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

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Volume 32, No. 4
Pages 92 - 101


Island Wakes Observed from High-Frequency Current Mapping Radar

Sophia T. Merrifield Patrick L. ColinThomas CookCarlos Garcia-MorenoJennifer A. MacKinnonMark OteroTravis A. SchramekMika SiegelmanHarper L. SimmonsEric J. Terrill
Article Abstract

The interaction of large-scale oceanic flows with remote island chains can lead to turbulent wakes, enhanced vorticity production, and significant increases in biological productivity. This study showcases the range of flow conditions captured by surface current mapping high-frequency (HF) radar systems deployed around the main island group of Palau in the western Pacific. The radar array captures strong tidal and inertial flows, both near- and offshore, as well as the spatial and temporal variability associated with the synoptic geostrophic flow interacting with the island group. Surface currents measured by HF radar are significantly correlated to currents in the upper 100 m of the ocean water column, as observed with a concurrent mooring, such that the resulting surface spatial maps provide insight on the wake flows of the island across a significant portion of the upper ocean. Composite averages of eastward and westward incident flow show flow-splitting and reconnection 60 km upstream and downstream of the island group, respectively. Surface current variability observed by the radar array includes topographically blocked flow, flow separation and acceleration through passages in the island chain, eddy dipole structure, and coastal eddies with Rossby numbers of 5. The range of variability near the island chain is reflective of the complex incident flow, which encounters Palau from all directions and changes on timescales of hours to weeks. A high-resolution model qualitatively agrees with the HF radar observations and shows vorticity filaments generated downstream of the island passages.


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.

Supplementary Materials

Andres, M., M. Siegelman, V. Hormann, R.C. Musgrave, S.T. Merrifield, D.L. Rudnick, M.A. Merrifield, M.H. Alford, G. Voet, H.W. Wijesekera, and others. 2019. Eddies, topography, and the abyssal flow by the Kyushu-Palau Ridge near Velasco Reef. Oceanography 32(4):46–55, https://doi.org/​10.5670/oceanog.2019.410.

Arístegui, J., P. Tett, A. Hernández-Guerra, G. Basterretxea, M.F. Montero, K. Wild, P. Sangrá, S. Hernández-Leon, M. Canton, J.A. García-Braun, and others. 1997. The influence of island-generated eddies on chlorophyll distribution: A study of mesoscale variation around Gran Canaria. Deep Sea Research Part I 44(1):71–96, https://doi.org/10.1016/S0967-0637(96)00093-3.

Barrick, D.E., M.W. Evans, and B.L. Weber. 1977. Ocean surface currents mapped by radar. Science 198(4313):138–144, https://doi.org/10.1126/science.198.4313.138.

Chapman, R.D., L.K. Shay, H.C. Graber, J.B. Edson, A. Karachintsev, C.L. Trump, and D.B. Ross. 1997. On the accuracy of HF radar surface current measurements: Intercomparison with ship-based sensors. Journal of Geophysical Research 102(C8):18,737–18,748, https://doi.org/​10.1029/97JC00049.

Chung, Y.J., Y.R. Chen, L.Z.H. Chuang, Y.J. Yang, and L.G. Leu. 2017. The correlation analysis of ionospheric clutter and noise using SeaSonde HF radar. In MTS/IEEE Kobe Techno-Oceans (OTO) 2018 OCEANS, pp. 1–4, https://doi.org/10.1109/OCEANSE.2017.8084968.

Crombie, D.D. 1955. Doppler spectrum of sea echo at 13.56 Mc/s. Nature 175:681–682, https://doi.org/​10.1038/175681a0.

Emery, B.M., L. Washburn, and J.A. Harlan. 2004. Evaluating radial current measurements from CODAR high-frequency radars with moored current meters. Journal of Atmospheric and Oceanic Technology 21(8):1,259–1,271, https://doi.org/10.1175/1520-0426(2004)021​<1259:ERCMFC>2.0.CO;2.

Gove, J.M., M.A. McManus, A.B. Neuheimer, J.J. Polovina, J.C. Drazen, C.R. Smith, M.A. Merrifield, A.M. Friedlander, J.S. Ehses, C.W. Young, and A.K. Dillon. 2016. Near-island biological hotspots in barren ocean basins. Nature Communications 7:10581, https://doi.org/10.1038/ncomms10581.

Gurgel, K.W. 1994. Shipborne measurement of surface current fields by HF radar. L’Onde Electrique 74:54–59.

Hsin, Y.C., and B. Qiu. 2012. Seasonal fluctuations of the surface North Equatorial Counter-current (NECC) across the Pacific Basin. Journal of Geophysical Research 117(C06), https://doi.org/​10.1029/​2011JC007794.

Hsu, P.C., K.H. Cheng, S. Jan, H.J. Lee, and C.R. Ho. 2019. Vertical structure and surface patterns of Green Island wakes induced by the Kuroshio. Deep Sea Research Part I 143:1–16, https://doi.org/​10.1016/j.dsr.2018.11.002.

