Field observations and numerical simulation of upwelling in the wake of Rottnest Island, south-western Australia

The redistribution of nutrients, pollution and sedi- ments, in the wake of islands or headlands has impor- tant applications in the areas of fisheries, pollutant dispersal and sediment transport. Recent studies have documented the two and three- dimensional structure of island wakes when there is flow separation leading to a recirculating eddy. However, the wake structure when flow separation does not occur (the attached flow condition) is relatively unknown. In this study we present results of extensive field observations to show an upwelling event in the summer wake of Rottnest Island, south west of Australia. During summer, the interaction between the northward wind-driven current and Rottnest Island does not lead to flow separation but a consistent cold water patch is observed to the north of the Island. The wake structure is also numerically sim- ulated using a three-dimensional baroclinic model. The results are shown to be in good agreement with the observations and reveal that upwelling may occur in the wake of an island even in the absence of a well- defined eddy. In addition, we show that curvature- induced secondary circulation at the tip of the island plays the dominant role in the generation of upwelling within the wake region.