Leeuwin Current System Research Articles

Transitions in surface thermal signatures during the evolution of long-lived eddies in the global ocean

Mesoscale eddies are quasi-circular ocean currents accompanied by dynamic and thermodynamic variations, and are classified as anticyclonic (AEs) and cyclonic eddies (CEs) according to their rotation direction. Eddies transition between surface cold CEs/surface warm AEs (“conventional” eddies) and surface warm CEs/surface cold AEs (“unconventional” eddies) during their lifetimes. However, the characteristics and mechanisms of these transitions remain unclear. In this study, satellite and in-situ data were used to explore the spatiotemporal variation and vertical structures of “conventional” and “unconventional” eddies during the evolution of long-lived eddies (>1 year) in the global ocean from 1993 to 2015. On average, long-lived eddies are “unconventional” for about 40% of their lifetimes, and these “unconventional” eddies are concentrated in the South Atlantic Ocean and off the western and southern coasts of Australia. “Unconventional” eddies show distinct temporal variation and subsurface temperature structures. Generally, cold AEs and warm CEs last <3 months and are more active in summer than in winter. They have cold or warm cores within the mixed layer depth of the water column, which is affected by eddy–wind interactions via eddy-induced Ekman pumping. However, some cold AEs in the South Atlantic and warm CEs in the South Indian Ocean can last for >3 months and display weak seasonal variation. In addition, their cold and warm cores can extend to ∼200 and ∼300 m, respectively, and are related to subduction and the Leeuwin Current System.

Variability of Heat Content and Eddy Kinetic Energy in the Southeast Indian Ocean: Roles of the Indonesian Throughflow and Local Wind Forcing

Abstract Prominent interannual-to-decadal variations were observed in both heat content and mesoscale eddy activity in the southeast Indian Ocean (SEIO) during 1993–2020. The 2000–01 and 2008–14 periods stand out with increased 0–700-m ocean heat content (OHC) by ∼4.0 × 1021 J and enhanced surface eddy kinetic energy (EKE) by 12.5% over 85°–115°E, 35°–12°S. This study provides insights into the key dynamical processes conducive to these variations by analyzing observational datasets and high-resolution regional ocean model simulations. The strengthening of the Indonesian Throughflow (ITF) and anomalous cyclonic winds over the SEIO region during the two periods are demonstrated to be the most influential. While the ITF caused prevailing warming of the upper SEIO, the cyclonic winds cooled the South Equatorial Current and attenuated the warming in the subtropical SEIO by evoking upwelling Rossby waves. The EKE increase exerts significant influence on OHC only in the Leeuwin Current system. Dynamical instabilities of the Leeuwin Current give rise to high EKEs and westward eddy heat transport in climatology. As the Leeuwin Current was enhanced by both the ITF and local winds, the elevated EKEs drove anomalous heat convergence on its offshore flank. This process considerably contributes to the OHC increase in the subtropical SEIO and erases the wind-driven cooling during the two warm periods. This work highlights the vital role of eddies in regional heat redistribution, with implications for understanding time-varying ocean heat storage in a changing climate.

Revisit the Vertical Structure of the Eddies and Eddy‐Induced Transport in the Leeuwin Current System

AbstractThe vertical structure of eddies in the Leeuwin Current system affects the eddy volume, heat, and salt transport, even the ecosystem. However, the understanding of eddy vertical structure and eddy‐induced transport here are still very limited. In this study, satellite observed sea surface heights were combined with decade‐long in situ measurements of Argo floats to study the vertical structure of mesoscale eddies in the LC system and their volume, heat, and salt transport. A novel eddy reconstruction method, which considers the influences of both the eddy and background flow, is devised to study the three‐dimensional structure of eddies. Result shows that, in LC system, anticyclonic eddies (AEs) are usually surface‐intensified, with the geostrophic velocity decreasing sharply below the mixed layer, while cyclonic eddies (CEs) are subsurface‐intensified, with a maximum speed at 240 m. The density anomaly core of the average AE (CE) is at a depth of 130 m (650 m) with a density anomaly of −0.51 (0.24) kg/m3. The volume‐integrated eddy kinetic energy and available potential energy of the average CE are much larger than those of the average AE. The average lifespan of CEs is significantly longer than that of AEs, which can be explained by the deeper vertical scale of CEs. The offshore volume transport by eddy drift across the coastal (107°E) section is 9.05 Sv (12.5 Sv). The heat and salt onshore transport by eddy drift across the coastal (107°E) section are, respectively, 10.6 Tw and 143.1 ton/s (17.1 Tw and 241.0 ton/s).

