Baroclinic Tides Research Articles

Internal Tides at the Coast: Energy Flux of Baroclinic Tides Propagating into the Deep Ocean in the Presence of Supercritical Shelf Topography

Abstract The generation of internal tides at coastal margins is an important mechanism for the loss of energy from the barotropic tide. Although some previous studies attempted to quantify energy loss from the barotropic tides into the deep ocean, global estimates are complicated by the coastal geometry and spatially and temporally variable stratification. Here, we explore the effects of supercritical, finite amplitude bottom topography, which is difficult to solve analytically. We conduct a suite of 2D linear numerical simulations of the barotropic tide interacting with a uniform alongshore coastal shelf, representing the tidal forcing by a body force derived from the vertical displacement of the isopycnals by the gravest coastal trapped wave (of which a Kelvin wave is a close approximation). We explore the effects of latitude, topographic parameters, and nonuniform stratification on the baroclinic tidal energy flux propagating into the deep ocean away from the shelf. By varying the pycnocline depth and thickness, we extend previous studies of shallow and infinitesimally thin pycnoclines to include deep permanent pycnoclines. We find that scaling laws previously derived in terms of continental shelf width and depth for shallow and thin pycnoclines generally hold for the deeper and thicker pycnoclines considered in this study. We also find that baroclinic tidal energy flux is more sensitive to topographic than stratification parameters. Interestingly, we find that the slope of the shelf itself is an important parameter but not the width of the continental slope in the case of these steep topographies. Significance Statement The objective of this study is to better understand how vertical density stratification, which can vary seasonally in the ocean, affects the interaction of tides with steep coastal topography and the generation of waves that travel away from the coast in the ocean interior. These waves in the interior can travel over long distances, carrying energy offshore into the deep ocean. Our results suggest that the amount of energy in these internal waves is more sensitive to changes in topography and latitude than to the vertical density profile. The scaling laws found in this study suggest which parameters are important for calculating global estimates of the energy lost from the tide to the ocean interior at the coastal margins.

Effects of internal waves on bottom thermal structures and turbulent mixing in the Xisha Islands

Using velocity and high-resolution temperature mooring data from the fore-reef slope of Yongxing Island in the northwest South China Sea (water depth of 69 m), we examine the effects of internal waves on the temporal variations in temperature, bottom mixed layer (BML) and turbulent mixing. The diurnal tide is found to be the dominant tidal force and the baroclinic tide is highly active, which would account for the 11 d abnormal spring-neap cycle in the barotropic tidal current. During the ebb period (tidal elevation decreases), the bottom diurnal baroclinic current transports cold water upslope, resulting in a decrease in temperature, and vice versa during the flood period. The BML thickness Hbml widely varies around 1.5m, approximately 2% of the water depth. The bottom turbulent mixing is not so active, indicated by the bulk dissipation Eɛ of 10 mWm−2 and turbulent diffusivity κz of 2×10−4m2s−1. Both Hbml and Eɛ approximate a log-normal distribution, demonstrating strong intermittency. The high-frequency (ω) internal bores can increase Hbml by four times and enhance the turbulent mixing by one order, which should be responsible for the slow cascade of Hbml(∼ω−1.5) and Eɛ(∼ω−1.0). In the downslope phase, Hbml is about 30% thicker, and turbulent mixing is enhanced by 3 times stronger than those in the upslope phase. It is revealed that under the forcing of internal waves, turbulent mixing corresponds to a thick BML with Hbml∼〈κz〉0.25, and stratification (N) has a significantly negative correlation with the development of BML, Hbml∼〈N〉−1.8.

Nutricline adjustment by internal tidal beam generation enhances primary production in idealized numerical models

