• Forristal, G. Z., K. J. Schaudt, and C. K. Cooper, 1992: Evolution and kinematics of a Loop Current eddy in the Gulf of Mexico during 1985. J. Geophys. Res., 97 , 21732184.

    • Search Google Scholar
    • Export Citation
  • Gill, A. E., 1982: Atmosphere–Ocean Dynamics. Academic Press, 662 pp.

  • Hamilton, P., 1990: Deep currents in the Gulf of Mexico. J. Phys. Oceanogr., 20 , 10871104.

  • Hamilton, P., 1992: Lower continental slope cyclonic eddies in the central Gulf of Mexico. J. Geophys. Res., 97 , 21852200.

  • Hamilton, P., G. S. Fargion, and D. C. Biggs, 1999: Loop Current eddy paths in the western Gulf of Mexico. J. Phys. Oceanogr., 29 , 11801207.

    • Search Google Scholar
    • Export Citation
  • Haney, R. L., 1991: On the pressure gradient force over steep topography in sigma coordinate ocean models. J. Phys. Oceanogr., 21 , 610619.

    • Search Google Scholar
    • Export Citation
  • Hogg, N. G., 1981: Topographic waves along 70°W on the continental rise. J. Mar. Res., 39 , 627649.

  • Hurlburt, H. E., and J. D. Thompson, 1980: A numerical study of Loop Current intrusions and eddy shedding. J. Phys. Oceanogr., 10 , 16111651.

    • Search Google Scholar
    • Export Citation
  • Johns, W. E., and D. R. Watts, 1986: Time scales and structure of topographic Rossby waves and meanders in the deep Gulf Stream. J. Mar. Res., 44 , 267290.

    • Search Google Scholar
    • Export Citation
  • Kirwan Jr.,, A. D., J. K. Lewis, A. W. Indest, P. Reinersman, and I. Quintero, 1988: Observed and simulated kinematic properties of Loop Current rings. J. Geophys. Res., 93 , 11891198.

    • Search Google Scholar
    • Export Citation
  • Levitus, S., and R. I. Gelfeld, 1992: NODC inventory of physical oceanographic profiles. Key to Oceanographic Records Doc. 18, NODC, 36 pp.

    • Search Google Scholar
    • Export Citation
  • Lighthill, M. J., 1978: Waves in Fluids. Cambridge University Press, 520 pp.

  • Louis, J. P., B. D. Petrie, and P. C. Smith, 1982: Observations of topographic Rossby waves on the continental margin off Nova Scotia. J. Phys. Oceanogr., 12 , 4755.

    • Search Google Scholar
    • Export Citation
  • Mellor, G. L., 1993: Princeton Ocean Model user's guide. [Available online at http://www.aos.princeton.edu/WWWPUBLIC/htdocs.pom.].

  • Mellor, G. L., T. Ezer, and L-Y. Oey, 1994: The pressure gradient conundrum of sigma coordinate ocean models. J. Atmos. Oceanic Technol., 11 , 11261134.

    • Search Google Scholar
    • Export Citation
  • Mellor, G. L., L-Y. Oey, and T. Ezer, 1998: Sigma coordinate pressure gradient errors and the seamount problem. J. Atmos. Oceanic Technol., 15 , 11221131.

    • Search Google Scholar
    • Export Citation
  • Oey, L-Y., 1996: Simulation of mesoscale variability in the Gulf of Mexico. J. Phys. Oceanogr., 26 , 145175.

  • Oey, L-Y., 1998: Eddy energetics in the Faroe–Shetland Channel. Cont. Shelf Res., 17 , 19291944.

  • Oey, L-Y., and P. Chen, 1992: A model simulation of circulation in the northeast Atlantic shelves and seas. J. Geophys. Res., 97C , 2008720115.

