Oceanography The Official Magazine of
The Oceanography Society
Volume 32 Issue 04

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Volume 32, No. 4
Pages 84 - 91


Dynamical Downscaling of Equatorial Flow Response to Palau

By Harper L. Simmons , Brian S. Powell, Sophia T. Merrifield, Sarah E. Zedler, and Patrick L. Colin 
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Article Abstract

The wake created by the interaction of equatorial currents with the Palau islands generates small-scale vortices. We aim to identify the significant scales of this vortex wake by studying its structure and variability. We utilize a subkilometer resolution numerical model integrated from May 2016 through April 2017 that is nested within a 2.5 km state estimate hindcast model. The near-Palau flow is highly variable and has very different wake generation characteristics depending on whether the flow is westward or eastward. During westward flow, numerous strong vortices are generated at the northern tip of Velasco Reef and advected hundreds of kilometers to the northwest. During less frequent and less persistent eastward flow, transport develops through the Euchelel Ngeruangl and Kekerel Euchel channels that separate Velasco Reef from the main Palau island group. As this flow encounters the atoll of Kayangel in the channel, it splits and emerges to the east as vortices. During our 2016–2017 study period, these Kayangel eddies were ejected from the channel and advected southward along the eastern coast of Palau while being sheared laterally, and thus the wake did not have a large eastward extent. We find that wake eddies were not well resolved in the 2.5 km state estimate and that more numerous and intense eddies were simulated at higher resolution. Accurate simulations of the wake around such ocean obstructions therefore require subkilometer resolving models.


Simmons, H.L., B.S. Powell, S.T. Merrifield, S.E. Zedler, and P.L. Colin. 2019. Dynamical downscaling of equatorial flow response to Palau. Oceanography 32(4):84–91, https://doi.org/10.5670/oceanog.2019.414.

    Bergh, J., and J. Berntsen. 2008. Numerical studies of wind forced internal waves with a nonhydrostatic model. Ocean Dynamics 59:1,025–1,041, https://doi.org/​10.1007/s10236-009-0226-1.
  1. Caldeira, R.M.A., P. Marchesiello, N.P. Nezlin, P.M. DiGiacomo, and J.C. McWilliams. 2005. Island wakes in the Southern California bight. Journal of Geophysical Research 110(C11), https://doi.org/10.1029/2004JC002675.
  2. 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. Journal of Geophysical Research 118(3):1,508–1,519, https://doi.org/10.1002/jgrc.20151.
  3. Courtier, P. 1997. Dual formulation of four-​dimensional variational assimilation. Quarterly Journal of the Royal Meteorological Society 123:2,449–2,461, https://doi.org/10.1002/qj.49712354414.
  4. Dong, C., J.C. McWilliams, and A.F. Shchepetkin. 2007. Island wakes in deep water. Journal of Physical Oceanography 37(4):962–981, https://doi.org/​10.1175/JPO3047.1.
  5. Dower, J., H. Freeland, and K. Juniper. 1992. A strong biological response to oceanic flow past Cobb seamount. Deep Sea Research Part A 39(7):1,139–1,145, https://doi.org/​10.1016/​0198-0149(92)90061-W.
  6. Gula, J., M.J. Molemaker, and J.C. McWilliams. 2015a. Topographic vorticity generation, submesoscale instability and vortex street formation in the Gulf Stream. Geophysical Research Letters 42(10):4,054–4,062, https://doi.org/​10.1002/2015GL063731.
  7. Gula, J., M.J. Molemaker, and J.C. McWilliams. 2015b. Topographic generation of submesoscale centrifugal instability and energy dissipation. Nature Communications 7:12811, https://doi.org/10.1038/ncomms12811.
  8. Hamner, W.M., and I.R. Hauri. 1981. Effects of island mass: Water flow and plankton pattern around a reef in the Great Barrier Reef lagoon, Australia. Limnology and Oceanography 26(6):1,084–1,102, https://doi.org/10.4319/lo.1981.26.6.1084.
  9. Hernandez-Léon, S. 1991. Accumulation of mesozooplankton in a wake area as a causative mechanism of the “island-mass effect.” Marine Biology 109:141–147, https://doi.org/10.1007/BF01320241.
  10. Heywood, K.J., D.P. Stevens, and G.R. Bigg. 1996. Eddy formation behind the tropical island of Aldabra. Deep Sea Research Part I 43(4):555–578, https://doi.org/10.1016/0967-0637(96)00097-0.
  11. Janekovi´c, I., and B.S. Powell. 2012. Analysis of imposing tidal dynamics to nested numerical models. Continental Shelf Research 34:30–40, https://doi.org/10.1016/j.csr.2011.11.017.
  12. Large, W.G., J.C. McWilliams, and S.C. Doney. 1994. Oceanic vertical mixing: A review and a model with non-local boundary layer parameterization. Review of Geophysics 32:363–403, https://doi.org/​10.1029/94RG01872.
  13. Mason, E., J. Molemaker, A.F. Shchepetkin, F. Colas, J.C. McWilliams, and P. Sangra. 2010. Procedures for offline grid nesting in regional ocean models. Ocean Modelling 35(1):1–15, https://doi.org/10.1016/​j.ocemod.2010.05.007.
  14. May, D., M. Parmeter, D. Olszewski, and B. McKenzie. 1998. Operational processing of satellite sea surface temperature retrievals at the Naval Oceanographic Office. Bulletin of the American Meteorological Society 79(3):397–407, https://doi.org/10.1175/1520-0477(1998)079​<0397:OPOSSS>2.0.CO;2.
  15. Merrifield, S.T., P.L. Colin, T. Cook, C. Garcia-Moreno, J.A. MacKinnon, M. Otero, T.A. Schramek, M. Siegelman, H.L. Simmons, and E.J. Terrill. 2019. Island wakes observed from high-frequency current mapping radar. Oceanography 32(4):92–101, https://doi.org/10.5670/oceanog.2019.415.
  16. Molemaker, M.J., J.C. McWilliams, and W.K. Dewar. 2015. Submesoscale instability and generation of mesoscale anticyclones near a separation of the California Undercurrent. Journal of Physical Oceanography 45(3):613–629, https://doi.org/​10.1175/JPO-D-13-0225.1.
  17. Perfect, B., N. Kumar, and J.J. Riley. 2018. Vortex structures in the wake of an idealized seamount in rotating, stratified flow. Geophysical Research Letters 45(17):9,098–9,105, https://doi.org/​10.1029/2018GL078703.
  18. Shchepetkin, A.F., and J.C. McWilliams. 2005. The Regional Oceanic Modeling System (ROMS): A split-explicit, free-surface, topography-following-coordinate ocean model. Ocean Modelling 9:347–404, https://doi.org/10.1016/​j.ocemod.2004.08.002.
  19. Smith, W.H.F., and D.T. Sandwell. 1997. Global sea floor topography from satellite altimetry and ship depth soundings. Science 277:1,956–1,962, https://doi.org/​10.1126/science.277.5334.1956.
  20. Zedler, S.E., B.S. Powell, B. Qiu, and D.L. Rudnick. 2019. Energy transfer in the western tropical Pacific. Oceanography 32(4):136–145, https://doi.org/​10.5670/oceanog.2019.419.
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