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The first coastal acoustic tomography (CAT) experiment site of the Neko-Seto Channel was revisited to elucidate the propagation and generation characteristics of the M2 and M4 tidal currents with a second CAT experiment, which was conducted from 3–6 April 2018 (local time). Two-dimensional flow fields of the M2 and M4 tidal currents and the residual current were reconstructed using a coast-fitting inversion model with the reciprocal travel-time data of five acoustic stations. The M2 tidal current is a progressive-type wave that propagates eastward at a speed of 0.7 ms−1, much slower than expected for free progressive tides in this region (19 ms−1). The M4 nonlinear current constructed an out-of-phase relationship between the western and eastern halves of the tomography domain, implying the generation of standing-type waves. Such nonlinear processes led to flood- and ebb-dominant tidal current asymmetries for the western and eastern halves of the model domain, respectively. The two-day mean residual currents constructed a northeastward current with a maximum speed of 0.3 ms−1 in the western half of the model domain and a clockwise rotation in the eastern half. The averaged inversion errors were 0.03 ms−1, significantly smaller than the amplitude of the aforementioned currents.

Coast-fitting tomographic inversion that is based on function expansion using three types of normal modes (the Dirichlet, Neumann, and open boundary modes) is proposed to reconstruct current fields from the coastal acoustic tomography (CAT) data. The superiority of the method was validated while using CAT data that were obtained in 2015 in the Dalian Bay. The semidiurnal tidal and residual current fields were accurately reconstructed over the entire model domain surrounded by coasts and open boundaries. The proposed method was effective, particularly around the peripheral regions of the tomography domain and the near-coast regions outside the domain, where accurate results are not expected from the conventional inverse method based on function expansion by Fourier function series with no coast fittings. The error velocity for the semidiurnal tidal currents was 2.2 cm s−1, which was calculated from the root-mean-square-difference between the CAT-observed and inverted range-averaged currents that were obtained along the nine peripheral transmission paths. The error velocity for the residual currents estimated from the 12-h mean net residual transport at the bay mouth was 0.9 cm s−1. The errors were significantly smaller than the amplitude of the tidal and residual currents.

Enhanced diapycnal mixing induced by the near-bottom breaking of internal waves is an essential component of the lower meridional overturning circulation. Despite its crucial role in the ocean circulation, tidally driven internal wave breaking is challenging to observe due to its inherently short spatial and temporal scales. We present detailed moored and shipboard observations that resolve the spatiotemporal variability of the tidal response over a small-scale bump embedded in the continental slope of Tasmania. Cross-shore tidal currents drive a nonlinear trapped response over the steep bottom around the bump. The observations are roughly consistent with two-dimensional high-mode tidal lee-wave theory. However, the alongshore tidal velocities are large, suggesting that the alongshore bathymetric variability modulates the tidal response driven by the cross-shore tidal flow. The semidiurnal tide and energy dissipation rate are correlated at subtidal time scales, but with complex temporal variability. Energy dissipation from a simple scattering model shows that the elevated near-bottom turbulence can be sustained by the impinging mode-1 internal tide, where the dissipation over the bump is O (1%) of the incident depth-integrated energy flux. Despite this small fraction, tidal dissipation is enhanced over the bump due to steep topography at a horizontal scale of O (1) km and may locally drive significant diapycnal mixing.

