Western Boundary Current System Research Articles

Deep western boundary current and southern frontal systems of the Antarctic Circumpolar Current southeast of the Kerguelen Plateau

The structures of the deep western boundary current and southern frontal systems of the Antarctic Circumpolar Current (ACC) southeast of the Kerguelen Plateau were investigated. Top‐to‐bottom hydrography was conducted around 58°S, and two underway observations were made around 56°S and 61°S. Middepth floats were also used to describe the subsurface flow and temperature fields. Along the 58°S section, a cold dome was located just off the plateau, and northwestward and southeastward flows were found in its western and eastern sides. The observed flow structure was highly barotropic. The maximum volume transport of the bottom water was 22 Sv equatorward, including a possible cyclonic recirculation of 10 Sv. The transport of the deep recirculation can be explained by the potential vorticity homogenization in the closed isopachs formed by the bottom slope off the plateau and the overlying ACC flowing southeastward through the Fawn Trough. The spatial scale of the cold dome, which showed a signature of the southern ACC front, was around 100 km at 58°S, and the similar cold domes were also found around 56°S and 61°S, along with a signature of the southern boundary of ACC around 61°S. The sharp excursion of the southern ACC front was also confirmed by the middepth float trajectories and the distribution of temperature maximum. This indicates the equatorward excursion up to 54°S, with the warmer ACC water just east of the southern frontal systems.

The Atlantic Subtropical Front/Current Systems of Azores and St. Helena

Abstract A large-scale climatic ocean circulation model was used to study the Atlantic Ocean circulation. This inverse model is an extension of the β-spiral formulation presented in papers by Stommel and Schott with a more complete version of the vorticity equation, including relative vorticity in addition to planetary vorticity. Also, a more complete database for hydrological measurements in the Atlantic Ocean was used, including not only the National Oceanographic Data Center database but also World Ocean Circulation Experiment data and cruises near the Azores, Angola, and Guinea-Bissau. A detailed analysis of the Northern Hemisphere Azores Current and Front shows that this new database and the model results were able to capture all major features reported previously. In the Southern Hemisphere, the authors have identified fully and described the subtropical front that is the counterpart to the Azores Current, which they call the St. Helena Current and Front. Both current systems of both hemispheres have similar intensities, depth penetration, volume transports, and zonal flow. Both have associated subsurface adjacent countercurrent flows, and their main cores flow at similar latitudes (∼34°N for the Azores Current and 34°S for the St. Helena Current). It is argued that both current systems and associated fronts are the poleward 18°C Mode Water discontinuities of the two Atlantic subtropical gyres and that both originate at the corresponding hemisphere western boundary current systems from which they penetrate into the open ocean interior. Thus, both currents should have a similar forcing source, and their origin should not be linked to any geographical peculiarities.

The Annual Cycle of Circulation of the Southwest Subtropical Pacific, Analyzed in an Ocean GCM*

Abstract An ocean GCM, interpreted in light of linear models and sparse observations, is used to diagnose the dynamics of the annual cycle of circulation in the western boundary current system of the southwest Pacific Ocean. The simple structure of annual wind stress curl over the South Pacific produces a large region of uniformly phased, stationary thermocline depth anomalies such that the western subtropical gyre spins up and down during the year, directing flow anomalies alternately toward and away from the boundary at its northern end, near 10°S. The response of the western boundary currents is to redistribute these anomalies northward toward the equator and southward to the subtropical gyre, a redistribution that is determined principally by linear Rossby processes, not boundary dynamics. When the subtropical gyre and South Equatorial Current (SEC) are strong (in the second half of the year), the result is both increased equatorward transport of the New Guinea Coastal Current and poleward transport anomalies along the entire Australian coast. Because of this opposite phasing of boundary current anomalies across 10°S, annual migration of the bifurcation point of the total SEC, near 18°S in the mean, has no significance regarding variability of transport from subtropics to equator.

