Circulation over the northeastern New Zealand continental slope, shelf and adjacent Hauraki Gulf, during spring and summer

Abstract

The physical oceanography of the northeastern New Zealand slope, shelf and adjacent Hauraki Gulf is investigated, to determine the forcing of the along- and cross-shelf non-tidal residual circulation. Cross-correlation and empirical orthogonal function analyses of current meter data clearly separated wind-forced upwelling and downwelling Ekman dynamics in the shelf circulation, from dynamics over the outer shelf and slope, which were dominated by longer-period variability of the East Auckland Current (EAUC). Moored thermistor and satellite temperature data mapped three inner-to-mid-shelf upwellings in early spring to early summer. Between early and late summer, however, upwelling ceased and there was a marked transition toward downwelling and strongly stratified conditions on the shelf and in Hauraki Gulf. The offshore extent and temporal response of the upwellings to winds were in approximate agreement with theory. Moored thermistor and CTD data obtained during four research voyages showed the internal hydrological structures of the shelf and slope watermasses from early spring to late summer. While the forcing for shelf circulation arose largely from local winds, far-field effects associated with the EAUC caused shallow (<60 m depth) subtropical surface water intrusions across the shelf/slope boundary in late summer and near-bed onshore flows at the shelf edge, possibly through bottom Ekman-layer effects. These results describe the physical dynamics underpinning the chemical and biological ecosystem responses to seasonally varying upwelling and downwelling, and surface water intrusion, described in an accompanying paper (Continental Shelf Res. (2004), this volume).