Complex geophysical wake flows
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Idealized studies of island wakes often use a cylinder-like island to generate the wake, whereas most realistic studies use a close representation of the oceanic bathymetry immersed in a complex representation of the “ambient” geophysical flows. Here, a system of multiple islands was placed into numerical and experimental channels, in order to focus on the complexity of the archipelago wake, including (a) the influence of small neighboring islands and (b) the role of the island-shelf. The numerical geostrophic and stratified channel was built using a three-dimensional primitive equation model, considering a realistic representation of the Madeira archipelago bathymetry, with prescribed initial and boundary conditions. Results from the simulations show that the neighboring islands alter the near-field wake. Small eddies generated by the neighboring islands lead to destabilization of the shear layers of the larger island. Laboratory experiments carried out in the Coriolis rotating tank corroborated this near-field disruptive mechanism. The neighboring island perturbation effect was present whatever the direction of the incoming flow, but under different regimes. North–south wakes produced geostrophic eddies (≥ R d), whereas west–east wakes produced (exclusively) ageostrophic submesoscale eddies (< < R d) which traveled offshore with wave-like motion. The archipelago shelf contributed to the asymmetric vertical migration of oceanic vorticity. Cyclonic vorticity dominated the surface dynamics, whereas anticyclonic circulation prevailed at the bottom part of the linearly stratified upper layer. This study identifies several likely wake scenarios induced by the Madeira archipelago, and may serve as guide for future multiscale numerical studies and in situ campaigns.
KeywordsIsland wake Submesoscale eddies Boundary layer disruption Hydrodynamic drafting Topographic trapped waves Vertical shear Subinertial instabilities
The authors are grateful for research funding from FCT-Portuguese National Science Foundation. This work was initiated as part of an FCT post-doctoral grant (SFRH/BPD/14871/2003) and continued in the scope of two subsequent research funding initiatives (POCI/MAR/57265/2004; PPCDT/MAR/57265/2004). We would like to thank the contributions of Changming Dong (Charles) and Dmitri Boutov for providing some of the scripts that were used for the pre- and post-processing of the ROMS solutions. We would also like to thank Stefan Rhia and Euclides Luis for their interest in the topic; regrettably, their contribution was not eligible to be included in the current work. Discussions with Alexandre Stegner and Olivier Cadot, during Rui Caldeira’s visit to the École Nationale Supérieure de Techniques Avancées (ENSTA), Paris, are reflected in the content of this report. Comments from David Dietrich and from an anonymous reviewer substantially contributed to the improvement of the first versions of this manuscript.
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