Spatiotemporal Variations of Mesoscale Eddies in the Southeast Indian Ocean

AbstractStrong eddy activities exist in the Leeuwin Current (LC) and the South Indian Countercurrent (SICC) systems. Through a detailed investigation based on satellite observations, we find a cyclonic preference for eddies with more frequent genesis, longer lifespan, smaller size and stronger intensity in both systems. The evolutions of amplitudes, radii and total vorticities of eddies can be classified as the generation, stable, and decay stages; however, those of the EKE and relative vorticities lack the stable stage, due to planetary vorticity changes related to the meridional deflections, as eddies drift westward. Eddy properties exhibit significant seasonal and interannual variations. In the LC system, more and stronger eddies tend to be generated in winter and in La Niña years; while in the SICC system, eddy properties generally reach their peaks in spring and in years when the negative Southern Annular Mode (SAM) occurs. And eddy intensities in the SICC system are also modulated by ENSO with about a one‐year phase lag. Through instability analysis, we find that baroclinic instability is the most dominant contributor to the development of eddies. By contrast, barotropic instability mainly acts to dampen eddies. Advection of EKE makes almost no net contribution when spatially averaged, although it is locally significant. Pressure work and dissipation may be nonnegligible EKE sinks in the LC system. The EKE variations in these two systems have close connections through the westward‐propagating baroclinic instability anomalies, which may explain the phase lags between these two systems.

Dynamics and Seasonality of the Leeuwin Current and the Surrounding Counter-Current System in the Region South of Western Australia

Surface circulation associated with the Leeuwin Current System off the southern coast of Western Australia was simulated using the Regional Ocean Model Systems (ROMS). The Leeuwin current (LC) and Flinders current (FC) were reproduced in two simulation: with and without wind stress. The inclusion of wind resulted in a strong LC during autumn and winter months with the LC flowing close to the shelf, accelerating after reaching the south-west corner at Cape Leeuwin. The geopotential gradient was present through all seasons, indicating that it is the major driving force of the currents. At the subsurface, continuation of the opposing undercurrent present at the southwest corner. Interchanging of strength and transport between LC and FC can be seen between seasons, where LC strength drops significantly in autumn and winter when the wind stress is low and this subsequently increases the FC transport. The FC strength declines in summer when there is no wind stress, which during this time LC is stronger. Meanwhile, the analysis shows an inshore presence of Cresswell current is evident along the coast when there is south-easterly wind in summer. The study provides comprehensive overview of the complex currents system where wind influence proves to be determining factors to the current system.

Influence of atmospheric rivers on the Leeuwin Current system

Previous work has demonstrated the strong ocean response to atmospheric rivers (ARs) in the northeast Pacific including coastal currents along the west coast of North America, because of strong surface winds associated with ARs. A recent study on the global distribution of ARs also suggests that the southeast Indian Ocean is one of the areas of relatively strong AR activity. This study investigates the influence of ARs on the Leeuwin Current system, which is one of the major boundary currents in the Indian Ocean. It is demonstrated that winds associated with typical ARs in the southeast Indian Ocean can generate strong poleward coastal currents and sea level rise along the west coast of Australia using a high-resolution ocean reanalysis (0.08° HYCOM). The composite of upper ocean currents and sea surface height (SSH) associated with landfalling ARs along the west coast of Australia is constructed using the HYCOM reanalysis, long-term AR data set, and tide gauge data. The enhancement of the poleward currents generated by ARs is found in the composite, and the magnitude of the enhancement is comparable to the strength of the Leeuwin Current itself. The results also indicate that the fluctuation of SSH and coastal currents along the west coast propagates along the southern coast all the way to the southeast coast (Pacific side) of Australia. The SSH propagation along the coasts is also detected in the tide gauge data in the west and southern coasts of Australia.

