Tidal Phase Research Articles

The influence of astronomical tide phases on urban flooding during rainstorms: Application to Macau

Study regionThe study area is the western part of the Macau Peninsula in China, with an area of 4.06 km2. Study focusThis study developed a coupled hydrological-hydraulic model that simulates the two-dimensional (2D) surface flow and one-dimensional (1D) drain pipe flow of compound inundation. The model was applied to Macau, China, as a study area, and the typhoon Mangkhut in 2018 was used as a case study to validate the model’s performance. Simulating compound inundation scenarios of extreme rainfall and astronomical tide and exploring the influence of astronomical tide on coastal urban inundation. New hydrological insights for the regionUrban flood disasters are profoundly influenced by tidal levels, albeit the height of tides alone does not solely exacerbate the phenomenon. Rather, the phase of astronomical tides during rainfall also plays a crucial role in determining the severity of urban flooding. This research has revealed that when rainfall synchronizes with the recession phase of the spring tide, urban flooding conditions become acute, 17.66 % increase in total surface water volume for the scenario with the highest tidal level impact compared to the scenario with the lowest tidal level impact. This paper presents the response of coastal cities to the flooding process under different combined rainstorm-astronomical tide scenarios, to provide scientific guidance for disaster preparedness planning in coastal areas, and to improve the resilience of disasters.

Phenomenological model of gravitational self-force enhanced tides in inspiraling binary neutron stars

Gravitational waves from inspiraling binary neutron stars provide unique access to ultradense nuclear matter and offer the ability to constrain the currently unknown neutron star equation-of-state through tidal measurements. This, however, requires the availability of accurate and efficient tidal waveform models. In this paper we present henom, a new phenomenological tidal phase model for the inspiral of neutron stars in the frequency domain, which captures the gravitational self-force informed tidal contributions of the time-domain effective-one-body model esum. henom is highly faithful and computationally efficient, and by choosing a modular approach, it can be used in conjunction with frequency-domain binary black hole waveform model to generate the complete phase for a binary neutron star inspiral. henom is valid for neutron star binaries with unequal masses and mass ratios between 1 and 3, and dimensionless tidal deformabilities up to 5000. Furthermore, henom does not assume universal relations or parametrized equations of state, hence allowing for exotic matter analyses and beyond standard model physics investigations. We demonstrate the efficacy and accuracy of our model through comparisons against esum, numerical relativity waveforms and full Bayesian inference, including a reanalysis of the binary neutron star observation GW170817. Published by the American Physical Society 2024

The Non‐Migrating DE3 Tide Response to the MJO Phenomenon at the MLT Altitudes

AbstractBased on TIMED/SABER temperature observations and the Real‐Time Multivariate Madden‐Julian Oscillation (MJO) Index, for the first time, we discussed the daily resolution non‐migrating DE3 temperature tide response to the MJO phenomenon during MJO phases between ±45° latitude from 2003 to 2012 in the Mesosphere and Lower Thermosphere (MLT) region. We found: (a) The DE3 tide significantly depends on different MJO phases in the four seasons above 100 km altitude. The DE3 tidal phase exhibits varying responses to the different MJO phases. (b) The DE3 tide response to the MJO phenomenon is stronger at the solstices than at the equinoxes, and the response at the June solstices is stronger. (c) The symmetric component of the DE3 response to the MJO phenomenon is stronger than the asymmetric component. In summary, the daily resolved DE3 tide response to the MJO phenomenon, especially the DE3 tidal phase dominates certain seasonal characteristics and symmetry in the MLT region.