When the barotropic tide encounters variable bathymetry, fluctuating flow along a topographic slope generates baroclinic tides, or internal tides. There is growing evidence that these internal tides can affect primary production in the euphotic zone, though the dominant mechanisms are unclear. Internal tides move passive phytoplankton through an exponentially varying light field, enhancing primary production near the base of the euphotic zone. In addition internal tides also increase primary production through vertical nutrient advection into the euphotic zone. Topographically generated internal tides can be separated into two regimes: 1) the often highly nonlinear near-field regime where tidal beams are observed and 2) the more linear far-field regime. This study examines the primary production response to these internal tide processes using the Regional Ocean Modeling System (ROMS) coupled to a simple Nutrient, Phytoplankton, Zooplankton, Detritus (NPZD) model configured for an oligotrophic system with the nutricline positioned below 50 m depth. These idealized simulations generate internal tide beams with an oscillating, horizontal body force at the M2 tidal frequency that is applied to domains with a bathymetric step and uniform stratification. Sensitivity of the primary production response to the energy content of the tidal beam is obtained by adjusting the height and slope of the bathymetric step. Simulation results reveal that primary production intensifies along tidal beams due to the local enhancement of parcel vertical displacement (light effect) and nutrient advective flux divergence (nutrient effect). In the near-field regime across the range of step heights and slopes in this study, the nutrient effect is an order of magnitude larger and explains 92% of the variance in primary production versus only 14% for the light effect. The geometry of the generating feature sets the kinematics of the tidal beam. The light effect is limited in the euphotic zone across our domains because realized changes in light experienced over a tidal cycle are small relative to the amount of light available at a particular depth. In contrast, the magnitude of the nutrient effect increases more substantially with tidal beam energy.

Mixing Processes at the Southwestern Entrance to the Japan/East Sea

Abstract Small-scale processes at the southwestern boundary of the Ulleung Basin (UB) in the Japan/East Sea (JES) were examined using combined ship-based and moored observations along with model output. Model results show baroclinic semidiurnal tides are generated at the shelf break and corresponding slope connecting the Korea/Tsushima Strait with the UB and propagate into the UB with large barotropic-to-baroclinic energy conversion over the slope. Observations show high-frequency internal wave packets and indicate strong velocity shear and energetic turbulence associated with baroclinic tides in the stratified bottom layer. Solitary-like waves with frequencies from 0.2N to 0.5N (buoyancy frequency N) were found at the edge of the shelf break with supercritical flow. For subcritical flow, a hydraulic jump formed over the shelf break with weakly dispersive internal lee waves with frequencies varying from 0.5N to N. These high-frequency lee waves were trapped in the stratified bottom layer, with wave stress similar to the turbulent stress near the bottom. The power loss due to turbulent bottom drag can be an important factor for energy loss associated with the hydraulic jump. Turbulent kinetic energy dissipation rates of ∼10−4 W kg−1 were found. Large downward heat and salt fluxes below the high-salinity core mix warm/salty Tsushima Current Water with cold/low-salinity JES Intermediate Water. Mixing over the shelf break could be very important to the JES circulation since the calculated diapycnal upwelling (1–6 m day−1) at the shelf break and slope is substantially greater than the basin-averaged estimate from chemical tracers and modeling studies. Significant Statement The Japan/East Sea (JES) is a marginal sea, enclosed by Japan, Korea, and Russia. This study describes mixing processes over the shelf break connecting the northern Korea/Tsushima Strait (KTS) with the southern Ulleung Basin (UB), where the warm, high-salinity Kuroshio water carried by the Tsushima Current interacts with southward-flowing subsurface water masses in the JES. Our analysis suggests that the shelf break and slope between the KTS and the UB are vital areas for water-mass exchange in the southern JES. The enhanced mixing at the shelf break may impact water masses and circulation over the entire JES.

Variability and energy budget of the baroclinic tides in the Arabian Sea

Variability and energy budget of the baroclinic tides in the Arabian Sea

Barotropic and baroclinic tides increase primary production on the Northwest European Shelf

