The ocean’s response to the termination of the major 2015/2016 El Niño event was captured from moorings and gliders deployed near Palau as part of the Office of Naval Research Departmental Research Initiative Flow Encountering Abrupt Topography (FLEAT). As the El Niño transitioned to neutral conditions in spring 2016, pulses of positive (warm) sea surface height anomalies (SSHAs) moved westward, deepening the thermocline and reaching Palau by the end of March. Observations collected nearly two months after the arrival of the warm water revealed intraseasonal oscillations (ISOs) with periods of ~30 days and vertical displacements of isotherms exceeding ~100 m in the deeper part of the thermocline. Arrival of these warm anomalies coincided with the disappearance of the eastward flow associated with the North Equatorial Countercurrent, and anomalously large meridional velocities (0.4 m s–1) and transports (~4–6 Sv) over the Kyushu-Palau Ridge on the northern edge of Palau. The 120-day, high-pass-filtered, satellite-acquired SSHAs showed packets of westward-moving waves with phase speeds of about 0.2 m s–1 at 8.625°N, with horizontal wavelengths and periods of about 550 km and 30 days, respectively. These waves, which appear to originate in the western Pacific, fall within the characteristics of mode-1 and mode-2 Rossby waves. Similar SSHAs were found near Palau following previous El Niño events, suggesting that formation of intraseasonal oscillations is part of an oceanic response to the termination of El Niño. The transition from El Niño to neutral conditions can affect the coral ecosystem around Palau by creating anomalous circulation and strong thermal anomalies extending from the surface to bottom waters deeper than 150 m, far below the depth limit of coral growth in Palau.
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.
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.
Bruno, J.F., C.E. Siddon, J.D. Whitman, P.L. Colin, and M.A. Toscano. 2001. El Niño related coral bleaching in Palau, western Caroline Islands. Coral Reefs 20:127–136, https://doi.org/10.1007/s003380100151.
Chassignet, E.P., L.T. Smith, G.R. Halliwell, and R. Bleck. 2003. North Atlantic simulations with the HYbrid Coordinate Ocean Model (HYCOM): Impact of the vertical coordinate choice, reference pressure, and thermobaricity. Journal of Physical Oceanography 33:2,504–2,526, https://doi.org/10.1175/1520-0485(2003)033<2504:NASWTH>2.0.CO;2.
Chelton, D.B., F.J. Wentz, C.L. Gentemann, R.A. de Szoeke, and M.G. Schlax. 2000. Satellite microwave SST observations of transequatorial tropical instability waves. Geophysical Research Letters 27(9):1,239–1,242, https://doi.org/10.1029/1999GL011047.
Clarke, A.J. 2014. El Niño physics and El Niño predictability. Annual Review of Marine Science 6:79–99, https://doi.org/10.1146/annurev-marine-010213-135026.
Colin, P.L. 2009. Marine Environments of Palau. Indo-Pacific Press, 414 pp, http://coralreefpalau.org/wp-content/uploads/2017/04/Colin-PL-2009-Marine-Environments-of-Palau.pdf.
Colin, P.L. 2018. Ocean warming and the reefs of Palau. Oceanography 31(2):126–135, https://doi.org/10.5670/oceanog.2018.214.
Cummings, J.A. 2005. Operational multivariate ocean data assimilation. Quarterly Journal of the Royal Meteorological Society Part C 131(613):3,583–3,604, https://doi.org/10.1256/qj.05.105.
Cummings, J.A., and O.M. Smedstad. 2013. Variational data assimilation for the global ocean. Pp. 303–343 in Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications, vol. II. S.K. Park and L. Xu, eds, Springer, https://doi.org/10.1007/978-3-642-35088-7_13.
Dee, D.P., S.M. Uppala, J.A. Simmons, P. Berrisford, P. Poli, S. Kobayashi, U. Andrae, M.A. Balmaseda, G. Balsamo, P. Bauer, and others. 2011. The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quarterly Journal of the Royal Meteorological Society 137:553–597, https://doi.org/10.1002/qj.828.
Farrar, J.T., and T.S. Durland. 2012. Wavenumber-frequency spectra of inertia-gravity and mixed Rossby-gravity waves in the equatorial Pacific Ocean. Journal of Physical Oceanography 42:1,859–1,881, https://doi.org/10.1175/JPO-D-11-0235.1.
Giering, R., and T. Kaminski. 1998. Recipes for adjoint code construction. ACM Transactions on Mathematical Software 24(4):437–474, https://doi.org/10.1145/293686.293695.
Gilbert, J., and C. Lemaréchal. 1989. Some numerical experiments with variable-storage quasi-Newton algorithms. Mathematical Programming 45(1):407–435, https://doi.org/10.1007/BF01589113.
Gill, Adrian E. 1982. Atmosphere-Ocean Dynamics. Academic Press, New York, 662 pp.