Pollution-source fractionation and quantification-based assessment of surface water quality in Saigon River, Vietnam: implications for sustainable management strategies

ABSTRACT The current study fractionated pollution sources and investigated the impacts of seasonal and tidal variations on Saigon River’s surface water quality. Ninety-six water samples were collected across dry and rainy seasons, covering ebb and flood tides, and analysed for 19 parameters. During the dry season, ebb tide showed higher levels of electrical conductivity, sulfate, chloride, sodium, and potassium. During the rainy season, ebb tide exhibited greater concentrations of ammonium, total nitrogen, and phosphorus. The water quality index was significantly lower during the dry season’s ebb tide compared to the flood tide but remained similar in both tidal phases during the rainy season. Four primary pollution sources were quantified, with seawater intrusion and agricultural activities contributing 62.68% and 28.54%, respectively, to water quality degradation. Briefly, the river’s surface water quality was inferior during the dry season compared to the rainy season, primarily due to seawater intrusion and agricultural activities, requiring remediation.

Thermal tides in the middle atmosphere at mid-latitudes measured with a ground-based microwave radiometer

Abstract. In recent decades, theoretical studies and numerical models of thermal tides have gained attention. It has been recognized that tides have a significant influence on the dynamics of the middle and upper atmosphere; as they grow in amplitude and propagate upward, they transport energy and momentum from the lower to the upper atmosphere, contributing to the vertical coupling between atmospheric layers. The superposition of tides with other atmospheric waves leads to non-linear wave–wave interactions. However, direct measurements of thermal tides in the middle atmosphere are challenging and are often limited to satellite measurements in the tropics and at low latitudes. Due to orbit geometry, such observations provide only a reduced insight into the short-term variability in atmospheric tides. In this paper, we present tidal analysis from 5 years of continuous observations of middle-atmospheric temperatures. The measurements were performed with the ground-based temperature radiometer TEMPERA (TEMPErature RAdiometer), which was developed at the University of Bern in 2013 and was located in Bern (46.95° N, 7.45° E) and Payerne (46.82° N, 6.94° E). TEMPERA achieves a temporal resolution of 1–3 h and covers the altitude range between 25–50 km. Using an adaptive spectral filter with a vertical regularization (ASF2D) for the tidal analysis, we found maximum amplitudes for the diurnal tide of approximately 2.4 K, accompanied by seasonal variability. The maximum amplitude was reached on average at an altitude of 43 km, which also reflected some seasonal characteristics. We demonstrate that TEMPERA is suitable for providing continuous temperature soundings in the stratosphere and lower mesosphere with a sufficient cadence to infer tidal amplitudes and phases for the dominating tidal modes. Furthermore, our measurements exhibit a dominating diurnal tide and smaller amplitudes for the semidiurnal and terdiurnal tides in the stratosphere.

Tidal Impacts on Zooplankton Dynamics in a Major Ocean-Lagoon Channel: Insights from a 25-Hour Intensive Survey in the Cotonou Channel, Benin

This study investigates the effects of tidal cycles on the zooplankton community within the Cotonou Channel, an important waterway connecting the large Nokoué Lagoon to the Atlantic Ocean in Benin. From the determination of zooplankton composition from 25-hour samples collected in July 2020, alpha diversity indices and abundance were assessed, while relationships between biotic and abiotic parameters were analyzed through Pearson correlation, analysis of variance, and principal component analysis. A total of 66 zooplankton taxa were identified, with rotifers exhibiting the highest species richness (35 taxa), while copepods dominated in abundance (71%). Zooplankton abundance varied significantly, ranging from 2 to 95 ind L−1 depending on the tidal phase. A negative correlation was found between species richness (r = −0.51, p < 0.01) and increasing salinity (3–37), indicating that higher salinity reduced diversity (r = 0.06, p > 0.05). Resilient species like Synchaeta bicornis persisted despite salinity changes. The tidal cycle structurally altered the zooplankton community, with abundance and diversity peaking at different phases, notably higher at high tide (15 ind L−1.) These initial findings underscore the complex interactions between tidal dynamics and estuarine biodiversity, suggesting the need for further research across different tidal and seasonal conditions to inform effective management and conservation efforts.

