Acceleration, Creation, and Depletion of Wind-Driven, Baroclinic Rossby Waves over an Ocean Ridge

Rémi Tailleux Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California

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James C. McWilliams Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California

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Abstract

The influences of topography on the propagation, spatial patterns, and amplitude variations of long baroclinic Rossby waves are investigated with a wind-forced, two-layer model above a midocean ridge. With steep topography the evolution equation for the baroclinic mode is shown to differ from that for a flat bottom in several ways: 1) The phase speed is systematically faster by the factor H/H2, where H is the total ocean depth and H2 is the lower layer thickness, though the propagation remains westward and nearly nondispersive; 2) an effectively dissipative transfer to the barotropic mode occurs whenever the baroclinic mode is locally parallel to f/H contours, where f is the Coriolis frequency; and 3) the wind-forced response is amplified in proportion to the topographic steepness, (f/H)(dH/dx)/(df/dy), for a longitudinally varying topography, which can be a large factor, but the amplification is only by the modest factor H/H2 for a latitudinally varying topography. Effects 2 and 3 are the result of energy exchanges to and from the barotropic mode, respectively. Effect 3 causes freely propagating, baroclinic Rossby waves to be generated west of the ridge. These effects collectively cause distortions of the baroclinic wave pattern as it traverses the ridge. These effects account qualitatively for several features seen in altimetric measurements in the vicinity of major topographic features: an increase in variance of baroclinic signals on the west side, an enhanced phase speed overall (compared to flat-bottom waves), and an abrupt change in the phase speed at midocean ridges.

Corresponding author address: R&eacute≳ Tailleux, LMD–UPMC Paris 6, Case Courrier 99, 4 Place Jussieu, Paris Cedex 75252, France.

tailleux@lmg.jussieu.fr

Abstract

The influences of topography on the propagation, spatial patterns, and amplitude variations of long baroclinic Rossby waves are investigated with a wind-forced, two-layer model above a midocean ridge. With steep topography the evolution equation for the baroclinic mode is shown to differ from that for a flat bottom in several ways: 1) The phase speed is systematically faster by the factor H/H2, where H is the total ocean depth and H2 is the lower layer thickness, though the propagation remains westward and nearly nondispersive; 2) an effectively dissipative transfer to the barotropic mode occurs whenever the baroclinic mode is locally parallel to f/H contours, where f is the Coriolis frequency; and 3) the wind-forced response is amplified in proportion to the topographic steepness, (f/H)(dH/dx)/(df/dy), for a longitudinally varying topography, which can be a large factor, but the amplification is only by the modest factor H/H2 for a latitudinally varying topography. Effects 2 and 3 are the result of energy exchanges to and from the barotropic mode, respectively. Effect 3 causes freely propagating, baroclinic Rossby waves to be generated west of the ridge. These effects collectively cause distortions of the baroclinic wave pattern as it traverses the ridge. These effects account qualitatively for several features seen in altimetric measurements in the vicinity of major topographic features: an increase in variance of baroclinic signals on the west side, an enhanced phase speed overall (compared to flat-bottom waves), and an abrupt change in the phase speed at midocean ridges.

Corresponding author address: R&eacute≳ Tailleux, LMD–UPMC Paris 6, Case Courrier 99, 4 Place Jussieu, Paris Cedex 75252, France.

tailleux@lmg.jussieu.fr

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  • Allen, J. S., and R. D. Romea, 1980: On coastal trapped waves at low latitudes in a stratified ocean. J. Fluid Mech.,98, 555–585.

  • Anderson, D. L. T., and A. E. Gill, 1975: Spin-up of a stratified ocean, with application to upwelling. Deep-Sea Res.,22, 583–596.

  • ——, and P. D. Killworth, 1977: Spin-up of a stratified ocean with topography. Deep-Sea Res.,24, 709–732.

  • Barnier, B., 1988: A numerical study on the influence of the Mid-Atlantic Ridge on nonlinear first-mode baroclinic Rossby waves generated by seasonal winds. J. Phys. Oceanogr.,18, 417–433.

  • Chang, P., and S. G. H Philander, 1989: Rossby wave packets in baroclinic mean currents. Deep-Sea Res.,36, 17–37.

  • Charney, J. G., and G. R. Flierl, 1981: Oceanic analogues of atmospheric motions. Evolution of Physical Oceanography, B. A. Warren and C. Wunsch, Eds., The MIT Press, 504–548.

