Numerical Simulation of the Kuroshio Flowing over the Hirase Seamount in the Tokara Strait in Autumn: Tidal Vortex Shedding in a Baroclinic Jet

Ryuichiro Inoue aJapan Agency for Marine-Earth Science and Technology, Yokosuka City, Japan

Search for other papers by Ryuichiro Inoue in
Current site
Google Scholar
PubMed
Close
,
Eisuke Tsutsumi bKagoshima University, Faculty of Fisheries, Kagoshima, Japan

Search for other papers by Eisuke Tsutsumi in
Current site
Google Scholar
PubMed
Close
, and
Hirohiko Nakamura bKagoshima University, Faculty of Fisheries, Kagoshima, Japan

Search for other papers by Hirohiko Nakamura in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

Idealized numerical simulations of the Kuroshio western boundary current flowing over the Hirase seamount were conducted to examine the mechanisms of phenomena observed by shipboard and mooring measurements. Along the Kuroshio, enhanced mixing [vertical diffusivity, Kρ = O(10−2) m2 s−1] was observed in a low-stratification layer between high-shear layers around low tide, and a V-shaped band of the negative vertical component of relative vorticity (ζz) was also observed. Those features were reproduced in simulations of the Kuroshio that included the D2 tide. In the simulation, a streak of negative ζz detached from the Hirase turned into vertically tilted 10-km-scale vortices. The buoyancy frequency squared (N2) budget at the mooring position showed that the low stratification was caused by vertical and horizontal advection and horizontal tilting. The Kρ tended to increase when the Ertel potential vorticity (PV) < 0, as expected given the inertial instability. However, the magnitude of Kρ also depended on the tidal phase near Hirase, and Kρ was increased in the high vertical shear zones at the periphery of vortices where a strain motion is large. These results indicate that not only inertial instability but also tidal and vertical shear effects are important for driving turbulent mixing.

Significance Statement

A basin-scale distribution of wind stress drives a strong surface-intensified current in the western part of each ocean basin, such as the Gulf Stream and the Kuroshio. This western boundary current is regarded as a place where the kinetic energy and vorticity generated by winds are dissipated, allowing the basin-scale circulation to keep a steady state, but its dissipation mechanisms are not well understood. To understand the mechanisms, we conducted idealized numerical simulations that isolate the interactions between a seamount and the current as well as tidal currents, and compared results with observations. Our findings provide insights into how the current transfers kinetic energy to smaller scales when it flows over a seamount.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Ryuichiro Inoue, rinoue@jamstec.go.jp

Abstract

Idealized numerical simulations of the Kuroshio western boundary current flowing over the Hirase seamount were conducted to examine the mechanisms of phenomena observed by shipboard and mooring measurements. Along the Kuroshio, enhanced mixing [vertical diffusivity, Kρ = O(10−2) m2 s−1] was observed in a low-stratification layer between high-shear layers around low tide, and a V-shaped band of the negative vertical component of relative vorticity (ζz) was also observed. Those features were reproduced in simulations of the Kuroshio that included the D2 tide. In the simulation, a streak of negative ζz detached from the Hirase turned into vertically tilted 10-km-scale vortices. The buoyancy frequency squared (N2) budget at the mooring position showed that the low stratification was caused by vertical and horizontal advection and horizontal tilting. The Kρ tended to increase when the Ertel potential vorticity (PV) < 0, as expected given the inertial instability. However, the magnitude of Kρ also depended on the tidal phase near Hirase, and Kρ was increased in the high vertical shear zones at the periphery of vortices where a strain motion is large. These results indicate that not only inertial instability but also tidal and vertical shear effects are important for driving turbulent mixing.