Interacting Influences of Diurnal Tides, Winds, and River Discharge on a Large Coastal Plume

AbstractThe dispersal of large river plumes in the coastal ocean depends on multiple factors, and in some cases, can be categorized into distinct dynamical regimes: a tidally dominated near‐field, a rotational mid‐field, and a coastal current far‐field. In this study, observations and modeling are used to evaluate the factors controlling the variability in the buoyant plume from Mobile Bay. Rather than distinct dynamical regimes, the Mobile Bay plume depends on forcings that act at overlapping temporal and spatial scales: diurnal tides, river discharge events, and winds. Satellite synthetic aperture radar images along with shipboard in‐situ sampling and marine radar are used to observe plume fronts in spring 2021. Hydrodynamic model simulations are compared with observations and used to characterize a large coastal plume at consistent tidal phase across a range of forcing conditions. The along‐shore position of the plume depends primarily on advection by wind‐driven surface currents. The cross‐shore extent and plume area depend primarily on the tidal amplitude and river discharge, and secondarily on northerly (seaward) winds. Along‐shore winds influence the buoyancy anomaly by altering salinity in the estuary and offshore. Upwelling winds increase the buoyancy anomaly and advect previous plumes away from the mouth. Downwelling winds reduce the buoyancy anomaly by trapping previous plumes near the coast and directing freshwater discharge toward a secondary outlet. Thus, the combined, overlapping influences of the tide, wind, and discharge dominate the variability in freshwater delivery to the shelf at time scales of days and distances of tens of km.

Modal Decomposition of Internal Tides in the Luzon Strait through Two-Dimensional Fourier Bandpass Filtering

Internal tides are pivotal dynamic processes enhancing the mixing of oceanic waters and facilitating energy transfer across various scales within the ocean. In recent years, the proliferation of satellite altimetry observations has enabled global predictions of the elevation and phase of internal tides. This study, leveraging the advanced global internal tide prediction model known as the Multivariate Inversion of Ocean Surface Topography-Internal Tide Model (MIOST-IT), employs a two-dimensional Fourier bandpass filtering approach to decompose the internal tides in the Luzon Strait, thereby addressing the east–west directional blind zones inherent in along-track satellite altimetry-based modal decomposition. To further elucidate the propagation trajectories of individual tidal modes in different directions, we introduce the directional Fourier filter method to characterize the spatial distribution features of each modal internal tide in the vicinity of the Luzon Strait. This work significantly enhances the accuracy and reliability of extracting parameters for distinct modal internal tides, furnishing a scientific basis for subsequent studies on internal tide dynamics and model refinement.

On reversal of wave-generated longshore currents at tidal frequencies on dissipative beaches contiguous to a mesotidal estuary, S.E coast of Nigeria

Reversals in wave-generated longshore currents in surf zones occur at different temporal and spatial scales. Along coastlines with tidal channel openings with well-developed mouth bars or ebb-tidal deltas, the reversal mechanism is often attributed to the mouth bar – induced refraction of the shoaling waves. This reversal mechanism is characterized by convergence of longshore currents from the adjoining surf zones at the mouth of the tidal channel. Simultaneous half-hourly monitoring of wave-generated longshore currents over 50 successive (daylight) semi-diurnal tidal cycles in beach surf zones adjoining the Qua Iboe River estuary, S.E coast of Nigeria showed the above reversal pattern during flooding stage only. The converse pattern, where the surf zone longshore currents diverged away from the mouth of the estuary, was observed during ebbing stage. Both surf zones showed flow direction inversion with respect to each other, with velocity vector correlation coefficient r > − 0.8 in over 80 % of the data set. Instances of comparable flow direction (<10 %) were also recorded. Tidal processes are implicated in the documented results. Direction-averaged longshore current velocities, typically in the 15–60 cm/s range, attained highest values in both surf zones at about spring tide phase. Also, tidal cycle-residual longshore current maximum and minimum velocities occurred close to spring and neap tide, respectively. Only 30 % of the residual velocities were eastward directed in the up-drift surf zone as against 80 % in the down-drift counterpart. Given the prevailing southwesterly waves, the present results negate the assertion that reversal in longshore current direction in this offset shoreline setting is exclusively a consequence of wave refraction by mouth bar morphology. The reversing pattern of the longshore currents over a tidal cycle is well explained by incorporating interacting effects of shoaling waves with tide-induced oscillations in water level as well as the estuarine flow.

