Variability Of Internal Tides Research Articles

Satellite Estimates of Mode-1 M2 Internal Tides Using Nonrepeat Altimetry Missions

Abstract Previous satellite estimates of internal tides are usually based on 25 years of sea surface height (SSH) data from 1993 to 2017 measured by exact-repeat (ER) altimetry missions. In this study, new satellite estimates of internal tides are based on 8 years of SSH data from 2011 to 2018 measured mainly by nonrepeat (NR) altimetry missions. The two datasets are labeled ER25yr and NR8yr, respectively. NR8yr has advantages over ER25yr in observing internal tides because of its shorter time coverage and denser ground tracks. Mode-1 M2 internal tides are mapped from both datasets following the same procedure that consists of two rounds of plane wave analysis with a spatial bandpass filter in between. The denser ground tracks of NR8yr make it possible to examine the impact of window size in the first-round plane wave analysis. Internal tides mapped using six different windows ranging from 40 to 160 km have almost the same results on global average, but smaller windows can better resolve isolated generation sources. The impact of time coverage is studied by comparing NR8yr160km and ER25yr160km, which are mapped using 160-km windows in the first-round plane wave analysis. They are evaluated using independent satellite altimetry data in 2020. NR8yr160km has larger model variance and can cause larger variance reduction, suggesting that NR8yr160km is a better model than ER25yr160km. Their global energies are 43.6 and 33.6 PJ, respectively, with a difference of 10 PJ. Their energy difference is a function of location. Significance Statement Our understanding of internal tides is mainly limited by the scarcity of field measurements with sufficient spatiotemporal resolution. Satellite altimetry offers a unique technique for observing and predicting internal tides on a global scale. Previous satellite observations of internal tides are mainly based on 25 years of data from exact-repeat altimetry missions. This paper demonstrates that internal tides can be mapped using 8 years of data made by nonrepeat altimetry missions. The new dataset has shorter time coverage and denser ground tracks; therefore, one can examine the impact of window size and time coverage on mapping internal tides from satellite altimetry. A comparison of models mapped from the two datasets sheds new light on the spatiotemporal variability of internal tides.

Interannual variability of internal tides in the Andaman Sea: an effect of Indian Ocean Dipole

A marginal sea in the north eastern Indian Ocean, the Andaman Sea, has been known for the presence of high-amplitude internal waves since the nineteenth century. In this study, we explored the interannual variations of the internal wave activity in this complex region. We found that the Dipole Mode Index, which represents the Indian Ocean Dipole (IOD), influences the circulation in the Andaman Sea, which in turn impacts its density stratification on interannual scales. Ocean Reanalysis System 5 data (1993–2018) is used to see an increasing trend in the sub-surface stratification, whereas it showed a decreasing trend in the near-surface waters. Numerical model simulations carried out from 2009 to 2018 have shown that the interannual variability in the generation of semidiurnal internal tides is governed by distinct parameters (tidal forcing and stratification) at different sites in different months. Enhanced upwelling (downwelling) is observed during positive (negative) IOD events. Sensitivity experiments conducted between extreme IOD events (2006 and 2016) revealed an increase in internal tide generation from positive IOD to negative IOD. Furthermore, a sharp decrease in local baroclinic dissipation is seen during negative IOD, increasing baroclinic flux into the Andaman Sea. An increase in the strength of positive IOD could lead to enhanced diapycnal mixing due to strong local dissipation, whereas an increase in the intensity of negative IOD could result in amplified propagation of internal waves.

Distinct Variability between Semidiurnal and Diurnal Internal Tides at the East China Sea Shelf

Breaking internal tides and induced mixing are critical to shelf dynamics, including heat and mass exchanges. Spatiotemporal variability of internal tides and modulation factors for the southern East China Sea shelf were examined based on a combination of a three-month mooring velocity and satellite altimeter data. Semidiurnal and diurnal internal tides exhibited distinct temporal trends, with the semidiurnal internal tides enhanced by an order of magnitude during the latter half of the record, while the diurnal internal tides followed quasi spring-neap cycles with a generally stable intensity except for two specific periods of strengthening. These internal tides probably originated remotely over the shelf-slope area northeast of Taiwan. Time-varying stratification was the most important factor for the internal tidal magnitude. In addition, varying background currents influenced the diurnal critical latitude band, which explains the slightly enhanced diurnal internal tides during the two periods. Although both semidiurnal and diurnal internal tides were mode-1 dominated, the semidiurnal internal tides were surface intensified while the diurnal tides were bottom intensified. The proportion of higher mode internal tides increased during robust eddy activities. Stronger background vertical shear corresponded to high-frequency events and energy transfers from tidal frequencies to high frequencies associated with turbulent mixing.