Modelo paramétrico analítico para a estrutura de velocidade do sistema corrente do Brasil

Modelos paramétricos de velocidade de correntes são utilizados na literatura oceanográfica para estudos teóricos envolvendo propriedades de instabilidade, ou como condições de contorno e iniciais para implementações regionais de modelos numéricos. Estes, no entanto, foram desenvolvidos na sua maioria para a Corrente do Golfo (Niiler & Robinson, 1967; Luther & Bane, 1985; Robinson et al., 1988; Arango et al., 1992; Xue & Mellor, 1993; Gangopadhyay et al., 1997). O sistema formado pela Corrente do Brasil (CB) e pela Corrente de Contorno Intermediária (CCI) apresenta características dinâmicas singulares enquanto flui ao largo do SE brasileiro, quando comparada as demais correntes de contorno oeste do oceano mundial. Neste trabalho, apresentamos um modelo paramétrico analítico original. O modelo parametriza a estrutura tanto da corrente de superfície (a CB) como da corrente intermediária (a CCI), visando preservar suas características essenciais. No caso da CB, suas principais características são a presença de um núcleo em superfície e o comportamento ocasionalmente assimétrico longitudinalmente devido ao fenômeno do meandramento. Fato que a faz aproximar e afastar o escoamento superficial da quebra de plataforma e do talude superior. Já para a CCI, a principal característica é apresentar o núcleo em torno dos 800-900 m de profundidade, geralmente junto ao talude intermediário. Adicionalmente ao modelo seccional de velocidades, um outro modelo de estrutura de densidade é obtido, assumindo que o campo de velocidades é estacionário e geostrófico: tratam-se, portanto, de campos básicos. Este modelo de densidade é construído a partir da integração da relação do vento térmico. Por fim, a parametrização é confrontada com três seções de velocidades baroclínicas obtidas a partir de observações dentro dos limites da bacia de Campos: 21ºS, 22ºS e 23ºS.

Resolution convergence and sensitivity studies with North Atlantic circulation models. Part I: The western boundary current system

The fidelity of numerical simulations of the general circulation of the North Atlantic Ocean in basin- to global-scale models have improved considerably in the last several years. This improvement appears to represent a regime shift in the dynamics of the simulated flow as the horizontal grid spacing decreases to around 10 km. Nevertheless, some significant biases in the simulated circulation and substantial uncertainties about the robustness of these results with respect to parameterization choices remain. A growing collection of simulations obtained with the POP primitive equation model allow us to investigate the convergence properties and sensitivity of high resolution numerical simulations of the North Atlantic, with particular attention given to Gulf Stream separation and the subsequent path of the North Atlantic Current into the Northwest Corner. Increases in resolution and reductions in dissipation both contribute to the improvements in the circulation seen in recent studies. We find that our highest resolution eddy-resolving simulations retain an appreciable sensitivity to the closure scheme. Our most realistic simulations of the Gulf Stream are not obtained at the lowest levels of dissipation, while the simulation of the North Atlantic Current continues to improve as dissipation is reduced to near the numerical stability limit. In consequence, there is a limited range of parameter space where both aspects of the simulated circulation can be brought into agreement with observations. This experience gained with the comparatively affordable regional North Atlantic model is now being used to configure the next generation of ocean climate models.

An Assessment of the Sea Surface Temperature Influence on Surface Wind Stress in Numerical Weather Prediction and Climate Models

