Tidal currents, mean flows, and upwelling on the north-east shelf of New Zealand

Abstract

Continuous time-series of near-bed and near-surface currents and temperature were recorded, at the shelf edge and on the shelf of northeast New Zealand, between June 1994 and January 1996. Tidal analysis of the current records show that typical tidal currents in the region are dominated by the Mj constituent, with amplitudes between 5 and 10 cm s-1. The amplitude of the M2 tide varies in response to the existence of an internal tide, and the tide was also found to include higher harmonics of the M2 tide during periods of strong internal tide activity. These higher harmonics are likely to be the result of a non-linear internal tide. Mean flows at the surface throughout the records were generally parallel to the bathymetry and towards the southeast, with typical speeds of between 20 and 30 cm s-1, sometimes reaching 60 cm s-1 near Cape Brett. It is suggested that flow curvature around Cape Brett could be a significant localised upwelling mechanism. Near-bed currents showed a strong cross-shelf component in response to along-shelf wind stress, indicating episodic wind-driven upwelling events driving slope water onto the shelf between the autumnal and vernal equinoxes. It is estimated that such upwelling introduces a net supply of 10 ± 3 mmol m-3of nitrate to the shelf waters between Cape Brett and Hauraki Gulf. © 1998, Copyright Taylor & Francis Group, LLC.