Geomorphological controls on estuary hydrodynamics with implications for diatom blooms in deglaciated coastal areas

Understanding local hydraulic conditions is imperative to coastal harmful algal bloom (HAB) monitoring. The research summarized herein describes how the locations and tidal phases selected for coastal hazard sampling can influence measurement results used to guide management decisions for HABs. Our study was conducted in Frenchman Bay, Maine, known for its complex deglaciated coastline, strong tidal influence, and shellfishing activities that are susceptible to problematic HABs such as those produced by some species (spp.) of the diatom genus Pseudo-nitzschia. In-situ measurements of current velocity, density, and turbulence collected over a semidiurnal tidal cycle and a companion numerical model simulation of the study area provide concurrent evidence of two adjacent counter-rotating sub-mesoscale eddies (2–4 km diameter) that persist in the depth-averaged residual circulation. The eddies are generated in the wake of several islands in an area with abrupt bathymetric gradients, both legacy conditions partly derived from deglaciation ∼15 kya. Increased concentrations of Pseudo-nitzschia spp. measured during the semidiurnal survey follow a trend of elevated turbulent dissipation rates near the water surface, indicating that surface sampling alone might not adequately indicate species abundance. Additional measurements of Pseudo-nitzschia spp. from two years of weekly sampling in the region show that algal cell abundance is highest where residual eddies form. These findings provide incentive to examine current practices of HAB monitoring and management by linking coastal geomorphology to hydraulic conditions influencing HAB sampling outcomes, coastal morphometric features to material accumulation hotspots, and millennial time scales to modern hydraulic conditions.

Developing a novel index for detection of optically shallow waters using multispectral satellite imagery and radiative transfer modelling

ABSTRACT Optical remote sensing of water quality has been problematic due to contamination of remote sensing observations by the sea-bottom effect over complex shallow or very clear waters. This has resulted in providing misleading information on the estimation of Water Constituent Concentrations (WCCs) retrieved from satellite images. In this research, we used Radiative Transfer (RT) Modelling to develop a simple and innovative index named the Sea-Bottom Effect Index (SEBEI) to readily determine water pixels contaminated by the sea-bottom effect, (hereafter named Optically Shallow Waters, (OSWs)) from remote sensing observations. To define the SEBEI, we initially assessed the influence of the sea-bottom on simulated water-leaving reflectance (R rs ) using RT Water-Sea-Bottom (WSB) modelling. This evaluation encompassed a range of water depths, seabed albedos, and WCCs (i.e. Chlorophyll-a (Chla) and Suspended Particulate Matter (SPM)). Next, we detected the most sensitive wavelengths (i.e. 750 nm, 810 nm, and 900 nm) to the seabed contribution on the simulated R rs spectra at the water surface level and developed the SEBEI. We validated the accuracy of the proposed SEBEI over a time series of MEdium Resolution Imaging Spectrometer (MERIS) and Ocean and Land Colour Instrument (OLCI) images captured during low and high tidal phases over shallow waters of the Dutch Wadden Sea, the Netherlands. The results indicate that the SEBEI effectively distinguishes between OSWs and Optically Deep Waters (ODWs) using satellite images (with an R2 value of ≥ 0.97 and a Root Mean Square Error (RMSE) of ≤ 0.65). The SEBEI can serve as an intermediate solution for detecting OSWs in multispectral and hyperspectral satellite images worldwide. It eliminates the need for ancillary datasets like bathymetry maps and significantly enhances the reliability of WCC maps produced over complex shallow coastal waters.