Observations of the Low‐Mode Internal Tide and Its Interaction With Mesoscale Flow South of the Azores

AbstractUnderstanding the temporal variability of internal tides plays a crucial role in identifying sources and sinks of energy in the ocean. Using a 10‐month‐long time series from moored instruments inside a tidal beam south of the Azores, the magnitude and the underlying causes of temporal variability in the first two modes of the internal tide energy flux was studied. We analyzed changes of the direction and coherence of the energy flux, its modal structure, and the impact of two eddies. Semidiurnal energy fluxes were further compared with estimates from a 1/10° ocean global circulation model, as well as with fluxes derived from satellite altimetry. All energy fluxes correlate reasonably well in direction, deviations from its fixed phase relation to astronomical forcing, and modal composition while model and satellite underestimate the total energy flux. A pronounced damping of the in situ fluxes coincides with the passing of two eddies. In the presence of a surface‐intensified eddy, the coherent part of the energy flux in the first two modes is lowered by more than 40%, a subsurface eddy coincides with a decrease of the energy flux mainly in the second mode. These observations support the hypothesis that eddy interactions increase the incoherent part of the energy flux and transfer energy from low modes into higher modes, which can lead to increased local dissipation. It remains an open question how much of the energy converted from lower to higher modes results in local dissipation, a crucial part in creating energetically consistent ocean‐climate models.

Spatial and Temporal Variability of Semidiurnal Internal Tide Energetics in the Western Bay of Bengal

The three-dimensional Massachusetts Institute of Technology general circulation model (MITgcm) with time-dependent forcing is implemented to investigate the spatial and temporal variability of internal tides over the eastern coast of India. Numerical simulations are conducted for a number of different months of the year for which in situ observations are available at high temporal resolution. During the months of November–December and March–April, peak spectral estimates of density variability are evident in the semidiurnal frequency band in both observations and simulations, while during August, variability is dominantly in the near-inertial frequency band. Empirical orthogonal functions (EOF) analysis is applied to decompose the baroclinic tidal currents into vertical modes. The results show that about 70–80% of the total variance is in the first mode, while the first three modes represent 90–95% of the total variance in all seasons. Internal tide characteristics such as phase speed, group speed, and wavelength are largest in the postmonsoon season. The magnitude of the computed energy flux is greatest during November, while the direction of propagation of internal tides is almost unchanged through the year. The available potential energy peaks in November (20 kJ m−2) and is smallest in August (14 kJ m−2). Calculation of the energy budget shows that the energy conversion rate from barotropic forcing to internal tides is about 85% in November but only about 46% in August.

Philippine Sea deep water acoustic observations : A new test for acoustic wave propagation through random media models

A primary goal of the Philippine Sea (2010–2011) experiment was to test the ability of models to predict stochastic acoustic fluctuations. The experiment had six different sources transmitting broadband pulses (200 Hz–300 Hz) to a water column spanning vertical line array. The transmission ranges varied from 130 km to 450 km, and took place across a 250 km x 250 km area. The observations are a new opportunity for two reasons. First, the source frequencies are twice that of the previous experiments that used less than 100 Hz. Second, the oceanography is more dynamic than previous sites in the North Pacific with energetic mesoscale and internal tide variability. This talk uses acoustic mode observations to analyze both broadband and narrowband statistics. Narrowband comparisons focus on mode energy estimates at different frequencies, and these results are compared with the Colosi-Morozov model. Broadband observations focus on mode pulse temporal spreads and these results are compared to predictions from a h...