High biological productivity and the efficient export of carbon-enriched subsurface waters to the open ocean via the continental shelf pump mechanism make mid-latitude continental shelves like the northwest European shelf (NWES) significant sinks for atmospheric CO2. Tidal forcing, as one of the regionally dominant physical forcing mechanisms, regulates the mixing-stratification status of the water column that acts as a major control for biological productivity on the NWES. Because of the complexity of the shelf system and the spatial heterogeneity of tidal impacts, there still are large knowledge gaps on the role of tides for the magnitude and variability of biological carbon fixation on the NWES. In our study, we utilize the flexible cross-scale modeling capabilities of the novel coupled hydrodynamic–biogeochemical modeling system SCHISM–ECOSMO to quantify the tidal impacts on primary production on the NWES. We assess the impact of both the barotropic tide and the kilometrical-scale internal tide field explicitly resolved in this study by comparing simulated hindcasts with and without tidal forcing. Our results suggest that tidal forcing increases biological productivity on the NWES and that around 16% (14.47 Mt C) of annual mean primary production on the shelf is related to tidal forcing. Vertical mixing of nutrients by the barotropic tide particularly invigorates primary production in tidal frontal regions, whereas resuspension and mixing of particulate organic matter by tides locally hinders primary production in shallow permanently mixed regions. The tidal impact on primary production is generally low in deep central and outer shelf areas except for the southwestern Celtic Sea, where tidal forcing substantially increases annual mean primary production by 25% (1.53 Mt C). Tide-generated vertical mixing of nutrients across the pycnocline, largely attributed to the internal tide field, explains one-fifth of the tidal response of summer NPP in the southwestern Celtic Sea. Our results therefore suggest that the tidal NPP response in the southwestern Celtic Sea is caused by a combination of processes likely including tide-induced lateral on-shelf transport of nutrients. The tidally enhanced turbulent mixing of nutrients fuels new production in the seasonally stratified parts of the NWES, which may impact the air–sea CO2 exchange on the shelf.

Internal Waves Near Half Moon Island, South Shetland Islands

We analyze internal waves based on measurements on cruise 87 of the R/V Akademik Mstislav Keldysh in the Bransfield Strait near Half Moon Island and calculations using a numerical model. The measurements were carried out on January 25, 2022 for four hours using a line of temperature and pressure sensors, along with the measurements with a CTD probe. Temperature fluctuations according to the sensors of the thermal line and the probe showed that the amplitude of internal waves is close to 5 m, sometimes reaching 15 m. According to the results of calculations of the global tidal model TPXO9, irregular semidiurnal tides predominate in the study area. Numerical calculations of the parameters of internal waves show that the baroclinic tide generated on a steep slope breaks up into higher frequency waves.

Diurnal Variability of Ocean Temperature Off Pt. Sal, Central California

AbstractDiurnal‐band temperature (DU‐T) variability is investigated over 30 days from measures of temperature over the vertical obtained by 78 moorings deployed in the Santa Monica Basin near Pt. Sal (PS), California. DU‐T variability accounts for 3.6%–23% of the total variability. Vertical empirical orthogonal function mode‐1 DU‐T variability accounts for an average of 86% of the total vertical DU‐T variability at each mooring, is greatest north of PS where its vertical structure is baroclinic, and weakest south of PS where its vertical structure is surface intensified. Spatial complex empirical orthogonal function mode‐1 DU‐T accounts for 73% of the total DU‐T over the region, reveals an onshore baroclinic DU‐T propagation of ∼0.5 m/s, and a previously observed equatorward baroclinic DU‐T propagation of ∼1.9 m/s along the coast north of PS. Dominant vertical mode‐1 diurnal‐band baroclinic velocity (DU‐w) variability at each mooring exhibits a two‐layer structure that is 180° out of phase with maxima near the surface and the bottom, and resembles the DU‐T propagation. DU‐T variability mechanisms include wind‐forced deepening of the surface mixed layer, inertial motions, upwelling, and barotropic and baroclinic tides. Among these concomitant diurnal mechanisms, wind‐driven inertial motions are likely the primary mechanism influencing DU‐T variability during upwelling‐favorable conditions owing to coherence between observed DU‐w and the predicted inertial response to wind‐forcing. Further, spatial mode‐1 DU‐T is correlated and coherent with spatial mode‐1 DU‐w and diurnal wind‐forcing. In addition to describing predominant DU‐T variability patterns, these findings have important implications regarding spatial patterns in biological recruitment, transport, and settlement.

SPATIAL DISTRIBUTION OF AMPLITUDES OF INTERNAL TIDAL WAVES ON THE NORTH-EASTERN SHELF OF SAKHALIN

The transformation of a multicomponent barotropic tide in the Sea of Okhotsk is considered in the framework of fully nonlinear non-hydrostatic calculations. The data of the tidal model and parameterized bottom topography and vertical profiles of sea water density based on data from open international atlases are used to initialize the model. Estimates of the wave amplitudes of the diurnal and semidiurnal baroclinic tides in terms of the displacement of isopycnal surfaces at different horizons are obtained and presented in the form of geographical maps. It is shown that the distribution of amplitudes significantly depends on depth, has a complex spatial structure with a noticeable predominance of the amplitudes of baroclinic waves of the diurnal period and the main extrema located on the shelf opposite Cape Elizabeth, Okhinsky Isthmus and Cape Patience. The implemented approach to mapping the amplitudes of internal tides can be applied to other shelf areas of the seas of the Russian Federation and used for predictions of these phenomena, including engineering assessments for the design and operation of marine infrastructure.

