Turbulence and vorticity in the Wake of Palau

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in St. Laurent, L., Ijichi, T., Merrifield, S. T., Shapiro, J., & Simmons, H. L. Turbulence and vorticity in the Wake of Palau. Oceanography, 32(4), (2019): 102-109, doi: 10.5670/oceanog.2019.416.
The interaction of flow with steep island and ridge topography at the Palau island chain leads to rich vorticity fields that generate a cascade of motions. The energy transfer to small scales removes energy from the large-scale mean flow of the equatorial current systems and feeds energy to the fine and microstructure scales where instability mechanisms lead to turbulence and dissipation. Until now, direct assessments of the turbulence associated with island wakes have received only minimal attention. Here, we examine data collected from an ocean glider equipped with microstructure sensors that flew in the island wake of Palau. We use a combination of submesoscale modeling and direct observation to quantify the relationship between vorticity and turbulence levels. We find that direct wind-driven mixing only accounts for about 10% of the observed turbulence levels, suggesting that most of the energy for mixing is extracted from the shear associated with the vorticity field in the island’s wake. Below the surface layer, enhanced turbulence correlates with the phase and magnitude of the relative vorticity and strain levels of the mesoscale flow.
We thank the Palau National Government for permission to carry out the research in Palau. We also thank the US Office of Naval Research for supporting this work. We especially thank Pat and Lori Colin of the Coral Reef Research Foundation and their team for accommodating our research team in Koror, Palau, and running vessel operations in support of glider deployments and recoveries. Sean Whelan of the Woods Hole Oceanographic Institution and Lance Braasch of Scripps Institution of Oceanography provided technical support in the field. Funding for the development of HYCOM has been provided by the National Ocean Partnership Program and the Office of Naval Research. Data assimilative products using HYCOM are funded by the US Navy. Computer time was made available by the Department of Defense High Performance Computing Modernization Program. The output is publicly available at https://www.hycom.org/.