthors, 2001: Deep circulation in the equatorial Atlantic Ocean. Geophys. Res. Lett., 28, 819–822, doi:10.1029/2000GL012326.

    • Search Google Scholar
    • Export Citation

  • Greatbatch, R. J., P. Brandt, M. Claus, S.-H. Didwischus, and Y. Fu, 2012: On the width of the equatorial deep jets. J. Phys. Oceanogr., 42, 1729–1740, doi:10.1175/JPO-D-11-0238.1.

    • Search Google Scholar
    • Export Citation

  • Hayes, S. P., and H. B. Milburn, 1980: On the vertical structure of velocity in the eastern equatorial Pacific. J. Phys. Oceanogr., 10, 633–635, doi:10.1175/1520-0485(1980)010<0633:OTVSOV>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Hua, B. L., D. W. Moore, and S. Le Gentile, 1997: Inertial nonlinear equilibration of equatorial flows. J. Fluid Mech., 331, 345–371, doi:10.1017/S0022112096004016.

    • Search Google Scholar
    • Export Citation

  • Johnson, G. C., and D. Zhang, 2003: Structure of the Atlantic Ocean equatorial deep jets. J. Phys. Oceanogr., 33, 600–609, doi:10.1175/1520-0485(2003)033<0600:SOTAOE>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Johnson, G. C., E. Kunze, K. E. McTaggart, and D. W. Moore, 2002: Temporal and spatial structure of the equatorial deep jets in the Pacific Ocean. J. Phys. Oceanogr., 32, 3396–3407, doi:10.1175/1520-0485(2002)032<3396:TASSOT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Leetmaa, A., and P. F. Spain, 1981: Results from a velocity transect along the equator from 125 to 159°W. J. Phys. Oceanogr., 11, 1030–1033, doi:10.1175/1520-0485(1981)011<1030:RFAVTA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Luyten, J. R., and J. C. Swallow, 1976: Equatorial undercurrents. Deep-Sea Res. Oceanogr. Abstr.,23, 999–1001, doi:10.1016/0011-7471(76)90830-5.

    • Search Google Scholar
    • Export Citation

  • Richardson, P. L., and D. M. Fratantoni, 1999: Float trajectories in the deep western boundary current and deep equatorial jets of the tropical Atlantic. Deep-Sea Res. II, 46, 305–333, doi:10.1016/S0967-0645(98)00100-3.

    • Search Google Scholar
    • Export Citation

  • Sadourny, R., 1975: The dynamics of finite-difference models of the shallow-water equations. J. Atmos. Sci., 32, 680–689, doi:10.1175/1520-0469(1975)032<0680:TDOFDM>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Schopf, P. S., D. L. T. Anderson, and R. Smith, 1981: Beta-dispersion of low-frequency Rossby waves. Dyn. Atmos. Oceans, 5, 187–214, doi:10.1016/0377-0265(81)90011-7.

    • Search Google Scholar
    • Export Citation
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