Significance Statement

A basin-scale distribution of wind stress drives a strong surface-intensified current in the western part of each ocean basin, such as the Gulf Stream and the Kuroshio. This western boundary current is regarded as a place where the kinetic energy and vorticity generated by winds are dissipated, allowing the basin-scale circulation to keep a steady state, but its dissipation mechanisms are not well understood. To understand the mechanisms, we conducted idealized numerical simulations that isolate the interactions between a seamount and the current as well as tidal currents, and compared results with observations. Our findings provide insights into how the current transfers kinetic energy to smaller scales when it flows over a seamount.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Ryuichiro Inoue, rinoue@jamstec.go.jp
Save
  • Chang, M.-H., T. Y. Tang, C.-R. Ho, and S.-Y. Chao, 2013: Kuroshio-induced wake in the lee of Green Island off Taiwan. J. Geophys. Res. Oceans, 118, 15081519, https://doi.org/10.1002/jgrc.20151.

    • Search Google Scholar
    • Export Citation
  • Chang, M.-H., S. Jan, C.-L. Liu, Y.-H. Cheng, and V. Mensah, 2019: Observations of island wakes at high Rossby numbers: Evolution of submesoscale vortices and free shear layers. J. Phys. Oceanogr., 49, 29973016, https://doi.org/10.1175/JPO-D-19-0035.1.

    • Search Google Scholar
    • Export Citation
  • Cushman-Roisin, B., and J.-M. Beckers, 2011: Introduction to Geophysical Fluid Dynamics: Physical and Numerical Aspects. Academic Press, 875 pp.

  • Dewar, W. K., J. C. McWilliams, and M. J. Molemaker, 2015: Centrifugal instability and mixing in the California Undercurrent. J. Phys. Oceanogr., 45, 12241241, https://doi.org/10.1175/JPO-D-13-0269.1.

    • Search Google Scholar
    • Export Citation
  • Dong, C., J. C. McWilliams, and A. F. Shchepetkin, 2007: Island wakes in deep water. J. Phys. Oceanogr., 37, 962981, https://doi.org/10.1175/JPO3047.1.

    • Search Google Scholar
    • Export Citation
  • Dong, J., B. Fox-Kemper, H. Zhang, and C. Dong, 2021: The scale and activity of symmetric instability estimated from a global submesoscale-permitting ocean model. J. Phys. Oceanogr., 51, 16551670, https://doi.org/10.1175/JPO-D-20-0159.1.

    • Search Google Scholar
    • Export Citation
  • Gula, J., M. J. Molemaker, and J. C. McWilliams, 2014: Submesoscale cold filaments in the Gulf Stream. J. Phys. Oceanogr., 44, 26172643, https://doi.org/10.1175/JPO-D-14-0029.1.

    • Search Google Scholar
    • Export Citation
  • Gula, J., M. J. Molemaker, and J. C. McWilliams, 2015: Topographic vorticity generation, submesoscale instability and vortex street formation in the Gulf Stream. Geophys. Res. Lett., 42, 40544062, https://doi.org/10.1002/2015GL063731.

    • Search Google Scholar
    • Export Citation
  • Gula, J., M. J. Molemaker, and J. C. McWilliams, 2016: Submesoscale dynamics of a Gulf Stream frontal eddy in the South Atlantic Bight. J. Phys. Oceanogr., 46, 305325, https://doi.org/10.1175/JPO-D-14-0258.1.

    • Search Google Scholar
    • Export Citation
  • Hasegawa, D., and Coauthors, 2021: How a small reef in the Kuroshio cultivates the ocean. Geophys. Res. Lett., 48, e2020GL092063, https://doi.org/10.1029/2020GL092063.

    • Search Google Scholar
    • Export Citation
  • Holmes, R. M., and L. N. Thomas, 2015: The modulation of equatorial turbulence by tropical instability waves in a regional ocean model. J. Phys. Oceanogr., 45, 11551173, https://doi.org/10.1175/JPO-D-14-0209.1.

    • Search Google Scholar
    • Export Citation
  • Jiao, Y., and W. K. Dewar, 2015: The energetics of centrifugal instability. J. Phys. Oceanogr., 45, 15541573, https://doi.org/10.1175/JPO-D-14-0064.1.

