Greg N. Ivey

We present 5-year long continuous mixing observations from four mooring locations, spanning 10° degrees of latitude, on the continental shelf of North-Western Australia. The sites are subject to a diverse range of atmospheric and oceanic forcing and each has distinct local bathymetry. We estimated the diapycnal diffusivity and vertical turbulent heat flux at all sites and throughout the water column using a fine-scale mixing length model. There were four dominant peaks in the power spectrum: annual, spring-neap, diurnal, and semi-diurnal. We examined the variability in the mixing at all sites in terms of the seasonal stratification, depth, and tidal range and phase. The highest annual variability occurred at the highest latitude sites which had the greatest seasonal variation in stratification. The mixing also varied with the strength of the dominant tidal constituents (both baroclinic and barotropic). Heat flux was a more useful measure of mixing than diffusivity due to the strong seasonal variations in density stratification on the shelf. Finally, we provide guidance on the determination of the appropriate sampling period to adequately describe the mixing, and discuss the effects of short sampling periods on the ability to describe the long-term mixing.

We present 5-year long continuous mixing observations from four mooring locations, spanning 10° degrees of latitude, on the continental shelf of North-Western Australia. The sites are subject to a diverse range of atmospheric and oceanic forcing and each has distinct local bathymetry. We estimated the diapycnal diffusivity and vertical turbulent heat flux at all sites and throughout the water column using a fine-scale mixing length model. There were four dominant peaks in the power spectrum: annual, spring-neap, diurnal, and semi-diurnal. We examined the variability in the mixing at all sites in terms of the seasonal stratification, depth, and tidal range and phase. The highest annual variability occurred at the highest latitude sites which had the greatest seasonal variation in stratification. The mixing also varied with the strength of the dominant tidal constituents (both baroclinic and barotropic). Heat flux was a more useful measure of mixing than diffusivity due to the strong seasonal variations in density stratification on the shelf. Finally, we provide guidance on the determination of the appropriate sampling period to adequately describe the mixing, and discuss the effects of short sampling periods on the ability to describe the long-term mixing.

We present 5-year long continuous mixing observations from four mooring locations, spanning 10° degrees of latitude, on the continental shelf of North-Western Australia. The sites are subject to a diverse range of atmospheric and oceanic forcing and each has distinct local bathymetry. We estimated the diapycnal diffusivity and vertical turbulent heat flux at all sites and throughout the water column using a fine-scale mixing length model. There were four dominant peaks in the power spectrum: annual, spring-neap, diurnal, and semi-diurnal. We examined the variability in the mixing at all sites in terms of the seasonal stratification, depth, and tidal range and phase. The highest annual variability occurred at the highest latitude sites which had the greatest seasonal variation in stratification. The mixing also varied with the strength of the dominant tidal constituents (both baroclinic and barotropic). Heat flux was a more useful measure of mixing than diffusivity due to the strong seasonal variations in density stratification on the shelf. Finally, we provide guidance on the determination of the appropriate sampling period to adequately describe the mixing, and discuss the effects of short sampling periods on the ability to describe the long-term mixing.

While mode‐2 nonlinear internal waves (NLIWs) can play an important role in mixing and transporting material, their generation and evolution remain less well‐understood than their mode‐1 counterparts. Field observations from a mooring in 330 m water on the Australian North West Shelf revealed the frequent occurrence of mode‐2 NLIWs, with leading wave amplitudes of 23 8 m, followed by a train of trailing waves, and a propagation direction of 54° 12° SE (i.e., inshore). While these mode‐2 NLIWs typically occurred every 12–12.4 hr, they were not directly linked to the local tidal forcing. Furthermore, the amplitude of the leading waves increased when the background stratification shifted from a 2‐layer to a 3‐layer regime (a structure influenced by the passage of TC Veronica ). We used a non‐hydrostatic (NH) ocean model to understand the mode‐2 generation and propagation process and its link to the background density stratification and forcing. We first ran a series of 2‐dimensional (2D) NH model runs, driven by both remote internal tides (RIT) with a low‐frequency phase modulation and local barotropic forcing. The 2D modeling demonstrated that non‐phase‐locked mode‐1 remotely‐generated internal tides (RITs) scattered energy to high mode beams at the shelf break. The shoreward‐propagating internal wave beams then interacted resonantly with the double‐pycnocline (3‐layer) stratification to ultimately generate mode‐2 NLIWs in shallower water at depths of around 500 m. Using knowledge from these 2D simulations, we then ran a 3D NH model with phase‐modulated RITs and local barotropic forcing, and the model results were in good agreement with the observations. The crest length of the mode‐2 NLIWs was ∼5 km and the crest width was less than 1 km in the shelf waters, as determined by the correlation length scales calculated from the numerical model results.