BACKGROUND AND OBJECTIVES: Understanding the correlation between tidal rhythms and marine organism behavior is crucial. This extends beyond fluctuations in chlorophyll a concentrations and includes various biological processes in the marine environment. Awareness is key for a comprehensive perspective on the role of tidal forces, affecting ocean's physical aspects and life form diversity. This study aims to explore the complex relationship between tidal movements and chlorophyll a concentrations in the northern Bay of Bengal, focusing on how tidal rhythms affect chlorophyll a concentrations. METHODS: The analyzed variables include tidal parameters, such as lunar semidiurnal tidal characteristics and Simpson-Hunter parameters, as well as sea level, tidal current, and current magnitude, obtained from the Tidal Model Driver. Additionally, hourly chlorophyll a data for January 2022 were acquired from the geostationary meteorological satellite Himawari-8, and the rate of change of chlorophyll a was determined through chlorophyll a calculations. This study employs wavelet analysis, applying continuous wavelet transform and wavelet transform coherence for chlorophyll a, rate of change of chlorophyll a, sea level, tidal current, and current magnitude, to explore oscillation patterns and temporal correlations within the marine ecosystem of the northern Bay of Bengal. FINDINGS: Lunar semidiurnal tidal amplitudes increase toward the north, peaking at the Sagar and Ramree Islands, and tidal phases rise from south to northeast. Most of the bay, categorized by <0.25 Formzahl values, experiences semidiurnal tides. Surface lunar semidiurnal elliptic currents, stronger in the north and east, flow clockwise and turn counterclockwise toward the south. The Simpson-Hunter parameter indicates heightened tidal mixing, particularly along the northern and eastern coasts. Region 2 showed the highest mean chlorophyll a concentration (12.58 milligram per cubic meter), whereas Region 1 showed the lowest mean chlorophyll a concentration (0.79 milligram per cubic meter). Similar trends were observed for tidal current and current magnitude. The continuous wavelet transform analysis provides data on chlorophyll a and the rate of change of chlorophyll a within 6, 12, and 24 hours, sea level changes within 8-16 hours, and consistent tidal effects on tidal current and current magnitude in the range of 5-7 hours. The wavelet transform coherence analysis highlights the relationships between chlorophyll a and sea level over 12 and 24 hours periods and between chlorophyll a and current magnitude. Furthermore, the wavelet transform coherence analysis examines the rate of change in chlorophyll a in relation to tidal currents over 6, 12, and 24 hours. CONCLUSION: Tides remarkably affect chlorophyll a concentrations. There are strong links between chlorophyll a concentrations and key tidal aspects, such as sea level and current magnitude. Higher tidal variables correlate with increased chlorophyll a concentrations and are related to the Simpson-Hunter parameter, indicating that regions with vigorous mixing show higher chlorophyll a concentrations. This finding highlights the major role of tidal forces and variations in the chlorophyll a concentrations in the Bay of Bengal. The wavelet transform coherence analysis of chlorophyll a, sea level, and current magnitude data in Regions 1, 2, and 3 show notable coherence in all areas.

Tidal asymmetry in estuaries with mixed, mainly semidiurnal tides arises from both the interaction of principal tides and the higher harmonics generated by distortions within the estuary. The duration asymmetry in rise and fall of water level caused by principal tides on the west coast of the continental United States is ebb‐dominant, and so the tide entering estuaries is also ebb‐dominant, prior to any internal distortions within the estuary. The interaction of higher harmonics with principal constituents either augments or cancels the duration asymmetry in the principal tides. In estuaries where tidal elevation and velocity phase are near quadrature (90° out of phase), the duration asymmetry in tidal elevation leads to asymmetries in tidal current magnitude. Asymmetry can be conveniently quantified in terms of the sample skewness, γ1, the normalized third sample moment about the mean. An analytic approximation to the skewness shows that traditional metrics of asymmetry, namely the ratio of constituent amplitudes and the relative constituent phase difference, arise from calculating the third sample moment. Observations from three California estuaries of different morphologies are presented as an illustration of how skewness can be used to quantify asymmetry in real systems. As in semidiurnal systems, morphology is a good predictor of whether higher harmonics engender ebb‐dominance or flood‐dominance, however asymmetry imposed by principal tides at the mouth must first be overcome and so there is a spatial evolution in the total asymmetry. Quantifying observations via skewness should be considered in addition to traditional metrics in estuaries with mixed tides.

Around Hopen Island, the satellite images and experiments with drifting buoys describe the movement of the drifting ice and depict tidally generated trapped motion. An analytical solution is applied to investigate the trapping phenomenon. A general solution is achieved by the superposition of the incident and reflected (scattered) waves for an elliptically shaped island above the critical latitude. The incident wave simulates the tidal wave propagation toward the island and its prominent feature, an amphidromic point located to the southeast from Hopen Island. The analytical solution for the reflected wave is constructed in elliptic coordinates. Tide amplitudes and cophase lines are analyzed in the island’s vicinity and compared to observations and numerical model results. A simulated drift of Lagrangian water particles constructed with the help of analytical solutions reproduces well the observed clockwise trapped motion of the drifting buoy near Hopen Island. Since the resonance may amplify the semidiurnal incident tide, we have also investigated the natural modes of water oscillations near the island. While this paper focuses on the details of the model used at the specific site of Hopen Island, a similar trapping analysis can be applied to circular or elliptic islands that have a small scale relative to the barotropic Rossby deformation radius.

