scholarly journals The Leeuwin Current south of Western Australia

1993 ◽  
Vol 44 (2) ◽  
pp. 285 ◽  
Author(s):  
GR Cresswell ◽  
JL Peterson
Keyword(s):  
Wind Stress ◽  
Western Australia ◽  
Satellite Image ◽  
Subtropical Water ◽  
Tropical Waters ◽  
Leeuwin Current ◽  
Low Salinity ◽  
Tropical Water ◽  
Autumn And Winter

Satellite images as well as data collected in situ were used to follow the seasonal changes of the Leeuwin Current south of Western Australia (WA) in 1986-87. The current has two major sources: salty subtropical water from west of WA, and fresher tropical water from north of WA. In summer, the tropical waters are excluded by the strong equatorward wind stress. In autumn and winter, this wind stress is reduced and tropical waters flood southward to dominate the flow. Nevertheless, salty subtropical water is entrained en route, and so, whatever the season, the Leeuwin Current is more saline than the 'local' subantarctic waters off southern WA. From a research vessel, observations were made on the current and one of its offshoots in June 1987. The Leeuwin Current had a maximum surface speed of more than 1 m s-1 just beyond the shelf edge. Its warm, low-salinity surface core rode on a sheath of higher-salinity subtropical water that it had entrained upstream. The first survey of the offshoot showed it to be 50 km across and 130 m deep (for water warmer than 17�C), and it extended 200 km seaward (as deduced from a satellite image). Velocities in the offshoot ranged up to 1 m s-1 southward and 1 m s-1 north-eastward on the western and eastern sides, respectively. Richardson numbers were, in places, as low as 0.25. On a second survey two days later, the offshoot was found to have pinched off and the remnant bulge on the edge of the parent stream to have moved 30 km eastward. The flow around this bulge reached 1.6 m s-'. The offshoot/bulge was possibly first formed in April, and it kept its identity at least until August. During this time, it moved eastward at speeds between 2 and 15 km day-1. In June, the offshoot was estimated to contain water equivalent to five days' transport of the parent current.

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

2020 ◽  
Vol 8 (8) ◽  
pp. 552
Author(s):  
Mohd Fadzil Akhir ◽  
Charitha Pattiaratchi ◽  
Michael Meuleners
Keyword(s):  
Wind Stress ◽  
Western Australia ◽  
Current System ◽  
Ocean Model ◽  
Model Systems ◽  
Comprehensive Overview ◽  
Easterly Wind ◽  
Leeuwin Current ◽  
Regional Ocean Model ◽  
Autumn And Winter

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.

Dynamics of the Ningaloo Current off Point Cloates, Western Australia

2006 ◽  
Vol 57 (3) ◽  
pp. 291 ◽  
Author(s):  
Mun Woo ◽  
Charitha Pattiaratchi ◽  
William Schroeder
Keyword(s):  
Numerical Model ◽  
Continental Shelf ◽  
Western Australia ◽  
Bottom Topography ◽  
Volume Transport ◽  
Relative Strength ◽  
Cross Sectional ◽  
Leeuwin Current ◽  
The North ◽  
Tropical Water

The Ningaloo Current (NC) is a wind-driven, northward-flowing current present during the summer months along the continental shelf between the latitudes of 22° and 24°S off the coastline of Western Australia. The southward flowing Leeuwin Current is located further offshore and flows along the continental shelf break and slope, transporting warm, relatively fresh, tropical water poleward. A recurrent feature, frequently observed in satellite images (both thermal and ocean colour), is an anti-clockwise circulation located offshore Point Cloates. Here, the seaward extension of the coastal promontory blocks off the broad, gradual southern shelf, leaving only a narrow, extremely steep shelf to the north. The reduction in the cross-sectional area, from the coast to the 50 m contour, between southward and northward of the promontory is ~80%. Here, a numerical model study is undertaken to simulate processes leading to the development of the recirculation feature offshore Point Cloates. The numerical model output reproduced the recirculation feature and indicated that a combination of southerly winds, and coastal and bottom topography, off Point Cloates is responsible for the recirculation. The results also demonstrated that stronger southerly winds generated a higher volume transport in the NC and that the recirculation feature was dependent on the wind speed, with stronger winds decreasing the relative strength of the recirculation.

