Jia-Lin Chen

Ocean currents represent continuous and promising source of hydrokinetic energy. Recent observations along the Kuroshio Current reveal elevated levels of turbulence kinetic energy (TKE), particularly in regions where strong currents interact with sudden topographic features. The Reynolds-averaged Navier–Stokes (RANS) model has been commonly employed to predict current velocities at prospective turbine sites. However, the standard RANS model can introduce significant discrepancies in predicting higher-order turbulence statistics, particularly for turbulence generated by Kelvin–Helmholtz billows. Accurate TKE predictions are critical for assessing hydrokinetic energy potential. This study employs a numerical model with Large Eddy Simulation (LES) to examine wake dynamics and the resulting TKE production/dissipation caused by flow-topography interactions (FTI). The LES model successfully reproduces Kolmogorov's − 5/3 law, indicating that the simulation resolves eddies within the inertial subrange, and the spatial–temporal variations due to breaking internal lee waves. As TKE is elevated in regions of breaking internal lee waves, incorporating ridge geometry is essential for identifying TKE hot spots. Model results under different ambient flow conditions and geometric slopes demonstrate that turbulence characteristics can be effectively predicted using the bifurcation slope, S _c (the height of the bifurcation cap versus the corresponding width of the cap). The turbulent characteristics discussed here will be used to evaluate turbulence levels for turbine site assessments.

Desalination contributes to the climate change adaption and increases resilience in all those circumstances in which water scarcity has been exacerbated severely in recent decades. Desalination is the process of removing salt from seawater or brackish water for a range of usage, including drinking. However, desalination produces a by- product, brine (a concentrated salt solution) that must be properly disposed of to avoid adverse impacts on the marine environment. In fact, emergency desalination facilities are popular when a sudden drought happens; it’s easy to install and can provide instant drinking water resource, but most of them are installed at harbor areas since there is no time to construct offshore brine outfall pipes, which means brine will be discharged into harbor water directly. In this study, tidal dispersion and the horizontal exchange of water between Kaohsiung Harbor and the surrounding ocean are examined with a three- dimensional high-resolution (the vertical/horizontal grid resolution is 1m/40 m) numerical model. The Port of Kaohsiung is the fifteenth largest container port in the world, and thus plays an important role to economic development. The harbor has two inlets connecting to the sea (indicated as North Boundary and South Boundary) and one fresh river inflow (indicated as River Boundary). There are 97 docks totalling 20.570 meters in length. The strongly varying bathymetry and coastline geometry of the harbor generate complex spatial patterns. In this study, Lagrangian exchange experiments were conducted to demonstrate that tidal currents exchange and mix material near the inlets of the harbor.

The spatial distribution of the flow field inferred from shipboard measurements often suffers from a spatial‐temporal aliasing effect. This study examined a detailed view of the horizontal divergence and relative vertical vorticity captured by a two‐vessel survey to characterize velocity gradients and the resulting process in the presence of nonlinear lee waves. The three‐dimensional structure of the horizontal velocity gradients and turbulent mixing within a nonlinear internal lee wave behind a shallow seamount was investigated. Synchronous two‐vessel Acoustic Doppler Current Profilers provide in‐situ measurements of velocity vectors that significantly minimize the spatial and temporal aliasing effect. The magnitude of horizontal divergence and relative vertical vorticity normalized by the planetary vorticity (, ∼ O (10)) is one order of magnitude greater than prior observations in the typical oceanic sub‐mesoscale flow field using a two‐vessel survey. Our analysis indicates that the spatial variations of horizontal divergence and relative vertical vorticity over the seamount are associated with flow‐topography interactions. Owing to the bottom Ekman effect, the deflected Kuroshio enhances the relative vertical vorticity, , and horizontal components of relative vorticity, and , resulting in vertical shear instability and symmetric instability. Instability hotspots are identified by the negative potential vorticity (PV) at the rear half of the nonlinear internal wave, where depressed isopycnals rebound. In situ observational surveys conducted on the lee of pinnacles further indicated that the enhanced turbulent eddy diffusivity and vertical nitrate gradient are collocated with the negative PV within a nonlinear internal lee wave.