    • Search Google Scholar
    • Export Citation
  • Oey, L-Y., D-P. Wang, T. Hayward, C. Winant, and M. Hendershott, 2001: Upwelling and cyclonic regimes of the near-surface circulation in the Santa Barbara Channel. J. Geophys. Res., 106C , 92139222.

    • Search Google Scholar
    • Export Citation
  • Paluszkiewicz, T., L. P. Atkinson, E. S. Posmentier, and C. R. McClain, 1983: Observations of a loop current frontal eddy intrusion onto the west Florida shelf. J. Geophys. Res., 88 (C14) 96399651.

    • Search Google Scholar
    • Export Citation
  • Pedlosky, J., 1977: On the radiation of meso-scale energy in the mid-ocean. Deep-Sea Res., 24 , 591600.

  • Pedlosky, J., 1979: Geophysical Fluid Dynamics. Springer-Verlag, 624 pp.

  • Pickart, R. S., 1995: Gulf Stream-generated topographic Rossby waves. J. Phys. Oceanogr., 25 , 574584.

  • Pickart, R. S., and D. R. Watts, 1990: Deep western boundary current variability at Cape Hatteras. J. Mar. Res., 48 , 765791.

  • Press, W. H., S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery, 1992: Numerical Recipes. 2d ed. Cambridge University Press, 963 pp.

    • Search Google Scholar
    • Export Citation
  • Schmitz Jr.,, W. J., 1996: On the World Ocean circulation: Vol. I, Some global features/North Atlantic circulation. WHOI Tech. Rep. WHOI-96-03, 141 pp.

    • Search Google Scholar
    • Export Citation
  • Smagorinsky, J., 1963: General circulation experiments with the primitive equations. Part I: The basic experiment. Mon. Wea. Rev., 91 , 99164.

    • Search Google Scholar
    • Export Citation
  • Sturges, W., J. C. Evans, S. Welsh, and W. Holland, 1993: Separation of warm core rings in the Gulf of Mexico. J. Phys. Oceanogr., 23 , 250268.

    • Search Google Scholar
    • Export Citation
  • Thompson, R. O. R. Y., and J. R. Luyten, 1976: Evidence for bottom-trapped topographic Rossby waves from single moorings. Deep-Sea Res., 23 , 629635.

    • Search Google Scholar
    • Export Citation
  • Vukovich, F. M., B. W. Crissman, M. Bushnell, and W. J. King, 1979: Some aspects of the oceanography of the Gulf of Mexico using satellite and in-situ data. J. Geophys. Res., 84 , 77497768.

    • Search Google Scholar
    • Export Citation
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Deep Eddy Energy and Topographic Rossby Waves in the Gulf of Mexico

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  • 1 Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey
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Abstract

Observations suggest the hypothesis that deep eddy kinetic energy (EKE) in the Gulf of Mexico can be accounted for by topographic Rossby waves (TRWs). It is presumed that the TRWs are forced by Loop Current (LC) pulsation, Loop Current eddy (LCE) shedding, and perhaps also by LCE itself. Although the hypothesis is supported by model results, such as those presented in Oey, the existence of TRWs in the model and how they can be forced by larger-scale LC and LCEs with longer-period vacillations have not been clarified. In this paper, results from a 10-yr simulation of LC and LCEs, with double the resolution of that used by Oey, are analyzed to isolate the TRWs. It is shown that along an east-to-west band across the gulf, approximately over the 3000-m isobath, significant EKE that accounts for over one-half of the total spectrum is contained in the 20–100-day periods. Bottom energy intensification exists in this east–west band with vertical decay scales of about 600–300 m decreasing westward. The decrease agrees with the TRW solution. The band is also located within the region where TRWs can be supported by the topographic slope and stratification used in the model and where wavenumber and frequency estimates are consistent with the TRW dispersion relation. The analysis indicates significant correlation between pairs of east–west stations, over distances of approximately 400 km. Contours of lag times suggest offshore (i.e., downslope) phase propagation, and thus the east–west band indicates nearly parabathic and upslope energy propagation. Ray tracing utilizing the TRW dispersion relation and with and without (for periods >43 days) ambient deep currents shows that TRW energy paths coincide with the above east–west high-energy band. It also explains that the band is a result of TRW refraction by an escarpment (with increased topographic gradient) across the central gulf north of the 3000-m isobath, and also by deep current and its cyclonic shear, and that ray convergence results in localized EKE maxima near 91°W and 94°–95°W. Escarpment and cyclonic current shear also shorten TRW wavelengths. Westward deep currents increase TRW group speeds, by about 2–3 km day−1 according to the model, and this and ray confinement by current shear may impose sufficient constraints to aid in inferring deep flows. Model results and ray paths suggest that the deep EKE east of about the 91°W originates from under the LC while farther west the EKE also originates from southwestward propagating LCEs. The near-bottom current fluctuations at these source regions derive their energy from short-period (<100 days) and short-wavelength (<200 km) near-surface fluctuations that propagate around the LC during its northward extrusion phase and also around LCEs as they migrate southwestward in the model.

