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Hypersaline Hamelin Pool, with mean salinity >65, is located in Shark Bay, Western Australia. The high salinity has reduced its biodiversity, but it is home to a diverse assemblage of modern marine stromatolites. The limited exchange of water between Hamelin Pool and the rest of Shark Bay, due to the presence of the shallow Faure Sill together with high evaporation and low rainfall-runoff have resulted in a hypersaline environment. With climate-change-induced mean sea level rise (MSLR), hydrodynamic processes that maintain the hypersaline environment may be affected and are the focus of this paper. Oceanographic observations, together with hydrodynamic model results, were used to examine the hydrodynamic processes under present and future MSLR scenarios. A large attenuation in the tidal range, changes in the tidal characteristics, and current speeds together with a strong salinity gradient were observed across the Faure Sill under present-day conditions. Under an MSLR scenario of 1 m, the tidal amplitude decreased by up to 10% to the north, whilst to the south, the tidal range increased by up to 15%. Regions of strong vertical stratification were present on both sides of the Faure Sill. The simulations indicated that, under MSLR scenarios, these regions expanded in area and exhibited higher levels of vertical stratification. The salt flux across the Faure Sill was maintained as a diffusive process under MSLR scenarios. [ABSTRACT FROM AUTHOR]

Observations of upper-ocean dynamics close to the critical latitude (ratio of the local inertial to diurnal frequency is 0.94) from a range of platforms (surface currents using HF radar, moored instruments, and satellite remote sensing data) off southwest Australia indicated the presence of energetic, near-inertial waves generated through the diurnal-inertial resonance. During the austral summer, when southerly winds and land-sea breeze (LSB) system dominated the wind regime, strong counterclockwise diurnal motions (amplitudes surpassing 0.3 m s−1) penetrated to 300-m depth with diurnal vertical isotherm fluctuations up to 60 m. The upward phase propagation speed of ~140 m day−1, deep penetration of diurnal currents below the mixed layer, and the ~180° phase difference between the upper and lower water column suggested that the local LSB system caused the resonant diurnal motions. Relative vorticity fluctuations along two cross-shore transects indicated changes to the local effective Coriolis frequency by more than 50% (±0.5 f). In the presence of strong and relatively consistent cross-shore diurnal wind forcing in the study area, the main factors that controlled the observed energetic but sporadic near-inertial oscillations were the Leeuwin Current strength and spatial-temporal variations. These variations controlled the effective Coriolis frequency and enabled the effective pumping of diurnal wind energy into the ocean particularly when the effective Coriolis frequency was ~24 h. [ABSTRACT FROM AUTHOR]

We measured water levels in Two Rocks Marina, Western Australia, to investigate infragravity-period (25-300s) oscillations and their forcing mechanisms. Spectral analyses identified four dominant oscillations in the infragravity band, which were generated through excitation of the marina's natural periods. The oscillations were present at all times, independent of the offshore conditions, indicating that they were forced by a continuous external energy source. The spectral energy of the oscillations increased by ~50 times during storm events (higher incident wave heights), in comparison to calm events (lower incident wave heights). Wave heights of oscillations within the marina were strongly correlated with offshore incident swell wave heights and reached maximum of 0.5m. The groupiness factor of swell waves around the marina was 0.6-0.85. Bound infragravity waves associated with swell wave groups were identified as potential forcing mechanism of infragravity-period oscillations within the marina. The bound infragravity waves have broad frequency spectrum without dominant periods matched the marina's natural periods however, bound infragravity waves of periods in the proximity of the marina NOPs were adequate to generate oscillations at the NOPs of the marina. Frequencies of the oscillations were independent of the forcing frequency, and determined by the marina׳s geometry. [ABSTRACT FROM AUTHOR]

Abstract: The oceanic circulation of the region between Carnarvon (25°S) and Jurien Bay (31°S) was examined using observational and remotely sensed data in conjunction with a detailed numerical modelling study. The model was validated using in situ ADCP and CTD data, and the horizontal eddy viscosity parameterization was tested against field observations. The results generated using the Regional Ocean Model (ROMS v1.8) simulated the observed mean surface flow''s spatial and temporal scales, particularly those of the eddy features. The modelled results also provided insight into the Leeuwin Current (LC)''s annually averaged alongshore transport and the Leeuwin Undercurrent (LUC)''s mean flow characteristics. Analysis of the model results, between 26.8°S and 30.8°S, revealed the annually averaged alongshore transport across the LC''s southern boundary was 5.7Sv, of which the northern boundary contributed 3.60Sv or 63%. The inflow of water through the western boundary due to geostrophic balance was estimated at 2.1Sv or 37%, and found to vary meridionally. A geopotential gradient, estimated at 1.9×10−7 and acting towards the equator, drives the LUC. Located at depths between 250 and 600m, the modelled undercurrent is characterized as an energetic flow with mesoscale variability, that is strongly influenced by the more energetic surface current. Coupling was evident in the region adjacent to the Abrolhos Islands’ western boundary (29°S) where the offshore movement of LC waters by a surface meander caused the vertical scale of the undercurrent to increase, inducing an anticlockwise vorticity response at a depth of ∼180m. The resulting subsurface eddy grew preferentially downwards before a surface response was observed. The model showed that the eddy increased its vertical scale before splitting into two forming a surface and subsurface eddy with a vertical scale characteristic of the two currents. The surface eddy intensified, detached, and moved offshore, while the subsurface eddy remained attached to the topography. [Copyright &y& Elsevier]

