Basil R. Stanton

Subantarctic oceanography around New Zealand: Preliminary results from an ongoing survey

Abstract An ongoing observational program focused on variability in the subantarctic currents and water masses around southern New Zealand was initiated in May 1998. This paper describes the preliminary results deduced from CTD (conductivity, temperature, depth) data collected during three hydrographic surveys, and moored current and temperature records between 1998 and 1999. An extensive archived data set has also been analysed to provide a hydrographic climatology of the region. The low‐frequency circulation within the subantarctic zone is described, revealing previously unreported flow features: persistent but weak anticyclonic and cyclonic circulations over the Campbell Plateau and a strong cyclonic flow around the western edge of the Bounty Trough. Flow within the Subantarctic Front (SAF) is strongly constrained by the New Zealand bathymetry; diverting to follow the south‐eastern flanks of the Campbell Plateau and crossing to the Bounty Plateau, before separating to join the basin‐scale circulation. The flow features deduced from recent data are consistent with the hydrographic climatology as well as mid‐depth float trajectories. Subantarctic Mode Water (SAMW) is observed year‐round over an extensive fraction of the subantarctic zone: along the equatorward side of the SAF, over the Campbell Plateau, and flowing within the cyclonic circulation around the Bounty Trough. There is a marked cooling and freshening of SAMW between the deep water along the western flanks of Campbell Plateau and waters further east, consistent with a blocking of the eastward flow carrying warm and salty SAMW by the plateau. Earlier ideas that a substantial volume of SAMW is formed over the Campbell Plateau by deep vertical mixing (Heath 1981) are not substantiated by our data, which include seasonal observations of the upper water column.

The East Auckland Current, 1994–95

Abstract The East Auckland Current (EAUC) was investigated in 1994–95 using data from three CTD surveys (conductivity, temperature, depth) and moored current meters. The strength and position of the EAUC was found to be highly variable with most of the current re‐circulating around an anticyclonic eddy north‐east of North Cape—the North Cape Eddy. The position and intensity of this eddy changed resulting in complex flow patterns with a south‐east flowing EAUC not always present as a contiguous feature. The current meter data showed a high level of mesoscale variability with low spatial coherence, suggesting that the along shore correlation length scale was c. 100 km in this current system. Near North Cape, a persistent but variable counter‐current was found inshore of the EAUC. To the south‐east a permanent anticyclonic East Cape Eddy found north of East Cape appears to dominate the flow field and the genesis of the East Cape Current.

Multiple techniques for determining stock relationships between orange roughy, Hoplostethus atlanticus, fisheries in the eastern Tasman Sea

Abstract Five techniques were used to determine stock relationships between four spatially isolated but geographically close orange roughy fisheries in the eastern Tasman Sea: the Lord Howe Rise (HOWE), Northwest Challenger (NWCH), Southwest Challenger (SWCH), and Westpac Bank (WPAC). The techniques included life history traits (age and length at maturity), population length frequency analysis, otolith shape analysis, genetic makers, and a comparison of spawning times. The estimated ages and lengths at onset of maturity did not identify clear stock differences between the four areas. Otolith shape revealed two groups: HOWE/NWCH and SWCH/WPAC. There were significant size differences between HOWE and NWCH (and between HOWE–SWCH and HOWE–WPAC) with larger fish on HOWE. One out of six nuclear DNA markers revealed significant heterogeneity among sites. Mitochondrial DNA polymorphisms of the control region revealed no heterogeneity among areas, but restriction digests of the whole mitochondrial genome revealed differences between HOWE and NWCH/SWCH. There was considerable between-year variation in the time of the onset of spawning at SWCH (3 weeks) and WPAC (4 weeks). In both areas, the time of spawning was later in the early 1990s than in the late 1990s. No major oceanographic features that might isolate stocks were identified in this region of the Tasman Sea. The biological differences between orange roughy taken from HOWE, NWCH, and SWCH indicate that these fisheries could be managed as independent stocks. There were no biological differences between SWCH and WPAC and these fisheries probably exploit one straddling stock.

Observed and modelled tidal currents in the New Zealand region

Abstract Tidal currents derived from current meter measurements are compared with the output from a barotropic tidal model of the New Zealand region. For the semi‐diurnal constituents there was very good agreement for the M2 tide and good agreement for the S2 tide. For the diurnal constituents (Kl, Ol) it was found that as the amplitude of the constituents decreased so did both the model/observation agreement and the accuracy of the observed tidal ellipse parameters. Consequently it was not possible to decide whether differences arose through shortcomings in the model or in the data. However, the overall performance of the model as a prognostic tool for ocean tidal current simulation appears to be good.

