Three-dimensionality of shallow island wakes

Abstract

Island wakes are thought to play a significant role in the vertical and cross-shelf mixing processes in strong tidally forced coastal regions. This paper describes a comprehensive laboratory study of shallow water wakes behind islands of circular cross section forced by a sinusoidal tidal flow. The wake structure and vertical circulation are determined through novel three-dimensional particle imaging velocimetry measurements. Four archetypal wake forms (symmetric, asymmetric, unsteady bubble, and vortex shedding) are observed. Through examination of the vertical structure of each of these wake forms, we demonstrate the dependence of vertical transport in island wakes on three key parameters: (1) the tidal excursion relative to the island size, (2) the bottom boundary layer thickness relative to the flow depth and (3) the aspect ratio of the island size to the flow depth. The importance of secondary vortices in island upwelling is highlighted by local peaks in vertical velocity that exceed 40% of the peak external tidal velocity. This study fundamentally changes the view of island wake upwelling from a weak ‘tea cup’-like recirculation process to one where primary and secondary flow structures vigorously stir the water column over the full depth. This has fundamental implications for the fate of passive biological tracers and the time scales that determine productivity in topographically-complex continental shelf regions.