Southern Australia Current System based on a gridded hydrography and a high-resolution model

We quantify the annual-mean structure, transport budget and coupling between surface and deep currents along the southern Australian shelves, that we propose be collectively known as the Southern Australia Current System. The system contains eastward Shelf Break Currents (SBC) including the Leeuwin Current Extension, the South Australian Current and the Zeehan Current; and a counter-flowing westward Flinders Current (FC). In this work, we use a high resolution climatological hydrography and a high resolution ocean model forced by an atmospheric climatology.The westward FC is dual structured with (1) the slope-FC, an undercurrent to the SBC trapped near 600 m depth; and (2) the offshore-FC, a large deep-reaching flow located offshore of the SBC. Along the slanted shelf (not zonally oriented), onshore flows feeding into the FC are weak, and the FC is disconnected from the Tasman Leakage south of Tasmania. Along the zonal shelf, where nearly 80% of total onshore flows occur, the FC intensifies resulting in 12.3 Sv transport at Cape Leeuwin.The eastward SBC are continuous flows in the upper 250 m. Over steep slopes, downwelling occurs from the SBC into the slope-FC and may be important in maintaining the slope-FC. The Leeuwin Current Extension transport decreases from 1.1 Sv at Cape Leeuwin to 0.1 Sv near the Great Australian Bight due to strong downwelling and offshore exports around Cape Leeuwin. The South Australian Current is weak and steady, and the Zeehan Current increases to 0.4 Sv due to onshore flows stronger than downwelling exports.We find that the SBC and offshore-FC are coupled through onshore Ekman drift and the SBC and slope-FC are coupled through downwelling. The combined effect is a conversion of widespread northward Ekman drift into downwelling with little impact on the eastward SBC transport. In contrast, the onshore flows feeding into the FC are largely converted into increasing westward FC transport. Even though the Leeuwin Current System off western Australia is fed by geostrophic flows and the Southern Australia Current System is fed by Ekman drift, the annual-mean circulation of the two systems are remarkably similar.In summer, when the winds are upwelling favourable, the SBC are weaker and partly reversed (to westward). In autumn, the winds become downwelling favourable and the FC is weaker and partly reversed (to eastward). In contrast, the Leeuwin Current System persists annually. The balance mechanism maintaining the seasonal opposition between the SBC and FC warrants further research.

Shallow water masses and their connectivity along the southern Australian continental margin

Four water masses are identified and described using hydrographic, nutrient and stable isotope data for the top 1000 m water depth of the southern Australian continental margin, from Cape Leeuwin to Tasmania. Three are identified from previous literature on the southeast Indian Ocean: Subtropical Surface Water (STSW), Tasmanian Subantarctic Mode Water (TSAMW) and Tasmanian Intermediate Water (TIW), and one is newly identified and named: South Australian Basin Central Water (SABCW). STSW (0–250 m) is transported east by the Leeuwin Current System and is modified by heating and evaporation along the subtropical continental shelf. SABCW (250–400 m) and TSAMW (400–650 m) form southwest of Tasmania from deep winter mixing in the region: SABCW at the Subtropical Front and TSAMW north of the Subantarctic Front. TIW (>650 m) forms southwest of Tasmania from mixing of warm, saline Antarctic Intermediate Water from the Tasman Sea and cool, fresh Antarctic Intermediate Water from the Antarctic Circumpolar Current. SABCW, TSAMW and TIW are transported west along the slope by the Flinders Current System, here defined as the western slope-trapped Flinders Current, Tasman Outflow and equatorward Sverdrup transport. Water mass distributions on the slope identify the interface between subantarctic water from the Southern Ocean and subtropical water transported by the Leeuwin Current System. This interface is ~300 m during winter and ~250 m during summer, but can be 150 m during summer in upwelling regions and off western Tasmania. Furthermore, stable isotope data of the water masses south and west of Australia show connectivity between the Subantarctic Zone, the southern Australian margin and the western Australian margin.

The three-dimensional structure of the Leeuwin Current System in density coordinates in an eddy-resolving OGCM

