Evidence for an eddy over the continental shelf in the downstream lee of Otago Peninsula, New Zealand

Circulation of coastal waters around Otago Peninsula, south-eastern New Zealand, was investigated by hydrographic surveys from 1978–1982, satellite imagery and a simple numerical model. Two distinct water masses were identified over the continental shelf: (i) Southland Current water occurred over the outer shelf at the surface but penetrated inshore to at least the mid-shelf region near the bottom; and (ii) Neritic waters occurred over the inner shelf, often with a surface band of low salinity water that extended offshore NE from Cape Saunders. Core salinities of this mid-shelf band were inversely related to flow of the Clutha River, located south of the Peninsula. Satellite images show that low salinity nearshore river plumes are squeezed into a core which is then deflected seawards as it flows northward past Cape Saunders. Further north, this water extends back towards shore and sheds a southward counter-current into Blueskin Bay, thereby forming an eddy. A numerical model indicated that asymmetric tidal flow around the Peninsula induces a counter-clockwise eddy within Blueskin Bay. The eddy was enhanced by northward flow of the Southland Current along the outer shelf and an associated meander at the Karitane Canyon. Inclusion of alongshore wind stress into the model suggested that the eddy is further enhanced under moderate steady wind conditions from the north-east but broken down by strong south-west winds. Analysis of local wind data indicates that the eddy is a relatively persistent feature in the northern lee of Otago Peninsula.