    • Search Google Scholar
    • Export Citation
  • Klymak, J. M., and S. M. Legg, 2010: A simple mixing scheme for models that resolve breaking internal waves. Ocean Modell., 33, 224234, https://doi.org/10.1016/j.ocemod.2010.02.005.

    • Search Google Scholar
    • Export Citation
  • Legg, S., 2021: Mixing by oceanic lee waves. Annu. Rev. Fluid Mech., 53, 173201, https://doi.org/10.1146/annurev-fluid-051220-043904.

    • Search Google Scholar
    • Export Citation
  • Liu, C.-L., and M.-H. Chang, 2018: Numerical studies of submesoscale island wakes in the Kuroshio. J. Geophys. Res. Oceans, 123, 56695687, https://doi.org/10.1029/2017JC013501.

    • Search Google Scholar
    • Export Citation
  • MacKinnon, J. A., M. H. Alford, G. Voet, K. L. Zeiden, T. M. S. Johnston, M. Siegelman, S. Merrifield, and M. Merrifield, 2019: Eddy wake generation from broadband currents near Palau. J. Geophys. Res. Oceans, 124, 48914903, https://doi.org/10.1029/2019JC014945.

    • Search Google Scholar
    • Export Citation
  • Marshall, J., A. Adcroft, C. Hill, L. Perelman, and C. Heisey, 1997: A finite-volume, incompressible Navier Stokes Model for studies of the ocean on parallel computers. J. Geophys. Res., 102, 57535766, https://doi.org/10.1029/96JC02775.

    • Search Google Scholar
    • Export Citation
  • Matsumoto, K., T. Takanezawa, and M. Ooe, 2000: Ocean tide models developed by assimilating TOPEX/POSEIDON altimeter data into hydrodynamical model: A global model and a regional model around Japan. J. Oceanogr., 56, 567581, https://doi.org/10.1023/A:1011157212596.

    • Search Google Scholar
    • Export Citation
  • Molemaker, M. J., J. C. McWilliams, and I. Yavneh, 2005: Baroclinic instability and loss of balance. J. Phys. Oceanogr., 35, 15051517, https://doi.org/10.1175/JPO2770.1.

    • Search Google Scholar
    • Export Citation
  • Molemaker, M. J., J. C. McWilliams, and W. K. Dewar, 2015: Submesoscale instability and generation of mesoscale anticyclones near a separation of California Undercurrent. J. Phys. Oceanogr., 45, 613629, https://doi.org/10.1175/JPO-D-13-0225.1.

    • Search Google Scholar
    • Export Citation
  • Nagai, T., D. Hasegawa, T. Tanaka, H. Nakamura, E. Tsutsumi, R. Inoue, and T. Yamashiro, 2017: First evidence of coherent bands of strong turbulent layers associated with high-wavenumber internal-wave shear in the upstream Kuroshio. Sci. Rep., 7, 14555, https://doi.org/10.1038/s41598-017-15167-1.

    • Search Google Scholar
    • Export Citation
  • Nagai, T., and Coauthors, 2021: The Kuroshio flowing over seamounts and associated submesoscale flows drive 100-km-wide 100-1000-fold enhancement of turbulence. Commun. Earth Environ., 2, 170, https://doi.org/10.1038/s43247-021-00230-7.

    • Search Google Scholar
    • Export Citation
  • Nakamura, H., A. Nishina, Z. Liu, F. Tanaka, M. Wimbush, and J.-H. Park, 2013: Intermediate and deep water formation in the Okinawa trough. J. Geophys. Res. Oceans, 118, 68816893, https://doi.org/10.1002/2013JC009326.

    • Search Google Scholar
    • Export Citation
  • Niwa, Y., and T. Hibiya, 2004: Three-dimensional numerical simulation of M2 internal tides in the East China Sea. J. Geophys. Res., 109, C04027, https://doi.org/10.1029/2003JC001923.

    • Search Google Scholar
    • Export Citation
  • Niwa, Y., and T. Hibiya, 2011: Estimation of baroclinic tide energy available for Deep Ocean mixing based on three-dimensional global numerical simulations. J. Oceanogr., 67, 493502, https://doi.org/10.1007/s10872-011-0052-1.