Effect of the Leeuwin Current upon sea surface temperatures off south-western Australia

1984 ◽  
Vol 35 (4) ◽  
pp. 487 ◽  
Author(s):  
DJ Rochford
Keyword(s):  
Western Australia ◽  
Coastal Waters ◽  
Sea Surface ◽  
Sea Surface Temperatures ◽  
Leeuwin Current ◽  
South Eastern ◽  
Surface Temperatures ◽  
Autumn And Winter

Comparison of long-term mean monthly sea surface temperatures of coastal waters at comparable latitudes off south-eastern and south-westem Australia shows that, during the duration of the Leeuwin Current in autumn and winter, sea surface temperatures are 1-3�C higher off south-western Australia.

Ring structure in the poleward boundary current off Western Australia in Summer

1983 ◽  
Vol 34 (4) ◽  
pp. 547 ◽  
Author(s):  
JC Andrews
Keyword(s):  
Structure Function ◽  
Western Australia ◽  
Large Scale ◽  
Function Analysis ◽  
Ring Structure ◽  
Boundary Current ◽  
Length Scales ◽  
Leeuwin Current ◽  
Tropical Water ◽  
Geopotential Anomaly

Data from five summer cruises off Western Australia are examined objectively using structure functions to establish principal length scales and amplitudes of mesoscale fields. Previous estimates of length scales using geopotential anomaly and geomagnetic electrokinetograph vectors as inputs to structure- function analyses gave length scales that differed by a factor of two. The present analysis shows that there are two length scales, which dominate in different parts of the flow, and this reconciles the two previous estimates. The shorter scale is λs = 157�25 km and the longer is λL = 309�28 km. Regions of strong large-scale currents have warm- and cold-core rings and mesoscale waves associated with them that assume the Rossby deformation scale. These are the λS structures. The longer, λL structures are found in regions of weak large-scale currents. Geopotential anomaly amplitudes and currents in the rings are, respectively, about 0.7 m2 s-2 (geopotential relief = 1 4 m2 s-1) and 70 cm s-1. Data from one summer cruise with a station density of approximately 12 per degree square are analysed in detail subjectively and the structure-function analysis is shown to be quantitatively meaningful. This cruise was near the shelf and shows the advection of low-salinity tropical water poleward over the slope in a narrow baroclinic current. Seaward cyclonic rings were associated with the current. The baroclinic structure of the current and of the rings is compatible with the winter behaviour of Lagrangian drifters released into the Leeuwin Current.

Origin and circulation of water types of the 25.00 sigma-t surface of the South-west Pacific

1969 ◽  
Vol 20 (2) ◽  
pp. 105 ◽  
Author(s):  
DJ Rochford
Keyword(s):  
Continental Margin ◽  
High Salinity ◽  
Vertical Mixing ◽  
Water Type ◽  
Subtropical Water ◽  
Water Types ◽  
Low Salinity ◽  
Tropical Water ◽  
South West ◽  
Mixed Transition

The salinity and oxygen of 25.00 sigma-t waters of the Tasman and Coral Seas have been averaged within 5� squares. From these average values four water types were identified by their salinity-oxygen relations. These were: a tropical high salinity (mean salinity 36.00‰, mean oxygen 3.50 ml/l.), a tropical low salinity (mean salinity 35.25‰, mean oxygen 3.10 ml/l.), a subtropical high salinity (mean salinity 35.75‰, mean oxygen 5.15 ml/l.), and a subtropical low salinity (mean salinity 35.46‰, mean oxygen 5.15 ml/l.). Waters north of 15�S. were all mixtures of the two tropical water types, and those south of 35� S. were all mixtures of the two subtropical water types. Topography of the 25.00 sigma-t surface showed easterly flow at around 15� S. and, as far as could be interpreted, easterly flow at 35� S. Between these two zones of tropical and subtropical water types there occurred a region of mixed transition waters in which large meridional gradients of oxygen but small gradients of salinity were found. This transition region occupied a much larger area off east Australia than elsewhere, mainly because there was greater meridional exchange of the tropical and subtropical water types off east Australia. North of 20� S. the high salinity subtropical water type spreads northward in the central Tasman and Coral Seas in summer and the low salinity tropical water type spreads southward along the Queensland continental margin in winter. South of 20� S. it was impossible to separate changes due to circulation from those caused by vertical mixing and winter formation of 25.00 sigma-t water at the surface.

