This article presents a pedagogical approach toward understanding vorticity generated in flow past an island. The ubiquity of eddies in the ocean motivates the use of vorticity to quantify their rotation, and we define two kinds of vorticity: relative vorticity, that is, the vorticity of an eddy relative to Earth, and planetary vorticity, Earth’s spin relative to local vertical. The ratio of relative to planetary vorticity is the Rossby number, Ro, a useful normalized measure of vorticity. Vorticity may be created in flow past an island as the alongshore current at the coast is forced to zero by friction. Observations from the Flow Encountering Abrupt Topography (FLEAT) program, including those from satellites, underwater gliders, ship surveys, and moorings, are used to measure vorticity as forced when the North Equatorial Current collides with the northern tip of Palau. The observations include time means over years, with Ro about 0.3 at horizontal length scales of tens of kilometers and snapshots of Ro as large as 30 from eddies of diameter 1 km. Vorticity can be broad-banded in time, with strong westward flow causing vorticity at frequencies as high as semidiurnal. Implications of this wide range of scales in vorticity are discussed in the conclusion.

Rudnick, D.L., K.L. Zeiden, C.Y. Ou, T.M.S. Johnston, J.A. MacKinnon, M.H. Alford, and G. Voet. 2019. Understanding vorticity caused by flow passing an island. Oceanography 32(4):66–73, https://doi.org/10.5670/oceanog.2019.412.

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.
1. Gula, J., M.J. Molemaker, and J.C. McWilliams. 2016. Topographic generation of submesoscale centrifugal instability and energy dissipation. Nature Communications 7, https://doi.org/10.1038/ncomms12811.
2. 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.
3. MacKinnon, J.A., M.H. Alford, G. Voet, K. Zeiden, T.M.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.
4. 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 45:613–629, https://doi.org/10.1175/Jpo-D-13-0225.1.
5. Pinkel, R. 2012. Velocity imprecision in finite-​beamwidth shipboard Doppler sonar: A first-​generation correction algorithm. Journal of Atmospheric and Oceanic Technology 29:1,569–1,580, https://doi.org/​10.1175/JTECH-D-12-00041.1.
6. Roshko, A. 1954. On the Development of Turbulent Wakes from Vortex Streets. National Advisory Committee for Aeronautics Report 1191, 25 pp.
7. Rudnick, D.L. 2001. On the skewness of vorticity in the upper ocean. Geophysical Research Letters 28:2,045–2,048, https://doi.org/​10.1029/​2000gl012265.
8. Rudnick, D.L. 2016. Ocean research enabled by underwater gliders. Annual Review of Marine Science 8:519–541, https://doi.org/10.1146/annurev-marine-122414-033913.
9. Salmon, R. 1998. Lectures on Geophysical Fluid Dynamics. Oxford University Press, New York, 378 pp.
10. Schönau, M.C., and D.L. Rudnick. 2015. Glider observations of the North Equatorial Current in the western tropical Pacific. Journal of Geophysical Research 120:3,586–3,605, https://doi.org/​10.1002/​2014JC010595.
11. Schönau, M.C., H.W. Wijesekera, W.J. Teague, P.L. Colin, G. Gopalakrishnan, D.L. Rudnick, B.D. Cornuelle, Z.R. Hallock, and D.W. Wang. 2019. The end of an El Niño: A view from Palau. Oceanography 32(4):32–45, https://doi.org/​10.5670/oceanog.2019.409.
12. Shcherbina, A.Y., E.A. D’Asaro, C.M. Lee, J.M. Klymak, M.J. Molemaker, and J.C. McWilliams. 2013. Statistics of vertical vorticity, divergence, and strain in a developed submesoscale turbulence field. Geophysical Research Letters 40:4,706–4,711, https://doi.org/10.1002/grl.50919.
13. Sherman, J., R.E. Davis, W.B. Owens, and J. Valdes. 2001. The autonomous underwater glider “Spray.” IEEE Journal of Oceanic Engineering 26:437–446, https://doi.org/10.1109/48.972076.
14. Tennekes, H., and J.L. Lumley. 1972. A First Course in Turbulence. The MIT Press, Cambridge, 300 pp.
15. 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.

This is an open access article made available under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution, and reproduction in any medium or format as long as users cite the materials appropriately, provide a link to the Creative Commons license, and indicate the changes that were made to the original content. Images, animations, videos, or other third-party material used in articles are included in the Creative Commons license unless indicated otherwise in a credit line to the material. If the material is not included in the article’s Creative Commons license, users will need to obtain permission directly from the license holder to reproduce the material.