Kilometric Scale Modeling of the North West European Shelf Seas: Exploring the Spatial and Temporal Variability of Internal Tides

AbstractThe North West European Shelf break acts as a barrier to the transport and exchange between the open ocean and the shelf seas. The strong spatial variability of these exchange processes is hard to fully explore using observations, and simulations generally are too coarse to simulate the fine‐scale processes over the whole region. In this context, under the FASTNEt program, a new NEMO configuration of the North West European Shelf and Atlantic Margin at 1/60° (∼1.8 km) has been developed, with the objective to better understand and quantify the seasonal and interannual variability of shelf break processes. The capability of this configuration to reproduce the seasonal cycle in SST, the barotropic tide, and fine‐resolution temperature profiles is assessed against a basin‐scale (1/12°, ∼9 km) configuration and a standard regional configuration (7 km resolution). The seasonal cycle is well reproduced in all configurations though the fine‐resolution allows the simulation of smaller scale processes. Time series of temperature at various locations on the shelf show the presence of internal waves with a strong spatiotemporal variability. Spectral analysis of the internal waves reveals peaks at the diurnal, semidiurnal, inertial, and quarter‐diurnal bands, which are only realistically reproduced in the new configuration. Tidally induced pycnocline variability is diagnosed in the model and shown to vary with the spring neap cycle with mean displacement amplitudes in excess of 2 m for 30% of the stratified domain. With sufficiently fine resolution, internal tides are shown to be generated at numerous bathymetric features resulting in a complex pycnocline displacement superposition pattern.

Observed tidal currents on the continental shelf off the east coast of India

In the present study, we analysed 9-month long data from Acoustic Doppler Current Profilers (ADCP) deployed on the shelf off the east coast of India to study the characteristics of tidal currents in the region. The ADCPs were deployed at about 100–150m depths off Cuddalore (CD, 12.0°N), Ramayapatnam (RM, 15.0°N), Kakinada (KN, 16.3°N) and South of Gopalpur (SG, 18.6°N). Tidal currents in the region are mainly semidiurnal in nature and dominant constituent is M2. Semimajor axes of barotropic tidal ellipses of M2 is about 1.9 cms−1 in the southernmost ADCP location (off CD) and it is about 4.0 cms−1 in the northernmost ADCP location (off SG), which indicate a northward increase of tidal currents. Baroclinic spectra show high energy at tidal frequencies, which suggests the presence of internal tides on the shelf, particularly in the semidiurnal band. Semidiurnal internal tides of about 8–15 cms−1 are observed at different locations and the magnitude is relatively large on the shelf off SG in the northern part of the shelf, which is primarily due to large barotropic forcing on the north. The semidiurnal internal tides are found to be intensified at the bottom, particularly on the shelf off SG and RM. The computed ratio of slope of internal wave characteristics (c) and topographic slope (γ) indicates that large near-critical regions present surrounding the ADCP locations off SG and RM could be the possible reason of bottom intensification at these two locations. EOF analysis shows that observed semidiurnal internal tides are dominated by the first baroclinic mode, where it accounts for about 70–80% of semidiurnal internal tide variability on the shelf off SG and CD, whereas the contribution of the first mode is relatively small (43–50%) on the shelf off KN and RM. Enhanced small-scale vertical shear is observed at the ADCP locations associated with multimode structure of semidiurnal internal tides. Semidiurnal internal tides show a spring-neap variability on the shelf off SG and CD, whereas they are highly incoherent and intermittent on the shelf off RM and KN. Temporal variability of semidiurnal internal tide magnitude on the shelf is mainly due to the change in stratification, which is explained based on monthly climatology data.

Semidiurnal internal tide energy fluxes and their variability in a Global Ocean Model and moored observations

AbstractWe examine the temporal means and variability of the semidiurnal internal tide energy fluxes in 1/25° global simulations of the Hybrid Coordinate Ocean Model (HYCOM) and in a global archive of 79 historical moorings. Low‐frequency flows, a major cause of internal tide variability, have comparable kinetic energies at the mooring sites in model and observations. The computed root‐mean‐square (RMS) variability of the energy flux is large in both model and observations and correlates positively with the time‐averaged flux magnitude. Outside of strong generation regions, the normalized RMS variability (the RMS variability divided by the mean) is nearly independent of the flux magnitudes in the model, and of order 23% or more in both the model and observations. The spatially averaged flux magnitudes in observations and the simulation agree to within a factor of about 1.4 and 2.4 for vertical mode‐1 and mode‐2, respectively. The difference in energy flux computed from the full‐depth model output versus model output subsampled at mooring instrument depths is small. The global historical archive is supplemented with six high‐vertical resolution moorings from the Internal Waves Across the Pacific (IWAP) experiment. The model fluxes agree more closely with the high‐resolution IWAP fluxes than with the historical mooring fluxes. The high variability in internal tide energy fluxes implies that internal tide fluxes computed from short observational records should be regarded as realizations of a highly variable field, not as “means” that are indicative of conditions at the measurement sites over all time.