Physical processes behind flow‐topography interactions and turbulent transitions are essential for parameterization in numerical models. We examine how the Kuroshio cascades energy into turbulence upon passing over a seamount, employing a combination of shipboard measurements, tow‐yo microstructure profiling, and high‐resolution mooring. The seamount, spanning 5 km horizontally with two summits, interacts with the Kuroshio, whose flow speed ranges from 1 to 2 m s⁻¹, modulated by tides. The forward energy cascade process is commenced by forming a train of 2–3 nonlinear lee waves behind the summit with a wavelength of 0.5–1 km and an amplitude of 50–100 m. A train of Kelvin‐Helmholtz (KH) billows develops immediately below the lee waves and extends downstream, leading to enhanced turbulence. The turbulent kinetic energy dissipation rate is O (10⁻⁷–10⁻⁴) W kg⁻¹, varying in phase with the upstream flow speed modulated by tides. KH billows occur primarily at the lee wave's trailing edge, where the combined strong downstream shear and low‐stratification recirculation trigger the shear instability, Ri < 1/4. The recirculation also creates an overturn susceptible to gravitational instability. This scenario resembles the rotor, commonly found in atmospheric mountain waves but rarely observed in the ocean. A linear stability analysis further suggests that critical levels, where the KH instability extracts energy from the mean flow, are located predominantly at the strong shear layer of the lee wave's upwelling portion, coinciding with the upper boundary of the rotor. These novel observations may provide insights into flow‐topography interactions and improve physics‐based turbulence parameterization.

The Zhuoshui River has one of the highest sediment yields in the world. The riverine sediment may have been transported away from the river mouth by strong tidal currents and deposited in a navigational channel and a harbor basin south of the Zhuoshui River. This study describes the dispersal of riverine sediments in mesotidal conditions, including transport processes of sediments via river plumes, initial deposition, resuspension, and long-term net accumulation. Our results revealed that the distribution of suspended sediment concentration (SSC) and the transport process of the river plume was markedly modulated by tidal currents in a north–south direction. Under high-discharge conditions, energetic winds and ocean currents interrupted the offshore propagation of the sediment-laden river plume with a high SSC to the ocean. The plume, induced by typhoon floods, was either propagated northward due to the Coriolis effect or was deposited near the river mouth. The sediments deposited within the inner shelf were resuspended by intensive tidal currents and transported through southward residual circulations to the navigation channel south of the river mouth.

Observations on the lee of a topographic ridge show that the turbulence kinetic energy (TKE) dissipation rate due to shear instabilities is three orders of magnitude higher than the typical value in the open ocean. Laboratory-scale studies at low Reynolds number suggest that high turbulent dissipation occurs primarily within the core region of shear instabilities. However, field-scale studies indicate that high turbulence is mainly populated along the braids of shear instabilities. In this study, a high-resolution, resolving the Ozmidov-scale, non-hydrostatic model with Large Eddy Simulation (LES) turbulent closure is applied to investigate dominant mechanisms that control the spatial and temporal scales of shear instabilities and resulting mixing in stratified shear flow at high Reynolds number. The simulated density variance dissipation rate is elevated in the cusp-like bands of shear instabilities with a specific period, consistent with the acoustic backscatter taken by shipboard echo sounder. The vertical length scale of each cusp-like band is nearly half of the vertical length scale of the internal lee wave. However, it is consistent with instabilities originating from a shear layer based on linear stability theory. The model results indicate that the length scale and/or the period of shear instabilities are the key parameters to the mixing enhancement that increases with lateral Froude number Fr_L, i.e. stronger shear and/or steeper ridge.

The physical processes governing coastal exchange between the surf zone, the inner shelf, and the open ocean is critical for estimating mass exchange and its impact on ecological processes. The present study combined field measurements and theoretical approaches to explore the hydrodynamics in the coastal boundary layer (CBL) in which both bottom drag and shore friction affect the transport and mixing processes. Observed drifter-cluster trajectories in a nearly alongshore-uniform coastal area showed that the occurrence of current reversal varies with cross-shore distance, which confirmed the tidal phase difference between different cross-shore distances predicted by the proposed CBL model. According to the CBL model, tidal phase difference is affected by the bottom drag coefficient and horizontal eddy viscosity coefficient. With the results of three experiments under different wave conditions, this study also discusses the effects of waves on the CBL. Data analysis based on observations indicates that the bottom drag term is closely related to the bottom shear stress induced by the interactions of waves and currents. The bottom drag coefficient under the more energetic wave condition was much greater than that under milder wave conditions during the experiment. The study also suggests that in addition to pressure gradient and bottom drag, flow structure is subject to lateral stress, which reflects the impact of shoreline roughness in the nearshore region and that the estimated eddy viscosity coefficient decreases linearly with distance from the shoreline.