The sensitivity of tidal asymmetry descriptors in the Ems estuary

Tidal asymmetry in estuaries and other tidally dominated coastal systems is commonly evaluated to assess system states or their development. Based on different methods, local states are classified as either flood or ebb dominant. An increasing number of descriptors for deriving tidal asymmetry in recent years calls for a comparison and discussion of their sensitivity on input data and its quality. We compared tidal asymmetry from water level and current velocity using various descriptors that deduce from harmonic, ratio, and skewness methods. Computed from one-year measurements at different stations along the Ems estuary, their comparability was enabled by a new approach of scaling. Our results on the variation of sampling intervals demonstrated a highly site-specific sensitivity of the descriptors that led up to changes in the asymmetry direction in tidal duration asymmetry and phase lag. The slack water asymmetry appeared most sensitive to the studied parameter settings. As expected, variability of tidal asymmetry reduced with an increasing number of analyzed tides. At the same time, uncertainty from the asymmetry during spring or neap phases compared to spring-neap periods remained in all analyzed descriptors. Hence, the characterization of the estuary in terms of flood- or ebb-dominance depends critically on the quality and extent of the input data. For all parameter settings, the impact of river discharge on tidal asymmetry was pronounced but varied depending on the location in the estuary. The actual characterization of the effect of asymmetry, e.g., on sediment transport, is not conducted in this study. We propose that this requires a more comprehensive dataset, such as depth and cross-sectional variability of currents and sediment concentrations.

Nonlinearities detection in river-tide interaction in Río Negro hydrographic lower basin (Argentina) using higher-order spectra

The nonlinear interactions between river discharge, astronomical tidal wave, and local geomorphology during storm passage or water release from upstream dams can produce severe floods in the Río Negro lower basin (Argentina). For this reason, this paper aims to detect and study nonlinear processes in this area. The watercourse hydrodynamics was described using hourly water level data from three limnigraphs during 2003 – 2021 and flow time series. The tide gauge dataset was employed to describe the influence of tidal cycles on the hydrological regimen. Nonlinear processes' impact on the astronomical tidal cycle and river discharge was analyzed using Harmonic Analysis, and Fourier higher-order spectra, also it was complemented with the selection of two study cases. Harmonic Analysis results showed that the tidal wave entry upstream of the Río Negro modulates its hydrological regime, presenting the water column semidiurnal variations. Also, high-order spectral analysis detected nonlinear interactions in the signal in storm conditions with an energetic redistribution among the linear tidal constituents toward shallow water harmonics. Additionally, nonlinear interactions provoked a delay in the tidal ebb phase with a consequential extension of flooding duration time. This type of study contributes to the knowledge of the flood mechanisms activated during a storm.

Environmental factors as drivers of the spatial distribution of the copepods Eurytemora affinis affinis and Eurytemora velox in the Scheldt tributaries

The main tributaries of the Scheldt estuary offer an interesting setting to study the response of organisms to various environmental conditions. The Bovenscheldt and the Dender are closed to tidal entrance by locks, and are hence essentially fresh water systems with low turbulence. The Durme and the Rupel, with its two sub-tributaries (the Beneden-Nete and the Dijle), receive estuarine water at each tide and as such, present a fresh-brackish water gradient according to the geographic location of their confluence with the estuary and the tidal phase. This paper aims (1) to determine the spatial-temporal distribution of two calanoid copepod species: Eurytemora affinis affinis (Poppe, 1880) and E. velox (Lilljeborg, 1853) in these tributaries and (2) the environmental conditions favoring each species development. Species distribution and abundance data, collected in the four tributaries between 2018 and 2021 are analyzed by Generalized Linear Mixed Models (GLMM) and the results are interpreted in the context of ecosystem functioning. A spatial structuring between E. affinis affinis and E. velox was revealed. Whilst E. affinis affinis develops best in the tidal tributaries such as the Durme and Rupel, E. velox is more typical of the non-tidal ones: the Bovenscheldt and the Dender. The tidal tributaries have high Particulate Organic Matter (POM) concentrations due to tidal turbulence, and hence generally lower oxygen concentrations than the non-tidal ones (<10–11 mg L−1). Factors favoring E. affinis affinis development in the tidal tributaries are higher chlorinity, pH and diatom abundance than in the non-tidal ones. In the non-tidal tributaries, with lower POM concentrations, phytoplankton activity more strongly influences the overall oxygen balance, with oxygen concentrations often rising above 10–11 mg L−1 and sometimes showing high variability. The results suggest that E. affinis affinis might suffer from hyperoxia-induced oxidative stress under these high oxygen conditions and advocate the necessity to include this aspect in further research. E. velox, which is most abundant in those tributaries, seems to be more tolerant to hyperoxia and strong fluctuations in O2 concentrations than E. affinis affinis.