The Leeuwin Current near Rottnest Island, Western Australia

1996 ◽  
Vol 47 (3) ◽  
pp. 483 ◽  
Author(s):  
G Cresswell
Keyword(s):  
Satellite Image ◽  
Acoustic Doppler Current Profiler ◽  
Maximum Speed ◽  
Temperature Structure ◽  
Near Surface ◽  
Fish Schools ◽  
Leeuwin Current ◽  
Low Salinity ◽  
Current Profiler ◽  
Good Agreement

Ship data and a satellite image in June 1987 showed the Leeuwin Current as a warm, low-salinity tropical stream travelling southward inshore of the 180-m isobath with near-surface speeds up to 0.9 m s-1. Farther offshore, where the waters became progressively more subtropical, the southward currents were also quite strong--0.75 m s-1 above the continental slope and over 0.4 m s-1 out to 70 km beyond the shelf edge. Beyond this, a doming of 150 m in the temperature structure at several hundred metres depth drove a cyclonic eddy that had its maximum speed of ~0.5 m s-1 in a ring at 200-400 m depth. The presence of the eddy was confirmed by the path of a drifter. Geostrophic currents and currents measured directly with an Acoustic Doppler Current Profiler showed good agreement. The warm 'shoulder' of the Leeuwin Current between the 105-m and 135-m isobaths was a biological oasis characterized by, inter alia, several fish schools at least 10 km long and 1 km wide and with vertical extents from 20 m to more than 100 m depth.

Outbreak densities of the coral predator Drupella in relation to in situ Acropora growth rates on Ningaloo Reef, Western Australia

Coral Reefs ◽  
2018 ◽  
Vol 37 (4) ◽  
pp. 985-993 ◽  
Author(s):  
C. Bessey ◽  
R. C. Babcock ◽  
D. P. Thomson ◽  
M. D. E. Haywood
Keyword(s):  
Western Australia ◽  
Growth Rates ◽  
Ningaloo Reef

scholarly journals Spatial variability of phytoplankton pigment distributions in the Subtropical South Pacific Ocean: comparison between in situ and predicted data

2008 ◽  
Vol 5 (2) ◽  
pp. 353-369 ◽  
Author(s):  
J. Ras ◽  
H. Claustre ◽  
J. Uitz
Keyword(s):  
High Performance ◽  
South Pacific ◽  
Hplc Method ◽  
Trophic Levels ◽  
Phytoplankton Assemblages ◽  
Tropical Water ◽  
South Pacific Ocean ◽  
Upwelled Water ◽  
Gyre System

Abstract. In the frame of the BIOSOPE cruise in 2004, the spatial distribution and structure of phytoplankton pigments was investigated along a transect crossing the ultra-oligotrophic South Pacific Subtropical Gyre (SPSG) between the Marquesas Archipelago (141° W–8° S) and the Chilean upwelling (73° W–34° S). A High Performance Liquid Chromatography (HPLC) method was improved in order to be able to accurately quantify pigments over such a large range of trophic levels, and especially from strongly oligotrophic conditions. Seven diagnostic pigments were associated to three phytoplankton size classes (pico-, nano and microphytoplankton). The total chlorophyll-α concentrations [TChlα] in surface waters were the lowest measured in the centre of the gyre, reaching 0.017 mg m−3. Pigment concentrations at the Deep Chlorophyll Maximum (DCM) were generally 10 fold the surface values. Results were compared to predictions from a global parameterisation based on remotely sensed surface [TChlα]. The agreement between the in situ and predicted data for such contrasting phytoplankton assemblages was generally good: throughout the oligotrophic gyre system, picophytoplankton (prochlorophytes and cyanophytes) and nanophytoplankton were the dominant classes. Relative bacteriochlorophyll-α concentrations varied around 2%. The transition zone between the Marquesas and the SPSG was also well predicted by the model. However, some regional characteristics have been observed where measured and modelled data differ. Amongst these features is the extreme depth of the DCM (180 m) towards the centre of the gyre, the presence of a deep nanoflagellate population beneath the DCM or the presence of a prochlorophyte-enriched population in the formation area of the high salinity South Pacific Tropical Water. A coastal site sampled in the eutrophic upwelling zone, characterised by recently upwelled water, was significantly and unusually enriched in picoeucaryotes, in contrast with an offshore upwelling site where a more typical senescent diatom population prevailed.

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