Corresponding author address: L.-Y. Oey, Program in Atmospheric and Oceanic Sciences, Princeton University, Forrestal Campus, Sayre Hall, Princeton, NJ 08544. Email: lyo@princeton.edu

Abstract

Observations suggest the hypothesis that deep eddy kinetic energy (EKE) in the Gulf of Mexico can be accounted for by topographic Rossby waves (TRWs). It is presumed that the TRWs are forced by Loop Current (LC) pulsation, Loop Current eddy (LCE) shedding, and perhaps also by LCE itself. Although the hypothesis is supported by model results, such as those presented in Oey, the existence of TRWs in the model and how they can be forced by larger-scale LC and LCEs with longer-period vacillations have not been clarified. In this paper, results from a 10-yr simulation of LC and LCEs, with double the resolution of that used by Oey, are analyzed to isolate the TRWs. It is shown that along an east-to-west band across the gulf, approximately over the 3000-m isobath, significant EKE that accounts for over one-half of the total spectrum is contained in the 20–100-day periods. Bottom energy intensification exists in this east–west band with vertical decay scales of about 600–300 m decreasing westward. The decrease agrees with the TRW solution. The band is also located within the region where TRWs can be supported by the topographic slope and stratification used in the model and where wavenumber and frequency estimates are consistent with the TRW dispersion relation. The analysis indicates significant correlation between pairs of east–west stations, over distances of approximately 400 km. Contours of lag times suggest offshore (i.e., downslope) phase propagation, and thus the east–west band indicates nearly parabathic and upslope energy propagation. Ray tracing utilizing the TRW dispersion relation and with and without (for periods >43 days) ambient deep currents shows that TRW energy paths coincide with the above east–west high-energy band. It also explains that the band is a result of TRW refraction by an escarpment (with increased topographic gradient) across the central gulf north of the 3000-m isobath, and also by deep current and its cyclonic shear, and that ray convergence results in localized EKE maxima near 91°W and 94°–95°W. Escarpment and cyclonic current shear also shorten TRW wavelengths. Westward deep currents increase TRW group speeds, by about 2–3 km day−1 according to the model, and this and ray confinement by current shear may impose sufficient constraints to aid in inferring deep flows. Model results and ray paths suggest that the deep EKE east of about the 91°W originates from under the LC while farther west the EKE also originates from southwestward propagating LCEs. The near-bottom current fluctuations at these source regions derive their energy from short-period (<100 days) and short-wavelength (<200 km) near-surface fluctuations that propagate around the LC during its northward extrusion phase and also around LCEs as they migrate southwestward in the model.

Corresponding author address: L.-Y. Oey, Program in Atmospheric and Oceanic Sciences, Princeton University, Forrestal Campus, Sayre Hall, Princeton, NJ 08544. Email: lyo@princeton.edu

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