We collected observations of ocean mixing from three moorings placed at the 330m, 200m, and 150m isobaths on a pelagic ridge on the Australian North West Shelf (NWS). The region is subject to energetic surface and internal tides, non-linear internal waves, flow-topography interactions, and episodic intense wind events (i.e., tropical cyclones) that collectively drive energetic diapycnal mixing. We identified five dominant internal wave categories: both low (time scales from double the buoyancy period to 4 hours) and high-frequency (time scales between buoyancy period and double the buoyancy period) mode-1 waves, mode-2 waves, internal bores, and internal hydraulic jumps. A small number of turbulent mixing events dominated the total vertical heat flux at each mooring, with 15% of estimates accounting for as much as 90% of the total observed heat flux. These turbulent mixing events often occurred during the passage of internal wave events, with the internal wave events accounting for as much as 60% of the total heat flux in some locations. High-frequency mode-1 waves were the most significant contributors to the total vertical heat flux (∼ 20%). Internal bores made significant but localized contributions to mixing, accounting for up to ∼ 50% of the total vertical heat flux in some regions but with a negligible influence elsewhere. The contributions of the different internal wave categories to the total flux became more heterogeneous at shallower sites, indicating an increasingly complicated relationship between the forcing internal wave field and the mixing.

Previous studies have highlighted the individual importance of diurnal warm layers (DWLs) and surface-layer fronts within the surface boundary layer (SBL) in regulating energy, momentum, and gas exchange between the atmosphere and the ocean. This study investigates the interactions between DWLs and surface-layer fronts using field observations and numerical turbulence models. Our study provides the real-ocean relevance of the coexistence of DWLs and surface-layer fronts in the SBL in an eddy-rich tropical ocean subjected to intense solar heating and weak winds. We found that the presence of a DWL isolates the deeper layers of the SBL from diabatic and frictional surface forcing, causing these layers to quickly become non-turbulent while remaining in a state of marginal stability. This condition suggests that small perturbations from local processes, such as internal tides and waves, can easily trigger instability and turbulence. Additionally, frontal dynamics were observed to deepen the SBL, allowing near-surface diurnal shear associated with DWL dynamics to penetrate to greater depths during nighttime, compared to conditions without a front, thereby facilitating the vertical transport of heat and tracers. Our findings underscore the necessity of accurately representing the interactions between DWLs and surface-layer fronts to enhance the precision of ocean circulation and climate models.

While it has been recognized for some time that large-amplitude nonlinear internal waves (NLIW) can mobilise and transport sediment, quantitative observations of this process are rare. Rarer still are accompanying measurements or even estimates of suspended sediment mass concentration (SSC) during the passage of NLIW. Here we present high resolution observations of NLIW and the SSC response within the bottom boundary layer. The observations were made in 2017 in the Browse Basin on Australia’s Northwest Shelf in 250 m of water. We compare two direct calibration methods designed to overcome the inherent difficulty of directly observing SSC in deeper ocean environments, and employ Bayesian methods to estimate the uncertainty in SSC. Both calibration methods were used as bench-marking to infer SSC from a range of instrumentation deployed on a bottom-lander frame (acoustics, optical, and laser scattering). Estimates of near-bed SSC, with uncertainty, during NLIW passages are presented for each instrument. During a large NLIW event, the peak mean SSC estimate was 102 mg L$^{-1}$ with 95\% credible intervals of 93 and 112 mg L$^{-1}$, 0.87 m above the sea bed. We also examine the propagation of uncertainty to several derived quantities, such as SSC gradients. This work is the first step towards the quantitative analysis of sediment dynamics needed to develop parameterized models associated with the passage of NLIW.

We describe a framework for the simultaneous estimation of model parameters in a partial differential equation using sparse observations. Monte Carlo Markov Chain (MCMC) sampling is used in a Bayesian framework to estimate posterior probability distributions for each parameter. We describe the necessary components of this approach and its broad potential for application in models of unsteady processes. The framework is applied to three case studies, of increasing complexity, from the field of cohesive sediment transport. We demonstrate that the framework can be used to recover posterior distributions for all parameters of interest and the results agree well with independent estimates (where available). We also demonstrate how the framework can be used to compare different model parameterizations and provide information on the covariance between model parameters.

We present an empirical model of the seasonal variability of the internal tide using seasonal harmonics to modulate the amplitude of the fundamental tidal constituents. Internal tide data, from both long-term, in-situ moorings and a mesoscale- and internal tide-resolving ocean model, are used to demonstrate the performance of the seasonal harmonic model for the Indo-Australian Basin Region. The seasonal model describes up to 15 % more of the observed (baroclinic) sea surface height variance than a fixed-amplitude harmonic mode at the mooring sites. The ocean model results demonstrate that the study region, which includes the Australian North West Shelf (NWS), Timor Sea and southern Indonesian Islands, is dominated by standing wave interference patterns due to the presence of multiple generation sites. The seasonal harmonic model reveals that temporal shifts in the standing wave patterns coincide with seasonal variations in density stratification. This shift is particularly evident within distances of 2 - 3 internal wave lengths from strong generation sites. The fraction of the variance of the internal tide signal explained by seasonal modulations is largest in standing wave node regions, contributing to differences in predictive skill of the seasonal harmonic model at two moorings separated by only 38 km. Output of the harmonic model also demonstrated that the seasonally-evolving M2 internal tide propagating southward from Lombok Strait had a small amplitude in October when shear from the Indonesian Throughflow was strongest.

We present an empirical model for describing the temporal variability of the internal tide, that uses seasonal harmonics to temporally modulate the amplitude of the fundamental tidal harmonics. Internal tide data, from both long-term, in-situ moorings and a mesoscale- and internal tide-resolving ocean model, are used to demonstrate the performance of the seasonal (non-stationary) harmonic model for the Indo-Australian Basin Region. The non-stationary model described up to 15 \% more baroclinic sea surface height and isotherm displacement variance than the fixed-amplitude harmonic model at some observation sites. The ocean model results demonstrate that the study region, which includes the Australian North West Shelf (NWS), Timor Sea and southern Indonesian Islands, is dominated by standing wave interference patterns produced by multiple generation sites. Application of the seasonal harmonic model demonstrates that temporal shifts in the standing wave locations coincide with seasonal variations in density stratification, particularly within 2 - 3 internal wave lengths from strong generation sites. It is shown that the variance fraction of internal tide signal explained by seasonal modulations is largest in standing wave node regions. This result helps explain the contrasting skill of the seasonal harmonic model at two moorings that were separated by only 38 km. Output of the harmonic model also demonstrates that the seasonally-evolving, southward propagating $M_2$ internal tide from Lombok Strait had a smaller amplitude in October when shear from the Indonesian Throughflow was strongest. Further applications for a regional internal tide climatology database are discussed.