Abstract The ability of six climate models to capture the observed coupling between SST and surface wind stress in the vicinity of strong midlatitude SST fronts is analyzed. The analysis emphasizes air–sea interactions associated with ocean meanders in the eastward extensions of major western boundary current systems such as the Gulf Stream, Kuroshio, and Agulhas Current. Satellite observations of wind stress from the SeaWinds scatterometer on NASA’s Quick Scatterometer and SST from the Advanced Microwave Scanning Radiometer clearly indicate the influence of SST on surface wind stress on scales smaller than about 30° longitude × 10° latitude. Spatially high-pass-filtered SST and wind stress variations are linearly related, with higher SST associated with higher wind stress. The influence of SST on wind stress is also clearly identifiable in the ECMWF operational forecast model, having a grid resolution of 0.35° × 0.35° (T511). However, the coupling coefficient between wind stress and SST, as indicated by the slope of the linear least squares fit, is only half as strong as for satellite observations. The ability to simulate realistic air–sea interactions is present to varying degrees in the coupled climate models examined. The Model for Interdisciplinary Research on Climate 3.2 (MIROC3.2) high-resolution version (HIRES) (1.1° × 1.1°, T106) and the NCAR Community Climate System Model 3.0 (1.4° × 1.4°, T85) are the highest-resolution models considered and produce the most realistic air–sea coupling associated with midlatitude current systems. Coupling coefficients between SST and wind stress in MIROC3.2_HIRES and the NCAR model are at least comparable to those in the ECMWF operational model. The spatial scales of midlatitude SST variations and SST-induced wind perturbations in MIROC3.2_HIRES are comparable to those of satellite observations. The spatial scales of SST variability in the NCAR model are larger than those in the ECMWF model and satellite observations, and hence the spatial scales of SST-induced perturbations in the wind fields are larger. It is found that the ability of climate models to simulate air–sea interactions degrades with decreasing grid resolution. SST anomalies in the GFDL Climate Model 2.0 (CM2.0) (2.0° × 2.5°), Met Office Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) (2.5° × 3.8°), and MIROC3.2 medium-resolution version (MEDRES) (2.8° × 2.8°, T42) have larger spatial scales and are more geographically confined than in the higher-resolution models. The GISS Model E20/Russell (4.0° × 5.0°) is unable to resolve the midlatitude ocean eddies that produce prominent air–sea interaction. Notably, MIROC3.2_MEDRES exhibits much weaker coupling between wind stress and SST than does the higher vertical and horizontal resolution version of the same model. GFDL CM2.0 and Met Office HadCM3 exhibit a linear relationship between SST and wind stress. However, coupling coefficients for the Met Office model are significantly weaker than in the GFDL and higher-resolution models. In addition to model grid resolution (both vertical and horizontal), deficiencies in the parameterization of boundary layer processes may be responsible for some of these differences in air–sea coupling between models and observations.

Development of a regional ocean model for New Zealand

A regional ocean model with a horizontal resolution of 1/6° encompassing the New Zealand Exclusive Economic Zone is described. The regional model successfully downscaled solutions from a high resolution, global, coupled model HadCEM. Transport estimates from the global and regional models were compared with observations, and both models supported largely consistent, climatological mean solutions. The regional model used monthly mean forcing at the surface. Nevertheless, the regional model eddy kinetic energy (EKE) spatial patterns compared favourably with long‐term mean satellite altimetric estimates, although the modelled background EKE amplitudes were much lower than observed. A series of permanent eddies associated with the western boundary current system around the top of the North Island of New Zealand were reproduced, and an eddy adjacent to Norfolk Ridge was identified in both the global and regional models. The western boundary current system around the North Island of New Zealand and the associated eddies were the most sensitive components of the model solutions, being influenced by initial conditions, wind forcing, and the model domain size.

Water Mass Export from Drake Passage to the Atlantic, Indian, and Pacific Oceans: A Lagrangian Model Analysis

Abstract The northward export of intermediate water from Drake Passage is investigated in two global ocean general circulation models (GCMs) by means of quantitative particle tracing diagnostics. This study shows that a total of about 23 Sv (Sv ≡ 106 m3 s−1) is exported from Drake Passage to the equator. The Atlantic and Pacific Oceans are the main catchment basins with 7 and 15 Sv, respectively. Only 1–2 Sv of the water exported to the Atlantic equator follow the direct cold route from Drake Passage without entering the Indian Ocean. The remainder loops first into the Indian Ocean subtropical gyre and flows eventually into the Atlantic Ocean by Agulhas leakage. The authors assess the robustness of a theory that relates the export from Drake Passage to the equator to the wind stress over the Southern Ocean. Our GCM results are in reasonable agreement with the theory that predicts the total export. However, the theory cannot be applied to individual basins because of interocean exchanges through the “supergyre” mechanism and other nonlinear processes such as the Agulhas rings. The export of water from Drake Passage starts mainly as an Ekman flow just northward of the latitude band of the Antarctic Circumpolar Current south of South America. Waters quickly subduct and are transferred to the ocean interior as they travel equatorward. They flow along the eastern boundaries in the Sverdrup interior and cross the southern basins northwestward to reach the equator within the western boundary current systems.

