Observations of Secondary Flow and Upwelling at Cape Saunders, Otago Peninsula.

Secondary flow is an important mechanism that can produce significant localised upwelling at headlands. Secondary flow develops in the curved flow around headlands. It flows radially outwards from the headland near the surface and radially inwards towards the headland near the bottom. The result is a helical flow pattern within the curved flow around the headland. The magnitude of secondary flow can be up to 10% of the depth average current. The driving forces behind secondary flow are the centripetal acceleration on curved stream-wise velocity and vertical shear in the stream-wise flow due to bottom friction. High resolution moving vessel ADCP measurements are made around the vicinity of Cape Saunders on the Otago Peninsula.

Around the Cape the northeastward flowing Southland current is generally stronger than tidal flows. Thus the currents are northeastward at most states of the tide. New radial basis function interpolation techniques for smoothing noisy data allows the weak secondary flow to be extracted from the much stronger along shore flows. During peak floods of 100 cms-1 the measured strength of the secondary flow is 10 cms-1. A region of secondary flow approximately 1500 m long and 800 m wide is observed down stream of the Cape. On the inside edge of this region, areas of upwelling are observed by inferring the vertical velocity from the horizontal velocity measurements using mass continuity. The strength of this upwelling is up to 50 m per hour at peak flood. A value comparable to the water depths around the Cape. Thus secondary flow induced upwelling of deep waters is significant at Cape Saunders.

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