Internal tides off the Amazon shelf during two contrasted seasons: interactions with background circulation and SSH imprints

Abstract. The Amazon shelf break is a key region for internal tide (IT) generation. It also shows a large seasonal variation in circulation and associated stratification. This study, based on a high-resolution model (1/36∘) explicitly forced by tide, aims to better characterize how the ITs vary between two contrasted seasons. During the season from March to July (MAMJJ) the currents and mesoscale eddies are weak while the pycnocline is shallower and stronger. From August to December (ASOND) mean currents and mesoscale eddies are strong, and the pycnocline is deeper and weaker than in MAMJJ. For both seasons, semi-diurnal M2 ITs are generated on the shelf break mainly between the 100 and 1000 m isobath in the model. South of 2∘ N, the conversion from barotropic to baroclinic tide is more efficient in MAMJJ than in ASOND. Local dissipation of the coherent M2 at the generation sites is higher in MAMJJ (30 %) than in ASOND (22 %), because higher modes are favourably generated (modes 2 and 3), making the internal wave packet more dispersive. The remaining fraction (70 %–80 %) propagates away from the generation sites and mainly dissipates locally every ∼ 100 km, which corresponds to the mode 1 reflection beams. About 13 %, 30 %, and 40 % of the M2 coherent IT dissipates at the first, second, and third beams. M2 coherent baroclinic flux propagates more northward during MAMJJ while it seems to be blocked at 6∘ N during ASOND. There is no intensified dissipation of the coherent M2 that could explain the disappearance of the coherent flux. In fact, the flux at this location becomes more incoherent because of strong interaction with the currents. This has been shown in the paper using 25 h mean snapshots of the baroclinic flux that shows branching and stronger eastward deviation of the IT when interacting with mesoscale eddies and stratification during ASOND. Finally, we evaluated sea surface height (SSH) frequency and wavenumber spectra for subtidal (f<1/28 h−1), tidal (1/28 h−1 < f<1/11 h−1), and supertidal (f>1/11 h−1) frequencies. Tidal frequencies explain most of the SSH variability for wavelengths between 250 and 70 km. Below 70 km, the SSH is mainly incoherent and supertidal. The length scale at which the SSH becomes dominated by unbalanced (non-geostrophic) IT was estimated to be around 250 km. Our results highlight the complexity of correctly predicting IT SSH in order to better observe mesoscale and sub-mesoscale eddies from existing and upcoming altimetric missions, notably the Surface Water Ocean Topography (SWOT) mission.

Impact of Horizontal Resolution (Submesoscale Permitting vs. Mesoscale Resolving) on Ocean Dynamic Features in the South China Sea

AbstractModel resolution is a key factor that impacts the model bias in simulating ocean dynamics. How would model resolution influence model bias is an important question. In this study, we set up two sets of experiments with identical settings except the grid resolution, one with a minimum spacing of 2 km, referred to as the submesoscale permitting (SP) model, and the other with a minimum spacing of 6 km, referred to as the mesoscale resolving (MR) model, to investigate the impact of horizontal resolution on simulated dynamic features in the South China Sea. Results show that the SP outperforms the MR model in simulating both barotropic and baroclinic tides, upper ocean circulation, sea surface height, and temperature in the South China Sea, and in simulating the evolution of the eddy pair east of Indo‐China Peninsula in summer and the Kuroshio loop current in the Luzon Strait in winter. The SP model also performs better in simulating thermal structure within the ocean and the winter mixed layer depth. Additionally, the SP model can capture more vertical motion features. The superiority of the SP model lies in its ability to better resolve terrain features that have a significant impact on ocean dynamical processes, and to resolve more physical processes. As a result, the SP model relies less on the uncertain parameterized processes. Our study has important implications for studies on the modeling of physical oceanography in the South China Sea, as well as for the reduction of climate model biases.