We present 5-year long continuous mixing observations from four mooring locations, spanning 10° degrees of latitude, on the continental shelf of North-Western Australia. The sites are subject to a diverse range of atmospheric and oceanic forcing and each has distinct local bathymetry. We estimated the diapycnal diffusivity and vertical turbulent heat flux at all sites and throughout the water column using a fine-scale mixing length model. There were four dominant peaks in the power spectrum: annual, spring-neap, diurnal, and semi-diurnal. We examined the variability in the mixing at all sites in terms of the seasonal stratification, depth, and tidal range and phase. The highest annual variability occurred at the highest latitude sites which had the greatest seasonal variation in stratification. The mixing also varied with the strength of the dominant tidal constituents (both baroclinic and barotropic). Heat flux was a more useful measure of mixing than diffusivity due to the strong seasonal variations in density stratification on the shelf. Finally, we provide guidance on the determination of the appropriate sampling period to adequately describe the mixing, and discuss the effects of short sampling periods on the ability to describe the long-term mixing.

We present 5-year long continuous mixing observations from four mooring locations, spanning 10° degrees of latitude, on the continental shelf of North-Western Australia. The sites are subject to a diverse range of atmospheric and oceanic forcing and each has distinct local bathymetry. We estimated the diapycnal diffusivity and vertical turbulent heat flux at all sites and throughout the water column using a fine-scale mixing length model. There were four dominant peaks in the power spectrum: annual, spring-neap, diurnal, and semi-diurnal. We examined the variability in the mixing at all sites in terms of the seasonal stratification, depth, and tidal range and phase. The highest annual variability occurred at the highest latitude sites which had the greatest seasonal variation in stratification. The mixing also varied with the strength of the dominant tidal constituents (both baroclinic and barotropic). Heat flux was a more useful measure of mixing than diffusivity due to the strong seasonal variations in density stratification on the shelf. Finally, we provide guidance on the determination of the appropriate sampling period to adequately describe the mixing, and discuss the effects of short sampling periods on the ability to describe the long-term mixing.

We present 5-year long continuous mixing observations from four mooring locations, spanning 10° degrees of latitude, on the continental shelf of North-Western Australia. The sites are subject to a diverse range of atmospheric and oceanic forcing and each has distinct local bathymetry. We estimated the diapycnal diffusivity and vertical turbulent heat flux at all sites and throughout the water column using a fine-scale mixing length model. There were four dominant peaks in the power spectrum: annual, spring-neap, diurnal, and semi-diurnal. We examined the variability in the mixing at all sites in terms of the seasonal stratification, depth, and tidal range and phase. The highest annual variability occurred at the highest latitude sites which had the greatest seasonal variation in stratification. The mixing also varied with the strength of the dominant tidal constituents (both baroclinic and barotropic). Heat flux was a more useful measure of mixing than diffusivity due to the strong seasonal variations in density stratification on the shelf. Finally, we provide guidance on the determination of the appropriate sampling period to adequately describe the mixing, and discuss the effects of short sampling periods on the ability to describe the long-term mixing.

While nonlinear internal wave (NLIW) trains are known to influence near‐sea bed suspended sediment dynamics, the mechanisms remain a topic of debate. We present near‐sea bed observations of suspended sediment concentration and estimates of vertical sediment flux, at high vertical‐ and temporal‐resolution, during trains of NLIW of depression. We quantify the contributions of vertical advection and turbulent mixing to . Vertical advection was important during the leading wave and the turbulent mixing flux was important over the entire wave train. Maximum was highly correlated with the maximum horizontal current speed squared and was only weakly correlated with the maximum vertical velocity. Boundary layer‐induced turbulence was thus inferred to be the key driver of net vertical sediment flux over wave trains of this type. Estimating the maximum total horizontal speed (i.e., wave‐induced plus background) is sufficient for modeling sediment vertical dynamics in shelf‐scale modeling studies.

Ocean currents flowing over the seabed produce a turbulent bottom boundary layer (BBL) which extracts energy from the overlying flow and mediates sedimentary, chemical, and biological processes at the seafloor. The role of the BBL is especially important in relatively shallow shelf seas where stratification, mean flow, and tidally‐driven oscillating currents are typically strong. Despite this importance, the dynamics and even the height of the BBL formed under stratified, tidal conditions are not well understood. We address this knowledge gap by performing large‐eddy simulations of the evolution of a tidally forced and stratified BBL, initialized from rest, over a range of geophysically relevant parameter values. The BBL has two distinct regions: an actively stirred, near‐well‐mixed bottom mixed layer (BML) and a weakly stirred and strongly stratified capping pycnocline. A simple steady‐state empirical model, dependent on stratification, tidal period, and bottom turbulence intensity, provides a useful estimate of BML height. Motivated by appreciable growth in the BML that can occur on timescales of days to weeks, we also develop a time‐dependent model for the BML height. This model accounts for the vertical structure of the BBL turbulence and reproduces the BML growth with high fidelity. We also present a simple model for the cycle‐averaged eddy diffusivity, which depends on the BML height. Both the steady‐state and time‐dependent models of BML height—and the accompanying expression for eddy diffusivity—can be used to develop tidal BBL parameterizations and inform grid resolution requirements within ocean models, as well as guiding sampling for future observational studies.