Coastal-Trapped Waves on the West Coast of South Island, New Zealand

Abstract The coastal ocean response to wind forcing on the west coast continental shelf of South Island, New Zealand is examined using current, sea level and wind observations. Weather band motions over the northern region of the shelf appear to be dominated by the response to wind-forced flux through Cook Strait. Through-strait forcing is still evident in the currents of the southern region but west coast alongshore wind-forcing is also important. Most of the variance of nearshore weather band currents in the northern region of the shelf can be described in terms of coastal-trapped waves (CTWs). A fit of CTW modes 1 and 2 to the observations in that region indicates the equal importance of both modes, in agreement with the theories of strait-generated CTWS. Farther south, the across-shelf depth profile alters dramatically such that CTW structures change from being almost barotropic in the northern region to highly baroclinic. This rapid change in topography suggests that considerable scattering of CTWs m...

A Shallow Zonal Jet South of Fiji

In a recent paper, Webb (2000) presented evidence for shallow zonal jets in the South Equatorial Current in the Southwest Pacific. This was based on numerical model results from the high-resolution Ocean Circulation and Climate Advanced Modelling (OCCAM) model driven by ECMWF wind stress. Model resolution was sufficient to indicate that the current was broken up into a series of zonal jets by the extensive shallow topography associated with islands and reefs. These jets were prominent near the northern and southern extremities of the major island groups—Fiji, New Caledonia, and Vanuatu. The model defined the broad-scale structure of the jets and their effect on the temperature and salinity fields. Webb (2000) states that ‘‘Such features are relatively unknown and unstudied, so these results should provide a strong incentive for further detailed experimental and theoretical research in the region.’’ This statement reminded the present authors of oceanographic data collected south of Fiji in 1992, which appeared to show such a zonal jet, and these results are presented here.

Velocity measurements in the East Auckland Current north‐east of North Cape, New Zealand

Abstract Flow in the East Auckland Current (EAUC) system to the north‐east of North Cape, New Zealand is examined using data from two current meter mooring arrays, with supporting data from conductivity/temperature/depth (CTD) and satellite altimetry. Variable currents up to 45 cm s–1 were observed. The variability was partly attributable to changes in position and strength of the North Cape Eddy whose centre lies some 150 km offshore. The observed flows across the mooring line correlate well with those estimated from satellite altimetry. This gives confidence in the use of the satellite data to estimate the transport variability in the EAUC in the 1992–2001 period. No seasonal cycle was found in the volume transports but rather broadband variability at periods longer than 100 days.

Estimating the East Auckland Current transport from model winds and the Island Rule

Abstract The Island Rule allows the wind‐driven ocean transport around New Zealand to be determined from wind stress over the South Pacific. The transport in the western boundary current can be derived from the difference between the island transport and the interior Sverdrup transport to the east. This method has been used to estimate the transport in the East Auckland Current (EAUC) using ECMWF and NCEP model wind‐stress data, which are compared with transports derived from TOPEX/Poseidon satellite altimetry and expendable bathythermograph (XBT) sections. The results suggest that the mean transport in the EAUC can be explained by the Sverdrup dynamics implicit in the model results. However the model estimates of interannual variability in the EAUC were generally unrealistic, and some of the reasons for this are examined and possible extensions to the model suggested.

Antarctic Intermediate Water variability in the northern New Zealand region

Abstract In the region between 30°S and northern New Zealand, vertical salinity profiles through the core of the Antarctic Intermediate Water (AAIW) show a high degree of spatial and temporal variability, and this variability is much larger than that found in nearby ocean areas. Characteristic features are interleaving of salinity layers and large changes in the salinity minimum between adjacent stations. Quantifying the changes through the calculation of an intrusion index highlights the degree of variability and the importance of boundary mixing along the New Zealand slope. However, the main cause of the variability is the meeting and mixing of higher salinity AAIW, arriving from the north‐west (having travelled around the subtropical gyre) with lower salinity AAIW arriving by more direct entry from the north‐east. These waters meet in the region through the action of the meso‐scale eddy field. Present data indicate that where strong salinity interleaving occurs, the length scales are of the order of 10 km and the time scales are of the order of a few days. Resolution of the processes at work will require studies on finer scales than presently available.

Variations in the East Auckland Current from satellite radar altimeter data

We analysed satellite radar altimeter data for the New Zealand area during the period November 1992 to June 1994. Sea‐surface height anomalies from the Topex‐Poseidon and ERS—1 radar altimeter data...