Hacker, P., E. Firing, R. Lukas, P.L. Richardson, and C.A. Collins. 1989. Observations of the low-latitude western boundary circulation in the Pacific during WEPOCS III. Pp. 135–143 in Proceedings of the Western Pacific International Meeting and Workshop on TOGA COARE, Nouméa, New Caledonia, Institute Français de Recherche Scientifique pour le Développement en Coopération/Centre ORSTOM.
Halliwell, G.R. 2004. Evaluation of vertical coordinate and vertical mixing algorithms in the HYbrid Coordinate Ocean Model (HYCOM). Ocean Modelling 7:285–322, https://doi.org/10.1016/j.ocemod.2003.10.002.
Hsin, Y.-C., and B. Qiu. 2012. The impact of Eastern-Pacific versus Central-Pacific El Niños on the North Equatorial Countercurrent in the Pacific Ocean. Journal of Geophysical Research 117(C11), https://doi.org/10.1029/2012JC008362.
Hu, D.X., L. Wu, W. Cai, A. Sen Gupta, A. Ganachaud, B. Qiu, A.L. Gordon, X. Lin, Z. Chen, S. Hu, and others. 2015. Pacific western boundary currents and their roles in climate. Nature 522:299–308, https://doi.org/10.1038/nature14504.
Jacox, M.G., E.L. Hazen, K.D. Zaba, D.L. Rudnick, C.A. Edwards, A.M. Moore, and S.J. Bograd. 2016. Impacts of the 2015–2016 El Niño on the California Current System: Early assessment and comparison to past events. Geophysical Research Letters 43:7,072–7,080, https://doi.org/10.1002/2016GL069716.
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.
Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J. Woollen, and others. 1996. The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77(3):437–471, https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.
Kashino, Y., A. Atmadipoera, Y. Kuroda, and Lukijanto. 2013. Observed features of the Halmahera and Mindanao eddies. Journal of Geophysical Research 118:6,543–6,560, https://doi.org/10.1002/2013JC009207.
L’Heureux, M.L., K. Takahashi, A.B. Watkins, A.G. Barnston, E.J. Becker, T.E. Diliberto, F. Gamble, J. Gottschalck, M.S. Halpert, B. Huang, and others. 2017. Observing and predicting the 2015/16 El Niño. Bulletin of the American Meteorological Society 98(7):1,363–1,382, https://doi.org/10.1175/BAMS-D-16-0009.1.
Lien, R.-C., B. Ma, C.M. Lee, T.B. Sanford, V. Mensah, L.R. Centurioni, B.D. Cornuelle, G. Gopalakrishnan, A.L. Gordon, M.-H. Chang, and others. 2015. The Kuroshio and Luzon Undercurrent east of Luzon Island. Oceanography 28(4):54–63, https://doi.org/10.5670/oceanog.2015.81.
Marshall, J., A. Adcroft, L. Perelman, and C. Heisey. 1997. A finite-volume, incompressible Navier Stokes model for studies of the ocean on parallel computers. Journal of Geophysical Research 102:5,753–5,766, https://doi.org/10.1029/96JC02775.
Masumoto, Y., and T. Yamagata. 1991. Response of the western tropical Pacific to the Asian winter monsoon: The generation of the Mindanao Dome. Journal of Physical Oceanography 21:1,386–1,398, https://doi.org/10.1175/1520-0485(1991)021<1386:ROTWTP>2.0.CO;2.
McPhaden, M.J., A.J. Busalacchi, R. Cheney, J.-R. Donguy, K.S. Gage, D. Halpern, M. Ji, P. Julian, G. Meyers, G.T. Mitchum, and others. 1998. The Tropical Ocean-Global Atmosphere observing system: A decade of progress. Journal of Geophysical Research 103(C7):14,169–14,240, https://doi.org/10.1029/97JC02906.
McPhaden, M.J. 1999. Genesis and evolution of the 1997–98 El Nino. Science 283:950–954, https://doi.org/10.1126/science.283.5404.950.
McPhaden, M.J. 2008. Evolution of the 2006–2007 El Nino: The role of intraseasonal to interannual time scale dynamics. Advances in Geoscience 14:219–230, https://doi.org/10.5194/adgeo-14-219-2008.
Meyers, G., P. McIntosh, L. Pigot, and M. Pook. 2007. The years of El Niño, La Niña, and interactions with the tropical Indian Ocean. Journal of Climate 20:2,872–2,880, https://doi.org/10.1175/JCLI4152.1.
Qiu, B., D.L. Rudnick, S. Chen, and Y. Kashino. 2013. Quasi-stationary North Equatorial Undercurrent jets across the tropical North Pacific Ocean. Geophysical Research Letters 40:2,183–2,187, https://doi.org/10.1002/grl.50394.