Density gradient effect on circulation patterns and mixing processes: Observations at the convergence front of a salt wedge, microtidal estuary

The convergence front in estuaries is a crucial area influenced by dynamic mixing processes which vary from one tidal cycle to another, with density gradients playing a key role in shaping circulation patterns. This significance is particularly pronounced in salt wedge estuaries, where discharge holds greater influence than tides. Despite this, there is a limited amount of research based on high-resolution in-situ data to analyze circulation and mixing processes affected by density gradients in the convergence front of a salt wedge estuary. In this study, continuous measurements of current velocities and CTD profiles were conducted over three tidal cycles to analyze the intratidal variability of circulation patterns (u, v, w) and mixing parameters in the convergence front. Stratification parameter (ηs) and Richardson numbers (RL and Ri) were calculated to analyze the mixing/stratification variability within a tidal cycle. The observations revealed a persistent Subsurface Velocity Maximum (SVM) in the longitudinal component, coinciding with density gradient increases > 4 kg m−3 along the water column. Similarly, lateral circulation patterns were influenced by those increases in density gradient, resulting in eastward-directed currents and the formation of two circulations cells. As expected in a salt wedge estuary, stratification occurs during flood and High Water (HW), when the density gradient increases due to intrusion of the salt wedge. Notably, in the Magdalena River estuary, when the discharge is below 3800 m3 s−1, the salt wedge remains consistently present irrespective of the tidal phase, resulting in a permanently stratified water column with ηs values between 1.60 – 1.95. It is expected that the steady presence of stratified conditions during periods of low discharge leads to an increased accumulation of sediments. These findings provide valuable quantitative insights into the complex interplay of density gradients, circulation patterns, and stratification dynamics in salt wedge estuaries.

Contribution of high turbidity to tidal dynamics in a curved channel in Zhoushan Islands, China

The curved tidal channel, Luotou Deep-water Navigational Channel, is the main channel of the Ningbo Zhoushan Port, which is ranked first in the world. Tidal dynamics in the channel are spatially and temporally asymmetric. In this study, the three-dimensional tidal dynamics in the channel were analyzed using field data and simulated using FVCOM. The results show that the tides in the channel flood/ebb along the northern/southern bank near the bottom/surface layer and these asymmetries are due to the imbalanced Coriolis force, centrifugal force, sea-level gradient, and density gradient. Residual current velocity peaks (0.7 m/s) in the middle of the channel as the same distribution as sediment flux. There are two high turbidity zones (>4 kg/m3) which are northern at flood than at an ebb in the channel. The drag reduction effect of fluid mud enhances the lateral circulation, which is strong near the Chuanshan Peninsula and frictional dissipation plays an important role in it. The presence of suspended sediment changes the contribution of acceleration terms through impacting density and bottom friction, and the centrifugal force term has the largest increases. This study provides the foundation for the morphology evolution and harbor management of macro-tidal turbid coastal zones. Highlights A baroclinic model was built to study the tidal dynamics in macro-tidal turbid Zhoushan Islands. Asymmetric tides and lateral circulation occur in the Luotou Deep-water Navigational Channel (DNC). Mechanism of lateral circulation depends on tidal phases and locations in the DNC. Sediment impacts water density and drag coefficient, and then changes currents and sediment fluxes in the DNC.