Abstract: Light attenuation (K d) of photosynthetically active radiation (PAR) by chromophoric dissolved organic matter (CDOM), total suspended solids (TSS) and chlorophyll a (Chl a) were measured at nine stations along an estuarine gradient in the Swan River, Western Australia, over 15 months. There were strong spatial gradients associated with the marine-freshwater transition along the 32km of estuary sampled, as well as seasonal gradients mainly associated with rainfall, ∼80% of which occurs between May and September. CDOM absorbances at 440nm reached a maximum of 10.9m−1 with the freshwater inflow but concentrations of suspended matter remained low throughout the sampling period (1.0–21.0mgl−1) under the diurnal tides of the estuary. CDOM was the dominant constituent of K d and a stepwise multiple regression showed that 66% (p <0.0001) of the variation in K d can be explained by CDOM and an additional 8% (p <0.0001) by TSS. As a consequence of this result, analysis into the influence of river discharge rates on CDOM absorbance levels was examined for 2002 using data collected during this study, and for 2000 and 2001 using historical dissolved organic carbon (DOC) and river discharge data. The outcome of this analysis infers that greater river discharge rates result in increased CDOM absorbances in the Swan River estuary. [Copyright &y& Elsevier]

Seasonally open tidal inlets usually occur in micro-tidal, wave-dominated coastal environments where strong seasonal variations of streamflow and wave climate are experienced. These inlets are closed to the ocean for a number of months every year due to the formation of a sand bar across the entrance. Because the estuaries connected to these inlets commonly serve as harbours or recreational facilities, the annual inlet closure is undesirable in most cases. Hence, there is increased interest in understanding the processes governing inlet closure and the hydrodynamics and exchange processes of these estuaries. This study was undertaken with the objectives of: (a) identifying the morphodynamic processes governing seasonal inlet closure, and (b) determining the effect of the seasonal closure of the inlet on the hydrodynamic characteristics of the estuary/lagoon connected to the inlet. Wilson Inlet, a typical seasonally open tidal inlet located along the southwestern coast of Australia, was selected as a case study. The inlet is usually closed to the ocean for 6–7 months every year. Field experiments and numerical models were employed to examine inlet morphodynamics and estuarine hydrodynamics. As cross-shore processes, which the field data indicated to be important, have not been included in coastal area models to date, a quasi-3D morphodynamic model, which includes longshore and cross-shore processes, was developed to determine processes governing seasonal inlet closure. Application of the model to Wilson Inlet clearly indicates that onshore sediment transport during summer is the cause of inlet closure. Further applications show that summer streamflow and storm events also exert significant control on inlet closure depending on the intensity and timing of the event. The results of the estuarine hydrodynamic study indicate that the hydrodynamic regime of seasonally open estuaries changes distinctively when the inlet state varies from "open" to "closed." [ABSTRACT FROM AUTHOR]

Estimates of extreme sea levels have mainly been derived using atmospheric and tidal forcing only, but surface gravity waves are a key process that can cause a substantial elevation of mean sea level at the coast. In this paper a coupled wave-circulation model was used to simulate storm surge and wave setup across a broad range of coastal topographies and storm types around Australia. The main aim was a practical assessment of the benefits and limitations of using a coupled wave-surge model to determine wave effects for extreme sea level studies on the continental scale. The simulations included: tropical cyclones Yasi (2011) and George (2007) on the northeast and northwest coasts of Australia; Cyclone Alby (1978) that underwent extratropical transition in southwest Australia; and a large extratropical winter storm in the Southern Ocean (2016). Predicted wave setup was highly variable both spatially and temporally, with values up to 0.45 m, 35% of maximum storm surge height, and 9% of incident significant wave heights. In general, where storm surge heights were smaller, the relative importance of wave setup increased in the presence of large waves. The model best resolved wave setup in shallow gently sloping coastal areas, whilst unrealistically low values were predicted for steep coastlines. Predictions improved when wave setup was included for most sites and events, however, these wave effects were often secondary to other assumptions implicit in modeling extreme events. This was especially true for tropical cyclones where reliable wind forcing was not available. The results showed similar contributions from wave setup to other studies but highlighted the high temporal and spatial variability of wave setup across a wider variety of storms and coastal morphologies. The results also revealed that the maximum wave setup did not always coincide with the maximum storm surge. Overall, the coupled wave-surge model presented a useful tool to predict extreme water levels including wave setup when accurate depth and atmospheric data are available, however, significant challenges still exist to model this process at the continental scale. [Display omitted] • Continental scale coupled wave storm surge model was applied to examine extreme events around Australia. • Maximum wave setup preceded arrival of slow-moving storms and co-occurred with fast-moving storms. • Predicted wave setup contributed up to 35% of total storm surge. [ABSTRACT FROM AUTHOR]

In Shark Bay, a large hypersaline bay in Western Australia, longitudinal density gradients force gravitational circulation that is important for Bay-ocean exchange. First-time observations of vertical stratification and velocity are presented, confirming the presence of a steady, near-bed dense water outflow from Shark Bay’s northern Geographe Channel that persisted through all stages of the tide. Outflow velocities were 2–3 times stronger than the outflows recorded previously in Naturaliste Channel (in the west), and were more resistant to breakdown by tidal mixing. Estimates of turbulent kinetic energy production derived from the variance method showed a more complex structure in the Geographe Channel, due to shear between surface and bottom layers. Turbulence varied between flood and ebb tide, with peak levels of turbulence occurring during reversal of tidal flows. For both channels, the main source of turbulence was tidal flow along the seabed, with the bottom current speed cubed, |Ub3|, providing a reasonable proxy for tidal mixing and prediction of dense water outflows from Shark Bay majority of the time. Orientation and deeper water of the Geographe Channel along the main axis of the longitudinal density gradient provided an explanation for the predominant outflow from the Bay’s northern entrance. These density-driven currents could potentially influence recruitment of commercially fished scallops and prawns through the dispersal and flushing of larvae. [ABSTRACT FROM AUTHOR]