The Oceanic Circulation off the East Coast of New Zealand between East Cape and Banks Peninsula

Abstract

The oceanic circulation off the east coast of New Zealand is examined mainly by the geostrophic method, other techniques being used where the geostrophic method is not applicable. A suitable reference level for use in the geostrophic method is chosen by Defant's Method, taking into account the direction of flow of Intermediate Water. The East Cape Current is formed when part of the subtropical eastward-flowing East Auckland Current is diverted southwards around East Cape by the bottom topography of the Ranfurly Bank, East Cape Ridge and Kermadec Trench. At about 41½°S, 178°E, the East Cape Current turns east then north, forming an anticyclonic eddy which has a radius of about 50-100km and is developed to a depth of at least 1500m. The Southland Current flows northwards along the east coast of the South Island. The boundary of this Current with the cooler, less saline water offshore is the Southland Front, which is developed to a depth of at least 800m. The deeper water of the Southland Current is brought towards the surface in passing northwards through the Mernoo Gap at the western end of the Chatham Rise. Thus, at the surface the Southland Current is recognized south of the Rise as a warm, saline tongue of water and north of the Rise as a cool, low salinity tongue. Near Kaikoura part of the Southland. Current turns offshore while the remainder continues northwards across Cook Strait and along the east coast of the North Island, before turning offshore near Cape Turnagain to combine with the East Cape Current. The offshore transport of the Southland. Current near Kaikoura is increased when a small eddy, cast off from the East Cape Current, occurs near Kaikoura. This eddy occurs about every two months and its formation appears to be closely linked with the circulation off the east coast of Australia, possibly as a result of pulses in the flow around East Cape producing meanders which become unstable as they approach the shallow depths of the Chatham Rise. In summer the current speeds decrease with depth down to at least 1000m whereas in winter they are nearly constant down to 300m and decrease below this depth. This seasonal difference appears to be related to summer thermocline. The Subtropical Convergence is situated along the Chatham Rise, which inhibits the southward movement of the East Cape Current. The seasonal change in the structure of this current is likely to be responsible for the slight southward movement of the Convergence which occurs in winter.