The mode-1 semidiurnal internal tides that emanate from multiple sources in the Sulu-Sulawesi Seas are investigated using multi-satellite altimeter data from 1993–2020. A practical plane-wave analysis method is used to separately extract multiple coherent internal tides, with the nontidal noise in the internal tide field further removed by a two-dimensional (2-D) spatial band-pass filter. The complex radiation pathways and interference patterns of the internal tides are revealed, showing a spatial contrast between the Sulu Sea and the Sulawesi Sea. The mode-1 semidiurnal internal tides in the Sulawesi Sea are effectively generated from both the Sulu and Sangihe Island chains, forming a spatially inhomogeneous interference pattern in the deep basin. A cylindrical internal tidal wave pattern from the Sibutu passage is confirmed for the first time, which modulates the interference pattern. The interference field can be reproduced by a line source model. A weak reflected internal tidal beam off the Sulawesi slope is revealed. In contrast, the Sulu Island chain is the sole energetic internal tide source in the Sulu Sea, thus featuring a relatively consistent wave and energy flux field in the basin. These energetic semidiurnal internal tidal beams contribute to the frequent occurrence of internal solitary waves (ISWs) in the study area. On the basis of the 28-year consistent satellite measurements, the northward semidiurnal tidal energy flux from the Sulu Island chain is 0.46 GW, about 25% of the southward energy flux. For M2, the altimetric estimated energy fluxes from the Sulu Island chain are about 80% of those from numerical simulations. The total semidiurnal tidal energy flux from the Sulu and Sangihe Island chains into the Sulawesi Sea is about 2.7 GW.

The evidence for, baroclinic energetics of, and geographic distribution of parametric subharmonic instability (PSI) arising from both diurnal and semidiurnal tides in a global ocean general circulation model is investigated using 1/12.5° and 1/25° simulations that are forced by both atmospheric analysis fields and the astronomical tidal potential. The paper examines whether PSI occurs in the model, and whether it accounts for a significant fraction of the tidal baroclinic energy loss. Using energy transfer calculations and bispectral analyses, evidence is found for PSI around the critical latitudes of the tides. The intensity of both diurnal and semidiurnal PSI in the simulations is greatest in the upper ocean, consistent with previous results from idealized simulations, and quickly drops off about 5° from the critical latitudes. The sign of energy transfer depends on location; the transfer is positive (from the tides to subharmonic waves) in some locations and negative in others. The net globally integrated energy transfer is positive in all simulations and is 0.5%–10% of the amount of energy required to close the baroclinic energy budget in the model. The net amount of energy transfer is about an order of magnitude larger in the 1/25° semidiurnal simulation than the 1/12.5° one, implying the dependence of the rate of energy transfer on model resolution.

Numerous internal solitary waves (ISWs) have been observed in the southern Andaman Sea. In this study, the two-dimensional Massachusetts Institute of Technology general circulation model is applied to investigate the dynamics of ISWs and explore the effects of the bottom topography and tidal forcing on the generation and propagation of ISWs in the southern Andaman Sea. The results show that the large-amplitude depression ISWs are mainly generated via the oscillating tidal flow over the sill of the Great Channel, and the generation of ISWs is subject to the lee wave regime. The Dreadnought Bank cannot generate ISWs itself; however, it can enhance the amplitudes of eastward-propagating ISWs generated from sill A, owing to constructive interference of internal tide generation between the sill of the Great Channel and the Dreadnought Bank The eastward-propagating ISWs generated by the eastern shallow sill near the continental shelf can propagate to the shelf, where they evolve into elevation waves because of the shallow water Sensitivity runs show that both the semidiurnal and diurnal tides over the sill of the Great Channel can generate ISWs in this area. However, the ISWs generated by diurnal tides are much weaker than those generated by semidiurnal tides. Mixed tidal forcing has no significant effect on the generation of ISWs.

A high-resolution primitive equation model simulation is used to form an energy budget for the principal semidiurnal tide (M2) over a region of the Hawaiian Ridge from Niihau to Maui. This region includes the Kaena Ridge, one of the three main internal tide generation sites along the Hawaiian Ridge and the main study site of the Hawaii Ocean Mixing Experiment. The 0.01°–horizontal resolution simulation has a high level of skill when compared to satellite and in situ sea level observations, moored ADCP currents, and notably reasonable agreement with microstructure data. Barotropic and baroclinic energy equations are derived from the model’s sigma coordinate governing equations and are evaluated from the model simulation to form an energy budget. The M2 barotropic tide loses 2.7 GW of energy over the study region. Of this, 163 MW (6%) is dissipated by bottom friction and 2.3 GW (85%) is converted into internal tides. Internal tide generation primarily occurs along the flanks of the Kaena Ridge and south of Niihau and Kauai. The majority of the baroclinic energy (1.7 GW) is radiated out of the model domain, while 0.45 GW is dissipated close to the generation regions. The modeled baroclinic dissipation within the 1000-m isobath for the Kaena Ridge agrees to within a factor of 2 with the area-weighted dissipation from 313 microstructure profiles. Topographic resolution is important, with the present 0.01° resolution model resulting in 20% more barotropic-to-baroclinic conversion compared to when the same analysis is performed on a 4-km resolution simulation. A simple extrapolation of these results to the entire Hawaiian Ridge is in qualitative agreement with recent estimates based on satellite altimetry data.