48 - Evaluating Geomorphic Features as Surrogates for Benthic Biodiversity on Australia’s Western Continental Margin

Hydrography and water masses off the western Australian coast

The 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.

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

The Leeuwin Current and its eddies: An introductory overview

Numerical modelling of the mean flow characteristics of the Leeuwin Current System

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.

Indonesian Throughflow (ITF) waters move along multiple pathways within the Indian Ocean. The western route is within the thermocline of the South Equatorial Current (SEC), and the southern route is via injection into the Leeuwin Current (LC) along western Australia. We use gridded Argo data to examine heat content anomaly (HCa) within three boxes in the eastern Indian Ocean, one adjacent to the ITF outflow from the Indonesian Seas (ITF box), the second in the eastern portion of the SEC (SEC box), and the third in the LC (LC box). Although interannual HCa variability in the SEC and ITF boxes is well correlated, a large increase in HCa within the ITF box does not appear in the SEC box in 2011 but is evident in the LC box. The 2011 change in the SEC–LC partitioning is investigated using GODAS reanalysis by examining the strength of the SEC and LC during a 2009 HCa increase within the ITF box and the subsequent increase in 2011. During 2009, a strong SEC and weakened LC spread the increased ITF HCa into the central Indian Ocean, whereas a weak SEC and strengthened LC during 2011 transmit the HCa signal to the south. Near-surface winds and mean sea level pressure from NCEP–NCAR reanalysis reveal that Ningaloo Niño events led to shifts in ocean circulation during 2000 and 2011. LC and SEC exports show a high negative correlation at interannual time scales, indicating that a reduction of outflow from one pathway is partially compensated by an increase from the other.

Dynamics of the Leeuwin Current: Part 2. Impacts of mixing, friction, and advection on a buoyancy-driven eastern boundary current over a shelf

Benthic foraminifera from the Carnarvon Ramp reveal variability in Leeuwin Current activity (Western Australia) since the Pliocene

The Leeuwin Current (LC) is an eastern boundary current flowing strongly southwards along the Western Australian coastline. The current varies seasonally, with weaker southwards flow occurring during the austral summer (November to March), when southerly winds are strongest. Nearly 9.5 years of TOPEX/Poseidon-derived sea-surface heights, in conjunction with the AUSGeoid98 gravimetric geoid model, have been used to investigate the temporal characteristics of the LC over the area bounded by 20 to 45°S and 108 to 130°E. The ocean dynamic height associated with the LC is used to compute the geostrophic parameters of volume transport, axis velocity, height jump, width, and axis locations of the current. These parameter estimates are then used to analyse the seasonal and interannual variability of the LC, as well as effects related to the El Nino Southern Oscillation (ENSO). Our results show that the LC is highly variable in terms of seasonal volume transport and is closely linked to ENSO events at interannual time-scales. Seasonally, when assuming an averaged isobath of 80 m, the LC has maximum volume transport of up to -7.2 Sv (106 m3 s-1, a negative sign means southwards transport) in April - June and -8.4 Sv in May - August in regions west and south of Western Australia, respectively. Inter-annually, the LC has higher volume transports than the average transport during 1993 - 2003 in most La Nina years, and the opposite in most El Nino years, which is linked to inter-basin exchange between the Pacific and Indian Oceans.

A continental shelf scale examination of the Leeuwin Current off Western Australia during the austral autumn–winter

Potential mechanisms of influence of the Leeuwin Current eddy system on teleost recruitment to the Western Australian continental shelf

Numerical simulation of the circulation within the Perth Submarine Canyon, Western Australia

Nutrients in an oligotrophic boundary current: Evidence of a new role for the Leeuwin Current

Sporadic upwelling on a downwelling coast: Phytoplankton responses to spatially variable nutrient dynamics off the Gascoyne region of Western Australia