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 loss in the formation of South Pacific Subtropical Mode Water. The mean heat balance shows good agreement between ocean heat flux convergence (42.3 W m−2), heat loss to the atmosphere from the NCEP–NCAR reanalysis (39.2 W m−2), and heat storage calculated from data in the box interior (1.3 W m−2). On interannual time scales, volume transport through the box ranges from about 1 to 9 Sv, with heat flux convergence ranging from about 20 to 60 W m−2. An interannual balance in the heat budget of the warm layer is achieved to within about 10 W m−2 (or 6 W m−2 for the upper 100 m alone). Maxima in the advective heat flux convergence occurred in 1993, 1997, and 1999–2000, and corresponded to maxima in air–sea heat loss. The evolution of surface-layer temperature in the region is the residual of nearly equal and opposing effects of ocean heat flux convergence and air–sea exchange. Hence, ocean circulation is a key element in the interannual heat budget of the air–sea climate system in the western boundary current region.

Linkages between coastal and open-ocean habitats and dynamics of Japanese stocks of chum salmon and Japanese sardine

Abstract Coastal-ocean–open-ocean migrations, prey–predator relations and long-term population dynamics of chum salmon (Oncorhynchus keta) and Japanese sardine (Sardinops melanostictus), associated with large-scale climate and oceanographic conditions, are reviewed. After early marine life in coastal waters in northern Japan, chum salmon of Japanese origin spend their first summer in the Okhotsk Sea, then move to the Western Subarctic Gyre for the first winter at sea. Thereafter, they migrate between summer feeding grounds in the Bering Sea and wintering grounds in the Alaskan Gyre for a period of usually up to four years, and finally return to their natal rivers to spawn. Carrying capacity (K) for chum salmon at an unfished equilibrium level was estimated from a Ricker spawner–recruitment curve, and the residual carrying capacity ( RCC = ( K - abundance ) K - 1 ). was positively correlated with body size at age 4, and negatively correlated with age at maturity. Marine survival of Hokkaido chum populations was affected by body size at release, but neither by Aleutian low pressure activity nor sea-surface temperature (SST) around coastal Hokkaido in spring, although there is some correlation between survival rate and coastal SST. Juveniles of the Pacific stock of Japanese sardine become broadly distributed in the Kuroshio Extension (KE) as far east as 180° longitude during spring. Adults disperse as far as the central Pacific and the southern areas of the Okhotsk Sea and Western Subarctic Gyre in years of high abundance. Somatic growth and age at maturation of sardine are density-dependent. We used catch, biomass and residuals of observed recruitment numbers from a Ricker curve (LNRR) as a measure of sardine population dynamics. LNRR was highly correlated with SST of KE in winter, which shifted in 1970 and 1988. Recent biomass and catch remain at extremely low levels due to a combination of adverse environmental conditions and intensive fishing. We suggest that Japanese populations of chum salmon and Japanese sardine have a broader geographic range than North American populations, perhaps because of differences between western and eastern boundary current systems, and associated larval drift (sardine) or avoidance of intra-species competition (chum salmon).