Tidally Driven Mixing “Hot Spot” at the Entrance of the Japan/East Sea

AbstractMixing at the southwestern boundary of the Ulleung Basin (UB) in the Japan/East Sea was examined using combined ship‐based, moored, and quasi‐autonomous observations. These observations revealed strong tidal currents, high‐frequency internal wave packets, and energetic turbulence over the sill connecting the Korea/Tsushima Strait and the UB, especially when a supercritical flow becomes subcritical over the sill. In addition, strong velocity shear associated with the semidiurnal baroclinic tide was found near the bottom, where the bathymetry slope was critical to the semidiurnal tide. Large temperature fluctuations were observed during the subcritical phase of the flow, as hydraulically controlled flow generated internal‐wave packets and energetic turbulence with a turbulent‐kinetic‐energy dissipation rate of 10−4 Wkg−1, and a diapycnal diffusivity of 10−2 m2s−1. Our analysis suggests that the sill is a hot spot of mixing, which in turn can play an important role in the Japan/East Sea circulation.

Seasonality and Spatial Dependence of Mesoscale and Submesoscale Ocean Currents from Along-Track Satellite Altimetry

Abstract Along-track wavenumber spectral densities of sea surface height (SSH) are estimated from Jason-2 altimetry data as a function of spatial location and calendar month to understand the seasonality of meso- and submesoscale balanced dynamics across the global ocean. Regions with significant mode-1 and mode-2 baroclinic tides are rejected, restricting the analysis to the extratropics. Where balanced motion dominates, the SSH spectral density is averaged over all pass segments in a region for each calendar month and is fit to a four-parameter model consisting of a flat plateau at low wavenumbers, a transition at wavenumber k0 to a red power law spectrum k−s, and a white spectrum at high wavenumbers that models the altimeter noise. The monthly time series of the model parameters are compared to the evolution of the mixed layer. The annual mode of the spectral slope s reaches a minimum after the mixed layer deepens, and the annual mode of the bandpassed kinetic energy in the ranges [2k0, 4k0] and [k0, 2k0] peak ∼2 and ∼4 months, respectively, after the maximum of the annual mode of the mixed layer depth. This analysis is consistent with an energization of the submesoscale by a winter mixed layer instability followed by an inverse cascade of kinetic energy to the mesoscale, in agreement with prior modeling studies and in situ measurements. These results are compared to prior modeling, in situ, and satellite investigations of specific regions and are broadly consistent with them within measurement uncertainties. Significance Statement This paper uses satellite observations to understand the source of ocean dynamics at the 1–100-km scales at which vertical motion becomes important and which are thus relevant for biology and for the exchange of heat and carbon with the atmosphere. The observations are consistent with a seasonal variation of dynamics at these scales, predicted by a specific theory of upper-ocean turbulence and confirmed by modeling studies and regional observations. We update prior satellite-based studies by excluding regions with competing effects, by our treatment of the noise, and by our characterization of the seasonality. This work provides a template for analyzing data from the upcoming Surface Water and Ocean Topography (SWOT) satellite.

Baroclinic Tidal Energetics Inferred from Satellite Altimetry

AbstractThe energetics of baroclinic tides are analyzed using the High Resolution Empirical Tide (HRET) model. The HRET model consists of maps of the sea surface height (SSH) anomaly associated with that component of the tides’ baroclinic pressure fields, which are phase locked with the gravitational tidal potential. The dynamical assumptions underpinning the transformation of SSH into corresponding baroclinic velocity and energy flux are examined critically through comparisons with independent information and term balances in the equations of motion. It is found that the HRET-derived phase speed of the mode-1 baroclinic tide agrees closely with the phase speed predicted by the theory for long waves propagating through the observed climatological stratification. The HRET SSH is decomposed into contributions from separate vertical modes, and the energy, energy flux, and energy flux divergence of mode-1 (for M2, S2, K1, and O1) and mode-2 (for M2) tides are computed, with an emphasis on the most accurately determined mode-1 M2. The flux divergence of HRET mode-1 M2, computed as the contour integral of the outbound normal flux around strong generation regions, is found to correspond with independent estimates of the area-integrated barotropic-to-baroclinic-mode-1 conversion, although, there is considerable uncertainty in both the flux divergence and the barotropic-to-baroclinic conversion. Further progress on mapping the baroclinic tidal energetics from altimeter observations will require more dynamically complete descriptions of the baroclinic tides than can be provided by kinematic models of SSH, such as HRET.

