Philip Sutton

The mean and variability of ocean circulation past northern New Zealand: Determining the representativeness of hydrographic climatologies

Eastward flow in the Tasman Sea, from the separated East Australia Current, reattaches to the shelf break near North Cape, New Zealand, and then continues alongshore to the southeast as the East Auckland Current. A series of three permanent warm core eddies occurs along the offshore side. The mean transport of the East Auckland Current is about 9 Sv, with an additional 10 Sv or more of circulation in the eddies. An extensive hydrographic data set, archived broad scale expendable bathythermograph (XBT) data, two repeating high-resolution XBT transects, neutrally buoyant float trajectories, and TOPEX altimetric data are used to estimate the temperature and absolute flow fields and to characterize variability. The aim is to examine the usefulness of time series information and absolute velocity measurements in the interpretation of hydrographic snapshots and climatologies, as well as to describe a region having intrinsic oceanographic interest and complexity. Issues of representativeness of the hydrographic data, of the magnitude and scales of the underlying variability, of the existence of permanent fine-scale features, and of the appropriateness of deep reference levels are addressed directly. The relatively well sampled hydrographic climatology is shown to contain the equivalent of as many as 10 independent realizations. Temperature errors, relative to the true mean, are typically a few tenths of a degree. Significant seasonal bias is identified in the surface layer, and interannual bias is seen in the position of an eddy near North Cape. The dynamic height field at 1000 dbar relative to 2000 dbar is similar to estimates based on float trajectories and the assumption of geostrophic dynamics. This study underlines the value of time series data in the interpretation of a hydrographic climatology, in quantifying the errors in the estimated mean field as well as determining the magnitude and nature of variability. It also highlights the fact that the mean circulation of the oceans contains significant mesoscale structure, unnoticed in coarsely smoothed climatologies.

The Southland Current: a subantarctic current

Abstract The Southland Current is a northward flow of water along the south‐east coast of New Zealand. This current has been studied many times since the early 1960s, with particular emphasis on an associated narrow coastal band of warm, salty water of subtropical origin, separated from offshore cold, fresh Subantarctic Water (SAW) by the Southland Front. Previous works on the Southland Current state that it advects modified warm, salty Subtropical Water (STW). This work quantifies the relative proportions of SAW and STW within the current. The Southland Current has a mean transport of 8.3 Sv comprising c. 90% SAW and 10% STW. The mean properties advected result from the current extending offshore of the Southland Front: in fact the core of the current is offshore of the Southland Front.

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.

Detailed structure of the Subtropical Front over Chatham Rise, east of New Zealand

A detailed description of the Subtropical Front (STF) over Chatham Rise is presented. Information is provided by conductivity-temperature-depth (CTD) profiler temperature and salinity transects, high-resolution (3–4 km spacing), near-synoptic expendable bathythermograph surveys, shipboard acoustic Doppler current profiler measurements, a 160 km constant depth undulator tow, and satellite SST measurements in austral autumn and spring (April and October) near 178°30′E. Detailed sections across the front reveal that the STF can be divided into northern and southern fronts (NSTF and SSTF) separated by a frontal zone, consistent with observations of the STF elsewhere. Comparison with a western CTD section along 176°E reveals that this structure breaks down close to the New Zealand landmass. The dominant impact of seasonally is the formation and decay of a seasonal thermocline of fairly constant depth over the entire region. Velocity measurements indicate eastward zonal flows on the northern and southern flanks of Chatham Rise. The flow over Chatham Rise is complicated by stirring processes but has a mean southward trend consistent with the less dense, warm, salty subtropical water (STW) overriding the more dense, cold, fresh subantarctic water (SAW) to the south. Horizontal-length-scale analyses of the temperature field for a constant depth tow at 21 m indicate a k−2.4 spectral falloff with no proof of preferential length scales. The temperature salinity gradients are largely density compensated in this mixed layer tow. The horizontal temperature standard deviation shows subsurface maxima concentrated at the locations of the NSTF and SSTF. Vertical length scales vary across the front, with large scales (80–100 m) in the STW north of the STF and smaller scales (20–40 m) in the subtropical frontal zone and SAW to the south. The surface vertical length scales are dominated by the presence of a seasonal thermocline.

The Southwest Pacific Ocean circulation and climate experiment (SPICE)

The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions.

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.

Closing the Time-Varying Mass and Heat Budgets for Large Ocean Areas: The Tasman Box

Abstract The role of oceanic advection in seasonal-to-interannual balances of mass and heat is studied using a 12-yr time series of quarterly eddy-resolving expendable bathythermograph (XBT) surveys around the perimeter of a region the authors call the Tasman Box in the southwestern Pacific. The region contains the South Pacific’s subtropical western boundary current system and associated strong mesoscale variability. Mean geostrophic transport in the warm upper ocean (temperature greater than 12°C) is about 3.8 Sv (1 Sv ≡ 106 m3 s−1) southward into the box across the Brisbane, Australia–Fiji northern edge. Net outflows are 3.3 Sv eastward across the Auckland, New Zealand–Fiji edge, and 2.7 Sv southward across Sydney, Australia–Wellington, New Zealand. Mean Ekman convergence of 2.2 Sv closes the mass budget. Net water mass conversions in the upper ocean consist of inflow of waters averaging about 26°C and 35.4 psu balanced by outflow at about 18°C and 35.7 psu, and reflect the net evaporation and heat los...