Relationship between hydro-environmental variables and Coilia nasus catch in a highly turbid macrotidal estuary in Japan

The effect of estuarine hydro-environmental parameters and tidal phases on the fish catch in the macrotidal Chikugo River estuary was investigated. This study focuses on the catch data of Etsu (Coilia nasus) fish, obtained during their spawning season from 2009 to 2020, employing traditional fishing techniques at a site situated 14.6–16 km upstream from the river mouth. It analyses various hydro-environmental parameters and identifies optimal ranges of environmental parameters. The optimal range of salinity (0.04–0.2) and suspended sediment concentration (SSC, 16–118 mg/L) indicated a higher fish catch at the saltwater-freshwater interface and upstream of the estuarine turbidity maximum (ETM) zone. By developing a generalized additive model (GAM) to analyze the relationship between CPUE and environmental parameters, the study assesses the relative importance of each variable in explaining CPUE variance. GAM exhibited a reasonable predictive capability (R2 = 0.509) and indicated the relative importance of each variable (SSC-24.69%, salinity-21.81%, temperature-24.41%, discharge-9.16%, and MTR-19.91%) in explaining the CPUE variance. Additionally, it aims to verify the maturity status and spawning migration patterns of C. nasus fish through standard-length distribution analysis during the spawning season. The standard-length distribution and comparison with previous literature confirmed that fishes caught by the fisherman belong to the category of matured fishes aged 2–3 years. The results indicated that the high CPUE of matured C. nasus at the upstream of ETM and freshwater-saltwater interface was related to the spawning migration and not to their feeding behaviour at the ETM. This research contributes to fisheries management strategies and conservation efforts in the Chikugo River estuary by providing valuable guidance for habitat restoration, informing fishing regulations, and understanding the behaviour of C. nasus.

The Influence of Time, Tide, and Place on Fine-Scale Nekton Distribution: Insights from the San Francisco Estuary

The location of estuarine organisms varies based on geophysical cycles and environmental conditions, which can strongly bias understanding of organism abundance and distribution. In the San Francisco Estuary, California, extensive monitoring surveys have provided insight into the life history and ecology of certain commercially important or legislatively protected fish species. However, there remains substantial uncertainty in factors influencing the vertical and lateral distributions of many other nekton species in the San Francisco Estuary, including longfin smelt Spirinchus thaleichthys, for whom such distributional information may highly influence interpretation of existing data. We carried out paired sampling using surface and demersal gears to address three questions: (1) Does diel phase influence the vertical position of nekton (e.g., surface versus demersal)? (2) Do environmental conditions, specifically turbidity, influence the vertical and lateral positions of nekton (e.g., center channel versus peripheral shoal)? (3) Does tidal variability influence vertical and lateral distributions of nekton? We documented variability in sampled nekton densities across diel phase (day/night), vertical position (surface/bottom), and lateral position (channel/shoal). Tidal phase and turbidity concentration influenced vertical and lateral distributions for some species at certain locations. Although infrequently encountered, we documented associations of longfin smelt with the lower water column and shoal habitats, with some evidence for upward vertical shifts in low light conditions brought about by nightfall or elevated turbidity. Observed habitat associations provide insight into how interacting geophysical and environmental factors may influence the distribution of nekton and thus the vulnerability of individual species to detection by sampling gears.