The Leeuwin Current System (LCS) consists of the Leeuwin Current (LC), a surface, poleward current along the west coast of Australia; the Leeuwin Undercurrent (LUC), a subsurface, equatorward current on the continental slope beneath the LC; and zonal flows in the interior ocean that interact with the LC and LUC. To understand the three-dimensional structure of the LCS circulation, we construct a mean flow over twenty years in z coordinates and density coordinates from three-day snapshots of an eddy-resolving ocean general circulation model (OGCM) and carry out a “leak-free” volume budget analysis on the mean flow field in both coordinate systems. The mean flow in z coordinates forms a zonal and a meridional overturning, where a near-surface eastward flow enters the LC, flows poleward and at the same time sinks to the LUC depths, flows equatorward, and finally moves westward out of the LCS, as found in a previous study. On the other hand, the mean flow in density coordinates is mostly isopycnic, except that the upper part of the LC becomes denser as it flows poleward partly owing to sea-surface cooling. The lower part of the LC becomes denser, not because of cooling but because denser water joins the LC from the interior ocean. The LUC becomes lighter as it flows equatorward because its lower part flows westward out of the LCS and its top part is joined by eastward flows from the interior. The mean downwelling in z coordinates is therefore isopycnic and is likely a result of temporal variability.

Marine debris pathways across the southern Indian Ocean

The distribution of marine debris both with the open oceans and along coastlines is largely governed by surface currents and thereby dependent upon seasonal circulation patterns and annual variability inherent in the major current systems. The recovery of a satellite-tracked current drifter on a South African beach some 22 months after release off Western Australia was an indicator of the ocean-wide processes operating in the southern Indian Ocean gyre. Using this single drift trajectory in combination with data from other current drifters released along the Western Australian coast between 2008 and 2013, from earlier satellite-tracked drifters and a selection of historical drift bottle returns, drift patterns across the southern Indian Ocean between Western Australia and east Africa were analysed to quantify inter-continental debris transport rates via the South Equatorial Current (northern east-west pathway) and the South Indian Ocean Current (southern west-east pathway). The northern east-west trans-ocean pathway can be considered in three segments: the complex and highly variable Leeuwin Current eddy system in the east, which can retain drifting debris for up to a year, the comparatively uniform westward travel in the South Equatorial Current, and the eddying components in the western boundary current system off southern Africa. A simple Eddy Activity Index is derived here to clearly distinguish the Leeuwin Current eddy region from the South Equatorial Current. The southern route to Australia is simpler via the Agulhas Return Current and the South Indian Ocean Current.

Mesoscale eddy characteristics in the interior subtropical southeast Indian Ocean, tracked from the Leeuwin Current system

Two oppositely rotating mesoscale eddies, originating from the Leeuwin Current, were captured during a research cruise in the interior southeast Indian Ocean in August 2012, a few hundred kilometers off the west coast of Australia. The two eddies had different vertical structures: the anticyclonic eddy had stronger vertical shear above 200 m and a less stratified thermocline below the mixed layer, while the cyclonic eddy had a more barotropic structure in the upper 500 m. Compared with the same types of eddies sampled in 2003 close to the Australian coast, the 2012 anticyclonic eddy had a larger radius, deeper isothermal depths, and higher peak velocity; whereas the 2012 cyclonic eddy had a similar structure with the 2003 survey. We use an Argo float and three surface drifters deployed and trapped in the anticyclonic eddy to track its evolution over a 4-month period. The eddy was observed to propagate 1000 km further to the west, indicating that the Leeuwin Current properties of the eddy may be preserved for over 2000 km off the west coast of Australia. The eddy appeared to have experienced radial excursions relative to the eddy center, with periodicity of 4–6 days and 15 days, during their anticlockwise motions. The radial movements of the drifters appeared to be constrained by the radial momentum balance within the eddy. The importance of eddy distortions to ocean primary production in anticyclonic eddies in the southeast Indian Ocean warrants further studies.

The Effect of the Leeuwin Current on Offshore Surface Gravity Waves in Southwest Western Australia

AbstractThe knowledge of regional wave regimes is critical for coastal zone planning, protection, and management. In this study, the influence of the offshore current regime on surface gravity waves on the southwest Western Australian (SWWA) continental shelf was examined. This was achieved by coupling the three dimensional, free surface, terrain‐following hydrodynamic Regional Ocean Modelling System (ROMS) and the third generation wave model Simulating WAves Nearshore (SWAN) using the Coupled Ocean‐Atmosphere‐WaveSediment Transport (COAWST) model. Different representative states of the Leeuwin Current (LC), a strong pole‐ward flowing boundary current with a persistent eddy field along the SWWA shelf edge were simulated and used to investigate their influence on different large wave events. The coupled wave‐current simulations were compared to wave only simulations, which represented scenarios in the absence of a background current field. Results showed that the LC and the eddy field significantly impact SWWA waves. Significant wave heights increased (decreased) when currents were opposing (aligning with) the incoming wave directions. During a fully developed LC system significant wave heights were altered by up to 25% and wave directions by up to 20°. The change in wave direction indicates that the LC may modify nearshore wave dynamics and consequently alter sediment patterns. Operational regional wave forecasts and hindcasts may give flawed predictions if wave‐current interaction is not properly accounted for.