On the baroclinic structure of the Brazil Current–Intermediate Western Boundary Current system at 22°–23°S

The baroclinic structure of the Brazil Current(BC)‐Intermediate Western Boundary Current (IWBC) at 22°–23°S was investigated. A reanalysis of the pioneer velocity profile measurements of the TRANSCOBRA Experiment [Evans and Signorini, 1985] revealed that the BC‐IWBC system is about 75–80% baroclinic. Mapped velocity structure showed flow reversal at about 450 m, an IWBC thickness of 1200 m and core velocities exceeding 0.30 m s−1. Total (baroclinic) transports for BC (southwestwards) and IWBC (northeastwards) were 5.6 (4.2) Sv and 3.6 (4.1) Sv, respectively. The strong baroclinic character of the BC‐IWBC system and the lack of direct velocity observations in the area yielded us to propose the use of the cross‐shelf version of the Princeton Ocean Model to generate absolute baroclinic velocities from hydrographic data. These velocities presented a similar reversal depth, and the transports of about 6–7 Sv for both BC‐IWBC were also comparable to values reported in the literature.

Reproductive Biology of Southwestern Atlantic Wreckfish, Polyprion americanus (Teleostei: Polyprionidae)

We report on the reproductive biology of southwestern Atlantic wreckfish. Females mature first at 77.9 cm total length (TL) (10.4 years) and all are mature by 90 cm TL (15.2 years). Males mature first at 74.9 cm (9 years) and all are mature by 80 cm TL (10.9 years). The wreckfish is a gonochoristic multiple spawner and the gonadal cycle is synchronized at the population level. Spawning occurs from late July to early October along the continental slope (<300 m). Ovarian fecundity varies from 3 to 11.9 million (135–311 oocytes×g−1) and increases exponentially with length. Spawning at western boundary current systems, maintained by homing of adults, is a basic requirement for self-sustaining populations of this species.

Instabilidade linear de cisalhamento vertical em jatos geofísicos meridionais

The study of linear vertical shear instability of meridional (north-south) jets is conducted via an unidimensional hydrodynamic model, based on quasi-geostrophic potential vorticity conservation. The shape of the vertical jet profiles are motivated by the Brazil Current-Intermediate Western Boundary Current System that flows off the Brazilian Southeast coast and are built with the aim to satisfy necessary conditions for occuring baroclinic instability. The hydrodynamic model is based on Johns (1988) and its formulation is presented without the omission of algebraic details. Its physics is explored through numerically-solved examples and comparison with the analytical part of the model. The purpose is to subsidize the diagnosis of forms in which instability occur: at the ocean surface and in the interior of the water column. Additionally, the effect of the continental slope on the instability properties is investigated and discussed, through its effects on the propagation of unstable waves generated by instability process.

Spawning patterns of four species of predominantly temperate pelagic fishes in the sub-tropical waters of southern Queensland

The diverse pelagic fish assemblage of sub-tropical southern Queensland includes fishes with predominantly temperate distributions, such as tailor Pomatomus saltatrix, sardine Sardinops sagax, round herring Etrumeus teres, and Australian anchovy Engraulis australis. The peak spawning seasons of P. saltatrix, S. sagax and E. teres occur during late winter and early spring (June–October). Eggs and larvae of these three species are widely distributed in shelf waters and comprise >50% of the ichthyoplankton assemblage during this period. Mean monthly sea surface temperatures (SSTs) during late winter and early spring range from 21 to 23 °C, and are thus similar to those recorded in southern Australia during summer and autumn, which is the spawning season of these three species in those temperate waters. E. australis eggs occur mainly in inshore waters, and comprise >50% of fish eggs collected during summer and autumn when mean monthly SSTs in southern Queensland exceed 27 °C. E. australis also spawns mainly during summer and autumn in temperate Australia. Hence, water temperature may be less important as a determinant of the spawning season of E. australis than it is for the other three species. The suitability of southern Queensland for spawning by predominantly temperate species during late winter and early spring may contribute to the high diversity of the region's pelagic fish assemblage. Adult P. saltatrix, S. sagax and E. teres appear to migrate northwards into southern Queensland during early winter to spawn, and larvae may be transported southwards into temperate waters by the East Australian Current. This dispersal-migration pattern is similar to those observed for several species, including P. saltatrix, in the western boundary current systems off the east coasts of North America and Africa. Hence, pelagic fishes in ecosystems off the east coast of three continents migrate into sub-tropical waters to spawn, and larvae are transported back into temperate nursery areas by the prevailing current.