Three-Dimensional Observations of a Deep Convective Chimney in the Greenland Sea during Winter 1988/89

Abstract All available temperature data, including moored thermistor, hydrographic, and tomographic measurements, have been combined using least-squares inverse methods to study the evolution of the three-dimensional temperature field in the Greenland Sea during winter 1988/89. The data are adequate to resolve features with spatial scales of about 40 km and larger. A chimney structure reaching depths in excess of 1000 m is observed to the southwest of the gyre center during March 1989. The chimney has a spatial scale of about 50 km, near the limit of the spatial resolution of the data, and a timescale of about 10 days. The chimney structure breaks up and disappears in only 3–6 days. A one-dimensional vertical heat balance adequately describes changes in total heat content in the chimney region from autumn 1988 until the time of chimney breakup, when horizontal advection becomes important. A simple, one-dimensional mixed layer model is surprisingly successful in reproducing autumn to winter bulk temperatur...

Poor Transferability of Species Distribution Models for a Pelagic Predator, the Grey Petrel, Indicates Contrasting Habitat Preferences across Ocean Basins

Species distribution models (SDMs) are increasingly applied in conservation management to predict suitable habitat for poorly known populations. High predictive performance of SDMs is evident in validations performed within the model calibration area (interpolation), but few studies have assessed SDM transferability to novel areas (extrapolation), particularly across large spatial scales or pelagic ecosystems. We performed rigorous SDM validation tests on distribution data from three populations of a long-ranging marine predator, the grey petrel Procellaria cinerea, to assess model transferability across the Southern Hemisphere (25-65°S). Oceanographic data were combined with tracks of grey petrels from two remote sub-Antarctic islands (Antipodes and Kerguelen) using boosted regression trees to generate three SDMs: one for each island population, and a combined model. The predictive performance of these models was assessed using withheld tracking data from within the model calibration areas (interpolation), and from a third population, Marion Island (extrapolation). Predictive performance was assessed using k-fold cross validation and point biserial correlation. The two population-specific SDMs included the same predictor variables and suggested birds responded to the same broad-scale oceanographic influences. However, all model validation tests, including of the combined model, determined strong interpolation but weak extrapolation capabilities. These results indicate that habitat use reflects both its availability and bird preferences, such that the realized distribution patterns differ for each population. The spatial predictions by the three SDMs were compared with tracking data and fishing effort to demonstrate the conservation pitfalls of extrapolating SDMs outside calibration regions. This exercise revealed that SDM predictions would have led to an underestimate of overlap with fishing effort and potentially misinformed bycatch mitigation efforts. Although SDMs can elucidate potential distribution patterns relative to large-scale climatic and oceanographic conditions, knowledge of local habitat availability and preferences is necessary to understand and successfully predict region-specific realized distribution patterns.

Ocean temperature climate off North‐East New Zealand

Abstract The ocean temperature field off the north‐east coast of New Zealand is studied to quantify the annual cycle and reveal the intra‐ and inter‐annual variability. The data used are repeat expendable bathythermograph (XBT) sections between Auckland and either Suva or Honolulu which have been collected quarterly since 1986. These sections give temperature measurements between the surface and 800 m and Auckland and 30°S from 1986 to August 1999. The mean and annual cycle are compared with those from the NOAA World Ocean Atlas (WOA98). The results are similar; however WOA98 lacks the horizontal resolution to fully discern the East Auckland Current and North Cape Eddy, while the XBT analysis lacks the temporal resolution to discern higher frequency intra‐annual signals. The temperature variability in the mixed layer is dominated by the annual cycle, which accounts for 80–90% of the variance. The amplitude of the annual cycle diminishes rapidly with depth, from 2.8°C at the surface, to c. 0.1°C at 180 m. The phase of the annual cycle is retarded with depth, with peak temperatures occurring in February at the surface and in June/July at 180 m. Removing the annual cycle from the time series reveals the more subtle inter‐ and intra‐annual variability. This variability is of the order of 1°C in the upper 50 m, decreasing to 0.3°C at 400–500 m. The surface layer was cold between 1991 and 1994 (c. 0.7°C cooler than average), and 0.7°C warmer than average in 1999. The deeper ocean shows a different signal, being up to 0.3°C cooler in 1990–92, 0.3°C warmer in 1998, and c. 0.2°C warmer than average in 1999. The inter‐annual mixed layer variability is highly correlated with the Southern Oscillation Index and also with inter‐annual terrestrial air temperature and wind measurements from northern New Zealand. In contrast, at higher intra‐annual frequencies, the mixed layer variability is not correlated with air and wind measurements. At these higher frequencies, the air temperature is better correlated with the sea surface temperature (SST) than with the bulk mixed layer temperature.