Complex geophysical wake flows. Madeira Archipelago case study

Abstract

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 (a parts per thousand yen 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.