On the Leeuwin Current System and Its Linkage to Zonal Flows in the South Indian Ocean as Inferred from a Gridded Hydrography

AbstractThe Leeuwin Current System (LCS) along the coast of Western Australia consists of the poleward-flowing Leeuwin Current (LC), the equatorward-flowing Leeuwin Undercurrent (LUC), and neighboring flows in the south Indian Ocean (SIO). Using geostrophic currents obtained from a highly resolved (⅛°) hydrographic climatology [CSIRO Atlas of Regional Seas (CARS)], this study describes the spatial structure and annual variability of the LC, LUC, and SIO zonal currents, estimates their transports, and identifies linkages among them. In CARS, the LC is supplied partly by water from the tropics (an annual mean of 0.3 Sv; 1 Sv ≡ 106 m3 s−1) but mostly by shallow (200 m) eastward flows in the SIO (4.7 Sv), and it loses water by downwelling across the bottom of this layer (3.4 Sv). The downwelling is so strong that, despite the large SIO inflow, the horizontal transport of the LC does not much increase to the south (from 0.3 Sv at 22°S to 1.5 Sv at 34°S). This LC transport is significantly smaller than previously reported. The LUC is supplied by water from south of Australia (0.2 Sv), by eastward inflow from the SIO south of 28°S (1.6 Sv), and by the downwelling from the LC (1.6 Sv) and in response strengthens northward, reaching a maximum near 28°S (3.4 Sv). North of 28°S it loses water by outflow into subsurface westward flow (−3.6 Sv between 28° and 22°S) and despite an additional downwelling from the LC (1.9 Sv), it decreases to the north (1.7 Sv at 22°S). The seasonality of the LUC is described for the first time.

Influence of the Leeuwin Current on the epipelagic euphausiid assemblages of the south-east Indian Ocean

The Leeuwin Current is an anomalous eastern boundary current which transports warm, low salinity water poleward off Western Australia. This study investigated epipelagic euphausiids in the Leeuwin Current system from 22°S to 34°S focusing on the latitudinal gradient in species richness and whether variability in euphausiid assemblages was associated with an increase in seawater density across latitude. Twenty-eight species of euphausiids (including five new records) were identified from the study area. Species richness remained relatively constant across latitude as the distribution of seven tropical species, including the dominant Pseudeuphausia latifrons, extended south of 30°S. Euphausiid assemblages from the northern shelf stations were distinct from the oceanic, shelf break and southern shelf assemblages which, though clustering together, showed evidence of latitudinal shifts and day/night influence, particularly at oceanic stations. Distance-based linear models confirmed that, of the environmental variables examined, mean seawater density was the only significant explanatory variable accounting for 32, 27 and 71% of the variation for shelf, shelf break and night oceanic assemblages, respectively. This study provides the first account of the diversity, distribution and abundance of euphausiids along the entire western seaboard of Australia and enhances understanding of the influence of the Leeuwin Current on holoplanktonic biota.

Chaetognaths of the Leeuwin Current system: oceanographic conditions drive epi-pelagic zoogeography in the south-east Indian Ocean

Chaetognaths are ubiquitous marine zooplankton strongly influenced by oceanographic conditions. Epi-pelagic plankton samples taken across the Leeuwin Current system, the poleward flowing eastern boundary current of the Indian Ocean, were examined for chaetognath diversity and distribution. Sampling occurred at shelf, shelf break, and oceanic environments at each degree of latitude from 22 to 34°S. Nineteen species were identified, of which six are new records for the region with one an undescribed species. Low-moderate abundances (19.0 ± 2.4 individuals m−3) were observed compared with other eastern boundary currents and can be attributed to the unique oligotrophic nature of the system. The anomalous oceanography of the Leeuwin Current resulted in two distinct zoogeographic regions with distributions of tropical and temperate species separated around 28°S. Here, tropical waters transitioned to cooler, saltier waters and multivariate analyses identified temperature and salinity as the best indicators of community change. Community variation across the continental shelf was largely driven by one species, Flaccisagittaenflata, which was numerically dominant. Despite spatial variations, widely distributed species caused high overall community similarity. As the first system-wide investigation of Leeuwin Current Chaetognatha, this study provides benchmark biological information and highlights an unusual oceanography that produces a distinct planktonic zoogeography.