  • Chelton, D. B., and M. G. Schlax, 1996: Global observations of oceanic Rossby waves. Science,272, 234–238.

  • Colin de Verdière, A., 1988: Buoyancy driven planetary flows. J. Mar. Res.,46, 215–265.

  • de Szoeke, R. A., and D. B. Chelton, 1999: The enhancement of planetary wave speeds by homogeneous potential vorticity layers. J. Phys. Oceanogr.,29, 500–511.

  • Dewar, W. K., 1998: On “too fast” baroclinic planetary waves in the general circulation. J. Phys. Oceanogr.,28, 1739–1758.

  • Frankignoul, C., P. M&uuml_er, and E. Zorita, 1997: A simple model of the decadal response of the ocean to stochastic wind forcing. J. Phys. Oceanogr.,27, 1533–1546.

  • Hallberg, R., 1997: Localized coupling between the surface and bottom-intensified flow over topography. J. Phys. Oceanogr.,27, 977–998.

  • Killworth, P. D., and J. R. Blundell, 1999: The effect of bottom topography on the speed of long extratropical planetary waves. J. Phys. Oceanogr.,29, 2689–2710.

  • ——, D. B. Chelton, and R. de Szoeke, 1997: The speed of observed and theoretical long extratropical planetary waves. J. Phys. Oceanogr.,27, 1946–1966.

  • Lighthill, J., 1978: Waves in Fluids. Cambridge University Press, 502 pp.

  • McWilliams, J. C., P. Gent, and N. Norton, 1986: The evolution of balanced, low-mode vortices on the β-plane. J. Phys. Oceanogr.,16, 838–855.

  • Mellor, G. L., and X. H. Wang, 1996: Pressure compensation and the bottom boundary layer. J. Phys. Oceanogr.,26, 2214–2222.

  • Milliff, R. A., and J. C. McWilliams, 1994: The evolution of boundary pressure in enclosed ocean basins. J. Phys. Oceanogr.,24, 1317– 1338.

  • Philander, S. G. H., 1979: Equatorial waves in the presence of the equatorial undercurrent. J. Phys. Oceanogr.,9, 254–262.

  • Qiu, B., W. Miao, and P. M&uuml_er, 1997: Propagation and decay of forced and free baroclinic Rossby waves in off-equatorial oceans. J. Phys. Oceanogr.,27, 2405–2417.

  • Reznik, G. M., and T. V. Tsybaneva, 1994: On the influence of topography and stratification on planetary waves in the ocean (two-layer model) (English translation). Oceanology,34, 1–9.

  • Rhines, P. B., 1970: Edge-, bottom-, and Rossby waves in a rotating stratified fluid. Geophys. Fluid Dyn.,1, 273–302.

  • ——, 1977: The dynamics of unsteady currents. The Sea, Vol. 6, E. Goldberg, Ed., Wiley, 189–318.

  • Sakamoto, T., and T. Yamagata, 1997: Evolution of baroclinic planetary eddies over localized bottom topography in terms of JEBAR. Geophys. Astrophys. Fluid Dyn.,84, 1–27.

  • Samelson, R. M., 1992: Surface-intensified Rossby waves over rough topography. J. Mar. Res.,50, 367–384.

  • Schlax, M. G., and D. B. Chelton, 1994: Detecting aliased tidal errors in altimeter height measurements. J. Geophys. Res.,99, 12603– 12612.

  • Straub, D. N., 1994: Dispersion of Rossby waves in the presence of zonally varying topography. Geophys. Astrophys. Fluid Dyn.,75, 107–130.

  • Sverdrup, H., 1947: Wind-driven currents in a baroclinic ocean: With application to the equatorial currents of the eastern Pacific. Proc. Natl. Acad. Sci. USA,33, 318–326.

  • Tokmakian, R. T., and P. Challenor, 1993: Observations in the Canary basin and the Azores frontal region using GEOSAT data. J. Geophys. Res.,98, 4761–4773.

  • Veronis, G., 1981: Dynamics of large-scale circulation. Evolution of Physical Oceanography, B. A. Warren and C. Wunsch, Eds., The MIT Press, 140–183.

  • White, W. B., 1977: Annual forcing of baroclinic long waves in the tropical North Pacific Ocean. J. Phys. Oceanogr.,7, 50–61.

  • Zheng, Q., X.-H. Yan, C.-R. Ho, and C.-K. Tai, 1994: The effects of shear flow on propagation of Rossby waves in the equatorial oceans. J. Phys. Oceanogr.,24, 1680–1686.

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