Stephen M. Chiswell

Variability in the Southland Current, New Zealand

Current meter moorings were deployed in six locations around the South Island, New Zealand, in 1993 to monitor the Southland Current. Off the eastern shelf of the South Island, the Southland Curren...

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.

Variability in sea surface temperature around New Zealand from AVHRR images

Abstract Multi‐channel Advanced Very‐High Resolution Radiometer (AVHRR) images of sea surface temperature (SST) in the New Zealand region have been archived since 1989. The first 2 years of data are used to investigate the variability in sea surface temperature (SST) and its gradients around New Zealand. Empirical orthogonal function (EOF) analyses show that the annual cycle accounts for 92% of the variance of SST, but for less of the variance in SST gradient. The Southland and Subantarctic Fronts have an annual modulation in their strengths, accounting for about 20% of their variance; these modulations are out of phase, with the Southland Front being strongest in winter. The Subtropical Convergence shows variability that cannot be easily described by one mode, but appears to migrate meridionally with an annual period, being furthest north in March.

THE EAST CAPE CURRENT AND TWO EDDIES : A MECHANISM FOR LARVAL RETENTION?

Abstract Current meters deployed near East Cape, New Zealand, for 1 year show large differences between the offshore flow and that inshore of the 1000 m isobath. Offshore, mean flows are to the south‐east, and are the East Cape Current. Inshore, mean flows are to the north‐east, indicating that there may be a persistent inshore counter current. Comparison between the offshore current meter measurements and geostrophic currents inferred from TOPEX/Poseidon altimetry shows good agreement. We use this agreement to argue that passive drifter trajectories can be modelled using the altimeter data. By simulating drifter releases into the region, and modelling their trajectories, we develop a statistical picture of likely retention times for passive drifters. Drifters can get retained in one or other of two permanent eddies: the East Cape and Wairarapa Eddies, and retention within the system can be as high as 2–3 years, but depends on release time and location. If weak‐swimming larvae such as rock lobster larvae ...

Mean and variability in the Wairarapa and Hikurangi Eddies, New Zealand

Abstract The mean and variability of the circulation off the east coast of the North Island, New Zealand are investigated using shipboard conductivity‐temperature‐depth (CTD) and satellite altimeter data collected between 1993 and 2003. The altimeter data are used to adjust the in situ observations for the mesoscale eddy variability before computing the 11‐year mean in dynamic height. Mean dynamic height shows two anticyclonic eddies, centred near 178.3°E, 41.2°S and 176.2°E, 42.4°S. These locations are consistent with previous historical observations of the Wairarapa and Hikurangi Eddies, respectively. A long‐term trend in both in situ and satellite data shows that dynamic height rose at an average rate of up to 2 dyn cm year‐1 in the centre of the Wairarapa Eddy which is consistent with a strengthening of the eddy over the 11 years. The satellite data show periodic shedding of the Wairarapa Eddy from near East Cape at a rate of between two and three eddies per year. Thus, rather than indicating a permanent stationary eddy, the mean eddy reflects a region where the eddies tend to stall out or merge with the previous eddy. Often, the eddies will continue up the Hikurangi Trough so that the Hikurangi Eddy can be regarded as an older Wairarapa Eddy.

Physical oceanography of the deep seas around New Zealand: a review

We review the advances in ‘blue water’ physical oceanography of the seas around New Zealand since the last major review in 1985. By 1985, a basic description had been made of the circulation around New Zealand. Since then, dramatic increases in data from satellites, hydrographic cruises, surface drifters and profiling floats have improved knowledge on the locations, strengths and variability of the currents, water masses and fronts in the region. We have better estimates of the surface and deep circulation, and a better understanding of the dynamical processes driving this circulation and its variability. This review covers the open ocean, including water masses, ocean currents, tides and numerical modelling, and discusses the future of New Zealand oceanography.

Coherence of internal tide modulations along the Hawaiian Ridge

Long time series of sea level from tide gauges along the north side of the Hawaiian Ridge and shorter series of dynamic heights inferred from inverted echo sounders moored just north of the main Hawaiian Islands are examined for evidence of internal tides at the M2 frequency. We find that the amplitudes and phases of the M2 tidal components have low-frequency variability, which is consistent with a superposition of an internal tide with the larger barotropic tide. Further, the low-frequency variability is correlated with low-frequency changes in the depth of the pycnocline, which suggests a simple physical mechanism to account for the low-frequency modulations in the internal tidal amplitude. These modulations are coherent for long distances along the Hawaiian Ridge, indicating a coherent generation of the internal tide that is consistent with acoustic observations in the North Pacific and with recent analyses of sea surface heights from satellite altimetry.

Vertical Structure of the Baroclinic Tides in the Central North Pacific Subtropical Gyre

Abstract Cruises are made monthly to the World Ocean Circulation Experiment deep-water station, about 100 km north of Oahu, Hawaii. During these cruises, the principal causes of variability in dynamic height are the internal tides having a range about 30% of the annual range in dynamic height. The internal tides have peak-to-peak vertical displacements of up to 30 dbar but show little vertical coherence. At the diurnal frequencies modal fits typically require 6–8 baroclinic modes to account for 90% of the variance, suggesting that the diurnal tide should not be described in terms of dynamic modes. The semidiurnal tide has about two to three times the amplitude of the diurnal tide and shows more vertical phase coherence, but still requires three or four modes to describe it. Generally, the dominant mode is the second or third mode.

Distribution and condition of pueruli of the spiny lobster Jasus edwardsii offshore from north-east New Zealand

Pelagic spiny lobster, Jasus edwardsii, pueruli and phyllosomas were sampled on offshore transects from the south-east coast of the North Island of New Zealand in February 1998. Carapace length, weight, and total lipid content of pueruli (n = 360) were assessed; 33 pueruli had soft carapaces, indicative of recent metamorphosis from final-stage (stage 11) phyllosoma larvae. The recently metamorphosed pueruli occurred 24–216 km offshore, much farther offshore than has been previously suggested for the location of metamorphosis. Their distribution was compared to information on their size and condition, as well as their distance offshore, water depth, temperature and salinity, and estimates of phytoplankton biomass. The results indicate that a threshold of larval energy reserve is unlikely to trigger metamorphosis, but rather some exogenous trigger may be involved, or metamorphosis may be part of a programmed developmental process. The distribution and condition of the new pueruli suggest that about 84% had the energetic capacity to swim inshore to settle. This result may have important implications for patterns of puerulus settlement and subsequent recruitment of lobsters to coastal populations and their important associated fisheries. It may influence both the seasonal and interannual variability in settlement observed in this species.

Acoustic Doppler Current Profiler measurements over the Chatham Rise

Abstract In September 1991, several north‐south transects were made across the Chatham Rise along 179°E, using a ship equipped with a 150 kHz Acoustic Doppler Current Profiler (ADCP). Current meters were deployed near the crest of the rise. There is good agreement between the ADCP and current meter measurements. Zonal (along‐rise) currents are dominated by diurnal tides, whereas meridian (cross‐rise) currents are dominated by semi‐diurnal tides. An estimate of the mean currents over 4 days of intensive observations over the northern flank of the Chatham Rise shows currents of up to 0.15 m s‐1, probably associated with meanders of the Subtropical Convergence.