A Dynamical Model for the Leeuwin Undercurrent

Abstract Recently, Furue et al. explored analytic solutions to a dynamical model for the Leeuwin Current system (LCS) off the coast of Western Australia. Their linear, variable density, two-layer model reduces to a one-layer system near the coast. The circulation is determined by matching solutions in the coastal and offshore regions across the “grounding line” and displays many features observed in the LCS. However, it does not include a Leeuwin Undercurrent (LUC). Here, that model is extended considering an across-shore density gradient (front) caused by relatively light, tropical water being carried poleward by the Leeuwin Current (LC). As a result of including the front, the LCS circulation changes considerably; the LC deepens and strengthens significantly toward the pole, and the LCS now includes an equatorward LUC on the offshore edge of the LC. Differences in density and sea surface height across the front both contribute to the pressure gradient driving the LUC.

On the factors influencing the development of sporadic upwelling in the Leeuwin Current system

[1] While there is no persistent upwelling along the West-Australian (WA) coastline, sporadic upwelling events have been documented primarily in summer. By analyzing comparatively the variability of both Ekman and geostrophic cross-shore transports over a seasonal cycle, we show that the situation is more contrasted. Based on a composite index computed from satellite data over a 15 year period, calibrated with well documented events, we investigate the factors influencing the development of sporadic upwelling in the region. Overall, the occurrence of transient upwelling events lasting 3–10 days varies largely in space and time. Shelf regions at 31.5 and 34°S are favored with up to 12 upwelling days per month during the austral spring/summer. Although being generally favored from September to April, sporadic upwelling events can also occur at any time of the year at certain locations north of 30°S. On average over 1995–2010, the Ningaloo region (22.5°S) cumulates the highest number of upwelling (∼140 days per year) and is characterized by longer events. The intensity of intermittent upwelling is influenced by the upwelling-favorable winds, the characteristics of the Leeuwin Current (e.g., onshore geostrophic flow, mesoscale eddies and meanders, stratification and nitracline) and the local topography. This suggests that short-living nutrient enrichment of variable magnitude may occur at any time of the year at many locations along the WA coast.

Larval fish assemblages and particle back-tracking define latitudinal and cross-shelf variability in an eastern Indian Ocean boundary current

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 460:127-144 (2012) - DOI: https://doi.org/10.3354/meps09730 Larval fish assemblages and particle back-tracking define latitudinal and cross-shelf variability in an eastern Indian Ocean boundary current D. Holliday1,*, L. E. Beckley1, N. Millar1, M. P. Olivar2, D. Slawinski3, M. Feng3, P. A . Thompson4 1School of Environmental Science, Murdoch University, 90 South St, Murdoch, 6150, Western Australia, Australia 2Institut de Ciènces del Mar (CSIC), Passeig Marítim 37-49, Barcelona 08003, Spain 3CSIRO Marine and Atmospheric Research, Private Bag 5, Wembley, Western Australia, 6913, Australia 4CSIRO Marine and Atmospheric Research, GPO Box 1538, Hobart, Tasmania 7701, Australia *Email: d.holliday@murdoch.edu.au ABSTRACT: Empirical and modelled data were used to examine the influence of the Leeuwin Current on larval fish assemblages along the Western Australian continental shelf and adjacent eastern Indian Ocean (22°S−34°S) during the late austral autumn. Larval fish assemblages, comprising >200 taxa from 114 neritic and oceanic teleost families, displayed significantly distinct latitudinal and cross-shelf variability, which was linked to meso-scale features of the Leeuwin Current, specifically anticyclonic eddies. Results of Lagrangian particle back-tracking showed good connection between the shelf and a developing eddy situated at 27°S; connectivity was highest in the 0−20 m depth stratum where >80% of the particles entering the eddy were derived from the shelf. This was supported by the high abundance (mean: 84 larvae m−2) of larval neritic taxa, especially small anchovy Engraulis australis larvae in the eddy. The lack of a significant difference in the size structure of these larvae between the shelf and eddy indicated continuous connection between these waters. In contrast, connectivity between the shelf and an older eddy further south was much lower (<40% of particles in the 0−20 m stratum) and the larval fish assemblage was dominated by meso-pelagic species. Further, the distribution of Myctophidae larvae (Diaphus spp.) highlighted areas of enhanced onshore transport in the south. The study has shown that, in the alongshore dominated Leeuwin Current system, cross-shelf transport can be similarly important in dispersal processes. KEY WORDS: Leeuwin Current · Western Australia · Eddy · Lagrangian · Myctophidae · Engraulidae Full text in pdf format Supplementary material PreviousNextCite this article as: Holliday D, Beckley LE, Millar N, Olivar MP, Slawinski D, Feng M, Thompson PA (2012) Larval fish assemblages and particle back-tracking define latitudinal and cross-shelf variability in an eastern Indian Ocean boundary current. Mar Ecol Prog Ser 460:127-144. https://doi.org/10.3354/meps09730 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 460. Online publication date: July 24, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Modelling the potential transport of tropical fish larvae in the Leeuwin Current