Johnston, T.M.S., J.A. MacKinnon, P.L. Colin, P.J. Haley Jr., P.F.J. Lermusiaux, A.J. Lucas, M.A. Merrifield, S.T. Merrifield, C. Mirabito, J.D. Nash, and others. 2019. Energy and momentum lost to wake eddies and lee waves generated by the North Equatorial Current and tidal flows at Peleliu, Palau. Oceanography 32(4):110–125, https://doi.org/10.5670/oceanog.2019.417.

Kundu, P.K., and I. Cohen. 2002. Fluid Mechanics, Academic Press, New York.

Lipa, B., and D. Barrick. 1983. Least-squares methods for the extraction of surface currents from CODAR crossed-loop data: Application at ARSLOE. IEEE Journal of Oceanic Engineering 8(4):226–253, https://doi.org/10.1109/JOE.1983.1145578.

MacKinnon, J.A., M.H. Alford, G. Voet, K. Zeiden, T.S. Johnston, M. Siegelman, S. Merrifield, and M. Merrifield. 2019. Eddy wake generation from broadband currents near Palau. Journal of Geophysical Research 124(7):4,891–4,903, https://doi.org/10.1029/2019JC014945.

Ohlmann, C., P. White, L. Washburn, E. Terrill, B. Emery, and M. Otero. 2006. Interpretation of coastal HF radar-derived surface currents with high-resolution drifter data. Journal of Atmospheric and Oceanic Technology 24:666–680, https://doi.org/​10.1175/JTECH1998.1.

Paduan, J.D., and L. Washburn. 2013. High-frequency radar observations of ocean surface currents. Annual Review of Marine Science 5:115–136, https://doi.org/10.1146/annurev-marine-121211-172315.

Pattiaratchi, C., A. James, and M. Collins. 1987. Island wakes and headland eddies: A comparison between remotely sensed data and laboratory experiments. Journal of Geophysical Research 92(C1):783–794, https://doi.org/10.1029/JC092iC01p00783.

Qiu, B., and R. Lukas. 1996. Seasonal and interannual variability of the North Equatorial Current, the Mindanao Current, and the Kuroshio along the Pacific western boundary. Journal of Geophysical Research 101:12,315–12,330, https://doi.org/​10.1029/95JC03204.

Schmidt, R. 1986. Multiple emitter location and signal parameter estimation. IEEE Transactions on Antennas and Propagation 34(3):276–280, https://doi.org/10.1109/TAP.1986.1143830.

Schramek, T.A. 2018. Ocean-Island Interactions in the Western Pacific. Doctoral dissertation, UC San Diego, 133 pp.

Schönau, M.C., and D.L. Rudnick. 2015. Glider observations of the North Equatorial Current in the western tropical Pacific. Journal of Geophysical Research 120(5):3,586–3,605, https://doi.org/​10.1002/2014JC010595.

Siegelman, M., M.A. Merrifield, E. Firing, J.A. MacKinnon, M.H. Alford, G. Voet, H.W. Wijesekera, T.A. Schramek, K.L. Zeiden, and E.J. Terrill. 2019. Observations of near-inertial surface currents at Palau. Oceanography 32(4):74–83, https://doi.org/​10.5670/oceanog.2019.413.

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.

Stewart, R.H., and J.W. Joy. 1974. HF radio measurements of surface currents. Deep Sea Research and Oceanographic Abstracts 21(12):1,039–1,049, https://doi.org/10.1016/0011-7471(74)90066-7.

Smith, W.H.F., and D.T. Sandwell. 1997. Global sea floor topography from satellite altimetry and ship depth soundings. Science 277:1,957-1,962, https://doi.org/10.1126/science.277.5334.1956.

Sugimoto, T., S. Kimura, and K. Tadokoro. 2001. Impact of El Niño events and climate regime shift on living resources in the western North Pacific. Progress in Oceanography 49(1–4):113–127, https://doi.org/10.1016/S0079-6611(01)00018-0.

Weller, R.A. 1982. The relation of near-inertial motions observed in the mixed layer during the JASIN (1978) experiment to the local wind stress and to the quasi-geostrophic flow field. Journal of Physical Oceanography 12(10):1,122–1,136, https://doi.org/10.1175/1520-0485(1982)012​<1122:TRONIM>2.0.CO;2.

Zeiden, K.L., D.L. Rudnick, and J.A. MacKinnon. 2019. Glider observations of a mesoscale oceanic island wake. Journal of Physical Oceanography 49(9):2,217–2,235, https://doi.org/​10.1175/JPO-D-18-0233.1.

Zhang, Z., H. Xue, F. Chai, and Y. Chao. 2017. Variability of the Pacific North Equatorial Current from 1993 to 2012 based on a 1/8° Pacific model simulation. Journal of Geophysical Research: Oceans 122(3):2,382–2,400, https://doi.org/​10.1002/2016JC012143.

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