4D-Var data assimilation using satellite sea surface temperature to improve the tidally-driven interior ocean dynamics estimates in the Indo-Australian Basin

Estimating ocean processes in regions with sparse observations, and where dynamics are dominated by strong barotropic and baroclinic tides, poses a significant modelling challenge. In this study, a Four-Dimensional Variational (4D-Var) data assimilation method, incorporating remotely sensed sea surface temperature (SST) data observations only, was used to improve estimates of the internal tide dynamics in the ocean between Australia and Indonesia. In this region, local internal tide generation, propagation and interference of the internal tides is highly variable and dependent on the 3D density field and mesoscale dynamics. The results of this study show that assimilation of remotely sensed SST data not only reduced the surface temperature root mean squared error (RMSE) from 0.8 °C to 0.3 °C, and removed a 0.4 °C cold bias (compared to a non-assimilative model), but also improved estimates of through-water-column variables — with the latter evaluated against ARGO profiling floats and two independent mooring sites. Overall, the depth-averaged temperature RMSE was reduced by as much as 38%, and ocean currents by ∼20% at the two mooring sites. The improvements were most significant in the top 100 m, where the internal tide signature was significantly corrected despite no depth-varying variables being assimilated. The study also showed that assimilating 2 km resolution SST observations resulted in more realistic SST wavenumber power spectral density estimates than those without assimilating data; in particular, the scheme reduced the energy at mesoscale length scales and improved the spectral slope towards the smaller submesoscale dynamics. It is concluded that 4D-Var assimilation using only SST improves estimates of upper 100 m ocean dynamics in tropical regions where there is sensitivity to the air–sea fluxes.

Transformation and mixing of North Pacific Water Mass in Sangihe-Talaud in August 2019

Along the pathway, ITF water is considered to be transformed due to strong diapycnal mixing. This study aims to describe the structure of ITF water and to estimate turbulent mixing. The number of 6 CTD casts and 9 repeated CTD “yoyo” measurements were obtained from the “Years of Maritime Continent” YMC cruise (a joint cruise between BPPT/IPB/UNUD-Univ. Tokyo/JAMSTEC) and onboard R.V. Baruna Jaya IV in August 2019. The CTD datasets are processed with SBE Data Processing and analyzed for water mass composition, as well as turbulent mixing with Thorpe method. The results showed that thermocline water of NPSW with S-max, and intermediate water of NPIW with S-min from North Pacific origin are dominant. Transformation of NPSW and NPIW along their pathway can be identified from decreasing S-max of NPSW and increasing S-min of NPIW. Estimates of ϵ and Kρ are O(10−5) m2s−2 and 10−2 m2 s−1, respectively. High mixing occur also in the interior layer with the e and the Kp O(10−6) m2s−2 and O(10−1) m2 s−1, respectively. This is related to barotropic tidal activity that interacts with the bottom topography where there are many sills, causing the formation of strong baroclinic tides.

Feedbacks between hydrodynamics and cold-water coral mound development

Cold-water corals rely on currents to transport food towards them and when external conditions are favourable, they can form coral mounds. These structures, which can be over 300 m high, influence the hydrodynamics around the reefs that grow on the mounds, which feeds back to affect coral- and therefore mound-growth. We investigated these feedbacks at the Logachev coral mound province, by running simulations with a 3D hydrodynamic model (Roms-Agrif), using different seafloor bathymetries that represent consecutive stages of mound development. Simulations ranged from a fully smoothened bathymetry without mounds, to a coral mound (Haas mound) at 1.5 times its current size. The effect of mound height on coral growth was investigated by looking at the baroclinic (internal) tide, turbulent energy dissipation, vertical velocities, and horizontal bottom currents. The simulations suggest that with increasing mound height horizontal velocities increase, while turbulent energy dissipation and vertical velocities around the mound foot decrease. This supposedly limits coral growth at the mound foot and hence lateral mound extension in later stages of development. An increase in turbulent energy dissipation and vertical velocities on the mound top and upper flanks, indicates vertical mound growth at all subsequent stages. Our findings of continued vertical mound growth provide an explanation for recently published data on benthic cover from a transect over Haas mound, that show a dominance of live corals on the mound top. We find areas of increased energy conversion rates from the barotropic (surface) to the baroclinic tide on the bathymetry where we artificially eliminated the mounds from (i.e. smoothened bathymetry). Interestingly, these areas overlap with the region where coral mounds are located at present. So, the baroclinic tide is likely an important mechanism in the process of coral mound establishment. Given the relative ease with which the energy conversion rate from the barotropic to the baroclinic tide can be deduced from hydrodynamic model simulations, our results provide opportunities to investigate where coral mounds may be initiated worldwide.