Eulerian and Lagrangian Statistics from Surface Drifters and a High-Resolution POP Simulation in the North Atlantic

Eulerian and Lagrangian statistics were calculated from the North Atlantic surface drifter dataset for the years 1993–97 and a high-resolution eddy-resolving configuration of the Los Alamos National Laboratory (LANL) Parallel Ocean Program (POP) model. The main purpose of the study was to statistically quantify the state of the surface circulation in the North Atlantic Ocean for this period and compare it with the equivalent modeled state. Diffusivities and time and length scales are anisotropic over most of the ocean basin, except in most of the subpolar regions. Typical time and length scales are 2–4 days and 20–50 km. Longest timescales are found in the energetically quiescent regions in the south and southeast sectors of the basin. The longest length scales are found in the energetic western boundary current system, the most dispersive region of the domain. In many respects the eddy-resolving model reproduced a surface circulation in good statistical agreement with that depicted by the drifters. Model time and length scales were also anisotropic, with typical timescales of 2–4 days and length scales of 20–50 km in the zonal direction, and 30–50 km in the meridional direction. An eddy-permitting POP simulation produced unrealistic time and length scales that were too long and too short relative to the drifter scales; these were attributed to the model being too stable hydrodynamically.

Eulerian and Lagrangian Statistics from Surface Drifters and a High-Resolution POP Simulation in the North Atlantic

Abstract Eulerian and Lagrangian statistics were calculated from the North Atlantic surface drifter dataset for the years 1993–97 and a high-resolution eddy-resolving configuration of the Los Alamos National Laboratory (LANL) Parallel Ocean Program (POP) model. The main purpose of the study was to statistically quantify the state of the surface circulation in the North Atlantic Ocean for this period and compare it with the equivalent modeled state. Diffusivities and time and length scales are anisotropic over most of the ocean basin, except in most of the subpolar regions. Typical time and length scales are 2–4 days and 20–50 km. Longest timescales are found in the energetically quiescent regions in the south and southeast sectors of the basin. The longest length scales are found in the energetic western boundary current system, the most dispersive region of the domain. In many respects the eddy-resolving model reproduced a surface circulation in good statistical agreement with that depicted by the drifters. Model time and length scales were also anisotropic, with typical timescales of 2–4 days and length scales of 20–50 km in the zonal direction, and 30–50 km in the meridional direction. An eddy-permitting POP simulation produced unrealistic time and length scales that were too long and too short relative to the drifter scales; these were attributed to the model being too stable hydrodynamically.

A general circulation model study of the dynamics of the upper ocean circulation of the South China Sea

The temporal‐spatial structure and the formation mechanisms of the South China Sea (SCS) circulation are studied using the Princeton Ocean Model (POM). The model well reproduces the observed sea surface height (SSH) annual cycle and some current system such as the SCS Western Boundary Current (SCSWBC) system and the Kuroshio Loop Current (KLC). Four sensitivity experiments are carried out to reveal the dynamic mechanisms of the SCS circulation. The results show that most of the seasonal variability of the SCS is controlled predominantly by wind forcing. The Kuroshio affects the mean SSH significantly but contribute little to the variability. The SCSWBC system consists of the SCS Warm Current (SCSWC) and the Vietnam Coastal Current (VCC). In winter the SCSWBC splits into two branches separated at ∼14°–18°N. The northern branch is the SCSWC flowing northward and the southern branch is the VCC flowing southward. In summer, however, the SCSWBC flows unidirectionally northward from the Karimata Strait to the Taiwan Strait. The temporal variation of the SCSWBC, especially the VCC, is determined by the wind forcing over the interior SCS. Buoyancy forcing can strengthen (weaken) the summer (winter) SCSWBC. The KLC is crucial to the SCS circulation north of 18°N. It ultimately determines the appearance of the SCSWC in winter.