Studying the Internal Wave Generation Mechanism in the Northern South China Sea Using Numerical Simulation, Synthetic Aperture Radar, and In Situ Measurements

The internal waves in the South China Sea are highly correlated with the tidal currents in the Luzon Strait, which makes it possible to establish an internal wave prediction model based on internal wave kinematics. However, the kinematic model requires the input of the exact location and time of the initial internal wave for which the generation mechanism of internal waves in the northern South China Sea must be well understood. By analyzing the internal wave field in the northern South China Sea (SCS) simulated using the MIT General Circulation Model (MITgcm) and observations from satellite synthetic aperture radar (SAR) and mooring temperature–salinity–depth (TSD) chains, the source regions and propagation initiation times of internal waves are identified for three typical tidal phases, i.e., the diurnal-tide-dominated phase (DTP), transition tide phase (TTP), and semidiurnal-tide-dominated phase (STP). The generation procedures of Type A and Type B internal waves are discussed in detail with those data. The present study reveals that Type A and Type B waves are generated at the eastern and western ridges, respectively, and both commence their westward propagation at the peak of the eastward tidal flow. The dynamics of lee waves and the resonance effect with double ridges constitute the generation mechanisms of internal waves in the northern SCS. Combined with varying configurations of tidal conditions, topography, and stratification, the generation procedures of Type A and Type B waves in the DTP, TTP, and STP are elucidated with the generation mechanism in a unified and self-consistent way. In short, during DTP, weaker A waves alternate with weaker B waves each day; during TTP, strong A waves and strong B waves appear alternately every day; and there are two weak A waves per day during the STP. The generation mechanism can help in developing future empirical models for generating internal waves using tidal currents, topography, and stratification without requiring complex fluid dynamics calculations.

Joint Effects of Winds and Tides on Near-Inertial Internal Waves in the Northern South China Sea: A Three-Dimensional Numerical Study

Abstract Joint effects of winds and tides on near-inertial internal waves (NIWs) are numerically investigated via a series of three-dimensional quasi-realistic simulations in the northern South China Sea (NSCS). Model results demonstrate that in the presence of wind-induced NIWs, more tidal energy is transferred to NIWs, while in the presence of tide-induced NIWs, the extreme wind (cyclone) would inject less near-inertial kinetic energy (NIKE). The interaction between wind-induced and tide-induced NIWs produces total NIKE more (or less) than a linear superposition of that generated by wind and tide forcing alone at different sites in the NSCS. Specifically, near the Luzon Strait, both tides and winds make positive contributions to the local near-inertial energy input, resulting in more than 30% enhancement of total NIKE (&gt;0.5 kJ m−2). However, in some deep-water regions along the cyclone paths, energy is transferred from cyclones to NIWs and also from NIWs to internal tides. Due to this “energy pipeline” effect, tide- and wind-induced NIWs contribute to weakening of total NIKE (∼0.3 kJ m−2 or 30%). Additionally, sensitivity experiments with varying initial tidal phases indicate that the interaction between wind-induced NIKE and tide-induced NIKE is robust in most of the model domain (over 80%) under different phase alignments between wind- and tide-induced NIWs. From the perspective of cyclones, tide-induced NIKE is comparable to wind-induced NIKE in the Luzon Strait before the arrival of cyclones, while tide-induced NIKE is two orders of magnitude smaller than wind-induced NIKE in most of the NSCS after the arrival of cyclones. Overall, our results highlight the joint effects of wind and tide forcing on the local NIW dynamics in the NSCS. Significance Statement Near-inertial internal waves (NIWs) are ubiquitous phenomena in stratified water, which can significantly influence the ocean mixing, especially across the thermocline. NIWs are generated mainly by wind forcing near the sea surface, but also by internal tide breaking in the ocean interior. Hence, a question that arises is whether winds or tides play dominant roles in generating NIWs. In fact, due to the interaction between wind- and tide-induced NIWs, the total near-inertial kinetic energy (NIKE) is not merely a linear superposition of that generated by winds and tides forcing alone. By carrying out numerical experiments, we find that extreme winds can help to transfer more energy from tides to NIWs, while tides would suppress energy transferring from winds to NIWs. As a result, this fact is crucial in accurately reproducing NIW dynamics in a targeted region, thereby determining redistribution of local ocean mixing intensity.