The Impact of Fortnightly Stratification Variability on the Generation of Baroclinic Tides in the Luzon Strait

The impact of fortnightly stratification variability induced by tide–topography interaction on the generation of baroclinic tides in the Luzon Strait is numerically investigated using the MIT general circulation model. The simulation shows that advection of buoyancy by baroclinic flows results in daily oscillations and a fortnightly variability in the stratification at the main generation site of internal tides. As the stratification for the whole Luzon Strait is periodically redistributed by these flows, the energy analysis indicates that the fortnightly stratification variability can significantly affect the energy transfer between barotropic and baroclinic tides. Due to this effect on stratification variability by the baroclinic flows, the phases of baroclinic potential energy variability do not match the phase of barotropic forcing in the fortnight time scale. This phenomenon leads to the fact that the maximum baroclinic tides may not be generated during the maximum barotropic forcing. Therefore, a significant impact of stratification variability on the generation of baroclinic tides is demonstrated by our modeling study, which suggests a lead–lag relation between barotropic tidal forcing and maximum baroclinic response in the Luzon Strait within the fortnightly tidal cycle.

Model Simulation of M2 Internal Tide at the Mariana Double Ridges

In this study, M2 tidal energy and tide-induced mixing in the Mariana double ridges are investigated with a high-resolution three-dimensional non-hydrostatic numerical model and baroclinic energy budget analysis. The interference effect of the double ridges on the internal tide in the Mariana is examined by omitting either the eastern or the western ridge. Our results show that the baroclinic velocity on the sides of the interior facing slopes of the double ridges is larger than that on the other sides. In the double ridges, high values of dissipation reaching O (10−6 W kg−1) are accompanied by diapycnal diffusivity reaching O (10−1 m2 s−1), which is several orders of magnitude higher than the mixing of the open ocean. The bottom diapycnal mixing in the inner region between the two ridges is one order of magnitude larger than the mixing outside the ridges, indicating the important role of the interference of the double-ridge topography on the mixing in the Mariana Arc. Omitting either the eastern or the western ridge would have a significant impact on tide current, baroclinic energy flux and dissipation, and diapycnal mixing. The internal tide conversion, dissipation, and flux divergence are amplified by the double ridge topography, especially in the central part of the double ridges. Through energy budgets analysis, we conclude that the eastern ridge is the main source of the baroclinic tide in the Mariana double ridges.

On Tidal Modulation of the Evolution of Internal Solitary-Like Waves Passing Through a Critical Point

AbstractInternal solitary-like waves (ISWs) evolve considerably when passing through a critical point separating the deep water where ISWs are waves of depression and shallower water where they are waves of elevation. The location of the critical point is determined by the background current and stratification. In this study, we investigate the influence of tidal currents on the cross-shelf movement of the critical point and elucidate the underlying processes via fully nonlinear numerical simulations. Our simulations reveal phase-locked tidal variations of the critical point, which are mainly attributed to stratification fluctuations that are modulated by the combined effects of cross-shelf barotropic tidal currents and locally generated baroclinic tides. The barotropic tidal currents drive isopycnal displacements as they flow over the slope, and as this occurs baroclinic tides are generated, modulating the stratification and inducing sheared currents. This results in a cross-shelf movement of the critical point, which moves onshore (offshore) when the pycnocline is elevated (depressed) by the flood (ebb) tide. Our idealized numerical simulations for the study region in the South China Sea suggest that the cross-shelf movement of the critical point reaches to O(10) km within a tidal cycle. This distance depends on the strength of tidal currents, stratification, and bathymetry. Because of tidal currents, ISWs of depression may undergo a complex evolution even in a stratification with a shallow pycnocline. For the stratification with a deep pycnocline, the critical point may be at a location deep enough so that its tidal movement becomes insignificant.