    • Search Google Scholar
    • Export Citation
  • Perfect, B., N. Kumar, and J. J. Riley, 2018: Vortex structures in the wake of an idealized seamount in rotating, stratified flow. Geophys. Res. Lett., 45, 90989105, https://doi.org/10.1029/2018GL078703.

    • Search Google Scholar
    • Export Citation
  • Perfect, B., N. Kumar, and J. J. Riley, 2020: Energetics of seamount wakes. Part 1: Energy exchange. J. Phys. Oceanogr., 50, 13651382, https://doi.org/10.1175/JPO-D-19-0105.1.

    • Search Google Scholar
    • Export Citation
  • Puthan, P., M. Jalali, J. L. Ortiz-Tarin, K. Chongsiripinyo, G. Pawlak, and S. Sarkar, 2020: The wake of a three-dimensional underwater obstacle: Effect of bottom boundary conditions. Ocean Modell., 149, 101611, https://doi.org/10.1016/j.ocemod.2020.101611.

    • Search Google Scholar
    • Export Citation
  • Puthan, P., S. Sarkar, and G. Pawlak, 2021: Tidal synchronization of lee vortices in geophysical wakes. Geophys. Res. Lett., 48, e2020GL090905, https://doi.org/10.1029/2020GL090905.

    • Search Google Scholar
    • Export Citation
  • Puthan, P., G. Pawlak, and S. Sarkar, 2022: Wake vortices and dissipation in a tidally modulated flow past a three-dimensional topography. J. Geophys. Res. Oceans, 127, e2022JC018470, https://doi.org/10.1029/2022JC018470.

    • Search Google Scholar
    • Export Citation
  • Srinivasan, K., J. C. McWilliams, M. J. Molemaker, and R. Barkan, 2019: Submesoscale vortical wakes in the lee of topography. J. Phys. Oceanogr., 49, 19491971, https://doi.org/10.1175/JPO-D-18-0042.1.

    • Search Google Scholar
    • Export Citation
  • Takahashi, A., and Coauthors, 2024: Energetic stratified turbulence generated by Kuroshio–seamount interactions in Tokara Strait. J. Phys. Oceanogr., https://doi.org/10.1175/JPO-D-22-0242.1, in press.

    • Search Google Scholar
    • Export Citation
  • Thomas, L. N., J. R. Taylor, R. Ferrari, and T. M. Joyce, 2013: Symmetric instability in the Gulf Stream. Deep-Sea Res. II, 91, 96110, https://doi.org/10.1016/j.dsr2.2013.02.025.

    • Search Google Scholar
    • Export Citation
  • Tsutsumi, E., T. Matsuno, R.-C. Lien, H. Nakamura, T. Senjyu, and X. Guo, 2017: Turbulent mixing within the Kuroshio in the Tokara strait. J. Geophys. Res. Oceans, 122, 70827094, https://doi.org/10.1002/2017JC013049.

    • Search Google Scholar
    • Export Citation
  • Varlamov, S. M., X. Guo, T. Miyama, K. Ichikawa, T. Waseda, and Y. Miyazawa, 2015: M2 baroclinic tide variability modulated by the ocean circulation south of Japan. J. Geophys. Res. Oceans, 120, 36813710, https://doi.org/10.1002/2015JC010739.

    • Search Google Scholar
    • Export Citation
  • Yan, X., D. Kang, E. N. Curchitser, and C. Pang, 2019: Energetics of eddy-mean flow interactions along the western boundary currents in the North Pacific. J. Phys. Oceanogr., 49, 789810, https://doi.org/10.1175/JPO-D-18-0201.1.

    • Search Google Scholar
    • Export Citation
  • Zhai, X., H. L. Johnson, and D. P. Marshall, 2010: Significant sink of ocean-eddy energy near western boundaries. Nat. Geosci., 3, 608612, https://doi.org/10.1038/ngeo943.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 362 362 26
Full Text Views 150 150 3
PDF Downloads 183 183 5