Qiu, B., D.L. Rudnick, I. Cerovecki, B.D. Cornuelle, S. Chen, M.C. Schönau, J.L. McClean, and G. Gopalakrishnan. 2015. The Pacific North Equatorial Current: New insights from the Origins of the Kuroshio and Mindanao Currents (OKMC) Project. Oceanography 28(4):24–33, https://doi.org/10.5670/oceanog.2015.78.
Qiu, B., S. Chen, B.S. Powell, P.L. Colin, D.L. Rudnick, and M.C. Schönau. 2019. Nonlinear short-term upper ocean circulation variability in the tropical western Pacific. Oceanography 32(4):22–31, https://doi.org/10.5670/oceanog.2019.408.
Rudnick, D.L., R.E. Davis, C.C. Eriksen, D.M. Fratantoni, and M.J. Perry. 2004. Underwater gliders for ocean research. Marine Technology Society Journal 38:73–84, https://doi.org/10.4031/002533204787522703.
Rudnick, D.L., and S.T. Cole. 2011. On sampling the ocean using underwater gliders. Journal of Geophysical Research 116, C08010, https://doi.org/10.1029/2010JC006849.
Rudnick, D.L., S. Jan, and C.M. Lee. 2015. A new look at circulation in the western North Pacific. Oceanography 28(4):16–23, https://doi.org/10.5670/oceanog.2015.77.
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.
Santoso, A., M.J. McPhaden, and W. Cai. 2017. The deﬁning characteristics of ENSO extremes and the strong 2015/2016 El Niño. Reviews of Geophysics 55:1,079–1,129, https://doi.org/10.1002/2017RG000560.
Santoso, A., H. Hendon, A. Watkins, S. Power, D. Dommenget, M.H. England, L. Frankcombe, N.J. Holbrook, R. Holmes, P. Hope, and others. 2019. Dynamics and predictability of El Niño-Southern Oscillation: An Australian perspective on progress and challenges. Bulletin of the American Meteorological Society 100:403–420, https://doi.org/10.1175/BAMS-D-18-0057.1.
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.
Schönau, M.C., D.L. Rudnick, I. Cerovecki, G. Gopalakrishnan, B.D. Cornuelle, J.L. McClean, and B. Qiu. 2015. The Mindanao Current: Mean structure and connectivity. Oceanography 28(4):34–45, https://doi.org/10.5670/oceanog.2015.79.
Schönau, M.C., and D.L. Rudnick. 2017. Mindanao Current and Undercurrent: Thermohaline structure and transport from repeat glider observations. Journal of Physical Oceanography 47:2,055–2,075, https://doi.org/10.1175/JPO-D-16-0274.1.
Schramek, T.A., P.L. Colin, M.A. Merrifield, and E.J. Terrill. 2018. Depth-dependent thermal stress around corals in the tropical Pacific Ocean. Geophysical Research Letters 45(18):9,739–9,747, https://doi.org/10.1029/2018GL078782.
Stammer, D., C. Wunsch, R. Giering, C. Eckert, P. Heimbach, J. Marotzke, A. Adcroft, C.N. Hill, and J. Marshall. 2002. Global ocean circulation during 1992–1997, estimated from ocean observations and a general circulation model. Journal of Geophysical Research 107(C9), 3118, https://doi.org/10.1029/2001JC000888.
St. Laurent, L., T. Ijichi, S.T. Merrifield, J. Shapiro, and H.L. Simmons. 2019. Turbulence and vorticity in the wake of Palau. Oceanography 32(4):102–109, https://doi.org/10.5670/oceanog.2019.416.
Tozuka, T., and T. Yamagata. 2003. Annual ENSO. Journal of Physical Oceanography 33:1,564–1,578, https://doi.org/10.1175/2414.1.
Vecchi, G.A., and D.E. Harrison. 2000. Tropical Pacific sea surface temperature anomalies, El Niño, and equatorial westerly wind events. Journal of Climate 13:1,814–1,830, https://doi.org/10.1175/1520-0442(2000)013<1814:TPSSTA>2.0.CO;2.
Wang, C., and J. Picaut. 2004. Understanding ENSO physics: A review. Pp. 21–48 in Earth’s Climate: The Ocean-Atmosphere Interaction. Geophysical Monograph Series, 147, C. Wang, S.-P. Xie, and J.A. Carton, eds, American Geophysical Union, Washington, DC.
Wolanski, E., P.L. Colin, J. Naithani, E. Deleersnijder, and Y. Golbuu. 2004. Large amplitude, leaky, island-generated, internal waves around Palau, Micronesia. Estuarine, Coastal and Shelf Science 60:705–716, https://doi.org/10.1016/j.ecss.2004.03.009.
Wyrtki, K. 1975. El Niño—The dynamic response of the equatorial Pacific Ocean to atmospheric forcing. Journal of Physical Oceanography 5:572–584, https://doi.org/10.1175/1520-0485(1975)005<0572:ENTDRO>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.
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.