Application of an individual-based particle tracking model to the migration of tropical fish larvae along the continental shelf between the Houtman Abrolhos Islands and Rottnest Island (Western Australia) has shown that there is potential for the southwards advection of passive particles/larvae in the Leeuwin Current system throughout the year. However, seasonal variations in the prevailing wind field result in corresponding seasonal changes in the surface current flow (both alongshore and cross-shelf) on the continental shelf, leading to a pulse of modelled particles arriving at Rottnest Island during the autumn months. This matches, within a month, the observed April/May peak in annual recruitment of 2 species of damselfish ( Abudefduf sexfasciatus and A. vaigiensis), at the time when the Leeuwin Current is strengthening. It is assumed that the larvae are in the uppermost 20 m of the water column and that there are no vertical diurnal movements. The model has a 10 km grid spacing, and so can resolve some of the current gradients across the continental shelf. Comparison of the modelled near-surface currents with ADCP measurements at 2 current mooring sites as well as with a broader range of historical current measurements off south-western Australia indicates that the alongshore net transport is reasonably well reproduced by the model, whereas agreement with the cross-shelf flow is not as good (this may be partly attributed to the paucity of high-quality near-surface current measurements in the area of study). Because of limitations in our knowledge of the swimming ability, choice of swimming direction and habitat selection of larval fish, as well as the inability of the model to reproduce the small-scale circulation around Rottnest Island, the swimming capacity of the late-stage larval fish is not specifically included; they are considered as potential settlers once they have reached within 20 km of the Island. The observed inter-annual variability in recruitment is not as well matched by the model as is the seasonal pattern, but this is almost certainly due to uncertainties in biological factors such as spawning strength, food and predation en route, which are not known. The modelled results are also applied to a more general discussion of the transport of eggs and larvae of commercial fish and invertebrate species on the Western Australian continental shelf, and it is shown that the seasonality and position on the shelf of the spawning may play a large role in the movement (and hence survival and ultimate recruitment) of different species.

Contrasting local retention and cross-shore transports of the East Australian Current and the Leeuwin Current and their relative influences on the life histories of small pelagic fishes

Transport between shelf and offshore environments supports a significant proportion of ocean primary productivity and is critical to the life cycle of many marine species. While fundamental differences in the underlying dynamics of eastern and western boundary currents have been recognized and studied for more than half a century, the implications for physical dispersal rates have received much less attention. In this study we explore how Australia’s two major boundary current systems, the East Australian Current and the Leeuwin Current, differ in their local retention and cross-shore transports in the upper water column and how these differences favor contrasting life histories of small pelagic fishes. The results suggest that the East Australian Current forms a partial barrier to onshore transport, but is effective in entraining shelf waters and transporting them offshore, particularly in the region where the current separates from the coast. Blue mackerel ( Scomber australasicus) spawn on the outer-shelf in this separation region and may thereby maximize the dispersion of eggs and larvae in the mainly oligotrophic waters of the southern Coral Sea. In contrast, the Leeuwin Current system promotes onshore transport through the combined effects of mean onshore flow and eddy-induced mixing. In the Great Australian Bight, sardine ( Sardinops sagax) and anchovy ( Engraulis australis) may exploit the high coastal retention of the Leeuwin Current system by spawning on the inner-shelf during summer when the current is weakest and winds assist retention and enhance production through local upwelling.