Cross-shelf intrusion of subtropical water into the coastal zone of northeast New Zealand

Abstract

Observations from a series of four CTD surveys along the northeast shelf of New Zealand, along with data recorded by an array of current meter moorings and a series of AVHRR images, are presented from the period August 1994 to January 1995. The temperature record from a near-surface current meter at the shelf edge showed that warm, subtropical, oceanic water from the East Auckland Current approached the edge of the shelf twice during the deployment. The second event, in late November 1994, was also recorded by near-surface current meters on the moorings shoreward of the shelf edge, and seen in the CTD survey observations. The AVHRR imagery shows the November intrusion to occur initially in the north of the region, as a broad onshore movement of the front that separates the coastal and the subtropical water. The observed time delays of the temperature increases recorded by the current meters, and the spatial pattern of salinity from the CTD surveys, suggests that this surface intrusion of subtropical water then propagated southward, eventually reaching the coastline over most of the shelf. Based on analysing the current meter records with the available meteorological data, it is suggested that the approach of the subtropical water towards the shelf edge is a general result of southeasterly wind events, typically requiring a wind impulse of 5–7 x 10 4 Pa s. Continuation of the water across the shelf edge and into the coastal zone may then be dependent on continuing southeasterly winds and could also require the strong thermal stratification of early summer decoupling the surface water from the topographic constraint of the shelf edge. Such summer intrusions could be an annual feature of the region, and the potential for this introduction of offshore water to modify the coastal environment is discussed.