Behavior of Warm-Core Rings in a Double-Gyre Wind-Driven Ocean Circulation Model

Numerical experiments have been performed to understand the movement of warm-core rings in the Kuroshio-Oyashio Transition region, considering the double-gyre ocean circulation represented by a 2.5-layer primitive equation model. Rings, eddies, and frontal disturbances are simulated, and analytical descriptions of warm-core ring behavior are examined. Warm-core rings are usually generated at the south of the zero wind stress curl line (ZWCL), either off-shore or near-shore, and a near-shore ring gradually moves northward along the coast, supplied with energy from the sub-tropical front and other rings. When they approach the ZWCL, two different types of movements occur, which is affected by the intensity of the rings and controlled by the wind stresses. In case the ring intensity is strong, it moves northward along the coast against the subarctic western boundary current system and cold-core rings, whereas a moderate ring is advected by them to the northeastward. The advection process of moderate rings seems to be important for northeastward movement of the Kuroshio warm-core rings in the middle of the Oyashio current system. The mechanism of large-scale forcing dynamics influences the cross-shore location of rings and implications for the fishing ground formation of the Pacific saury are described.

Contourites of the Nova Scotian continental rise and the HEBBLE area

Abstract The Nova Scotian continental rise is swept by a Deep Western Boundary Current system comprising layers of Labrador Sea Water overlying a core of Norwegian Sea Overflow Water at depths of 3100-3900 m, and below about 4600 m a cold stream of southern-source water. Seismic-reflection data show that the rise contains sediments transported downslope in channels and debris lobes, but there is also evidence of current-controlled deposition and erosion in the post-Eocene sequence. The rise is now mantled by Holocene contourites that have accumulated at a rate of c . 6 cm ka -1 and are &lt;1 m thick. Bottom photographs show a zonation in current effects and bedform types, with longitudinal ripples and strong currents prevalent at 4800-5000 m, smaller bedforms and progressively weaker currents up to c . 4000 m, and mostly tranquil seafloor above 4000 m. Bedform scales and orientations also suggest significant short-term (hours to weeks) variability in current velocity but a mean contour-following flow to the southwest at longer time scales (months to years). These structures are not preserved in the sediment because of pervasive bioturbation and the uppermost layers have negligible preservation potential. The sediments display clear current controlled effects in their grain-size structure involving both percentage of (foraminiferal) sand, and size and percentage in the 10-63 µm range, the 'sortable silt'. There is a sand-rich zone at 4800-4900 m and below 5000 m, and a decreasing silt/clay ratio from 5100 m up to 4000 m. Although much of the sedimentary sequence probably has been emplaced by downslope processes, it has been significantly modified by the Deep Western Boundary Current. Particularly strong and variable currents which rework sediments below c . 4800 m probably are engendered by interaction of Gulf Stream eddies with the DWBC. Although strong currents and upstream input from tur-bidites and debris flows might be thought to favour a coarse-grained deposit, the facies at the HEBBLE (High Energy Benthic Boundary Layer Experiment) site is muddy contourite with ≤ 12% sand.

Life cycles, oceanography and variability: ommastrephid squid in variable oceanographic environments

Exploited populations of ommastrephid squid are found under a wide range of oceanographic regimes. However, to date most scientific attention has focussed on those found in northern hemisphere western boundary current systems, and these systems have become the paradigm for theoretical work. Dosidicus gigas, in the east central-southeast Pacific, and Martialia hyadesi, in the southwest Atlantic, provide examples of the interactions between squid life cycles and regional oceanography outside of the paradigm. Illex argentinus, also in the southwest Atlantic, provides a southern hemisphere example of a western boundary current species. These examples are used to highlight the key issues involved in understanding squid population variability within the context of variable oceanographic environments. The issues include the fundamental influence of oceanographic variability on population variability, and the importance of a thorough understanding of the life cycle of the study species in order to detect and understand the relationships between it and the environment. The importance of understanding the relative temporal and spatial scales on which the environmental and biological factors interact is also considered. Overall, a broader understanding of the interactions between oceanographic variability and squid life cycles is necessary to interpret successfully the adaptation of ommastrephid squid species to their environment and to allow the effective forecasting and management of fishery resources.