Long-range propagation and associated variability of internal tides in the South China Sea

The variability of internal tides during their generation and long-range propagation in the South China Sea (SCS) is investigated by driving a high-resolution numerical model. The present study clarifies the notably different processes of generation, propagation and dissipation between diurnal and semidiurnal internal tides. Internal tides in the SCS originate from multiple source sites, among which the Luzon Strait is dominant, and contributes approximately 90% and 74% of the baroclinic energy for M2 and K1, respectively. To the west of the Luzon Strait, local generation of K1 internal tides inside the SCS is more energetic than the M2 tides. Diurnal and semidiurnal internal tides from the Luzon Strait radiate into the SCS in a north-south asymmetry but with different patterns because of the complex two-ridge system. The tidal beams can travel across the deep basin and finally arrive at the Vietnam coast and Nansha Island more than 1000-1500 km away. During propagation, M2 internal tides maintain a southwestward direction, whereas K1 exhibit complicated wave fields because of the superposition of waves from local sources and island scattering effects. After significant dissipation within the Luzon Strait, the remaining energy travels into the SCS and reduces by more than 90% over a distance of ∼1000 km. Inside the SCS, the K1 internal tides with long crests and flat beam angles are more influenced by seafloor topographical features and thus undergo apparent dissipation along the entire path, whereas the prominent dissipation of M2 internal tides only occurs after their arrival at Zhongsha Island. This article is protected by copyright. All rights reserved.

Inter-seasonal variability of internal tides in the western Bay of Bengal

MITgcm is configured in a realistic situation to study the inter-seasonal variability of internal waves (IWs) in the western Bay of Bengal. Monthly climatology of the density fields from WOA09, modified ETOPO2 topography and barotropic tidal forcing are feed to the model for simulating IWs. The preliminary objective of the study is to identify IWs through the stratification and topographic configuration that support IWs. In this context numerical experiments are performed for January, April, July and October representing the winter, pre-monsoon, summer monsoon and post-monsoon, respectively, and analyses are made extensively over the western Bay of Bengal. Mixing caused by density overturning and shear instability is studied at the Gopalpur cross section for all seasons. The density time series are subjected to spectral analysis to find the energy associated with IWs of semi-diurnal frequency. Analysis depicts that the spectral estimates are high in the concave coastline pockets in the southern part and over a broader region on the shelf slope in the northern part. October shows higher estimates in response to highly stratified waters. Several cross sections are analyzed to obtain the spectral energy extent over the shelf slope. It is noticed in October that the peak estimate is within the shelf for shelf angles greater than 0.12° and it falls outside the shelf for shelf angles less than 0.12°.

Structure and Variability of Internal Tides in Luzon Strait

AbstractThe Luzon Strait is the generation region for strong internal tides that radiate westward into the South China Sea and eastward into the western Pacific. Intrusions of the Kuroshio and strong mesoscale variability in the Luzon Strait can influence their generation and propagation. Here, the authors use eight moorings and two numerical models to investigate these relationships by quantifying the coherence of the diurnal and semidiurnal internal tides in the Luzon Strait. This study finds that the level of coherence of internal tide generation, energy, and energy flux is quite variable, depending on the specific location within the Luzon Strait. Large-scale spatial patterns in internal tide pressure and velocity exist across the region, shaped by the bathymetry, mean flow, and stratification. Internal tide coherence is lower (<30%) near large gradients in this pattern (predominantly along the eastern ridge), which are shifted by the variable Kuroshio and mesoscale fields. At other locations within the Luzon Strait, the internal tide is largely coherent (>80%), and simple calculations suggest that remote sources of internal tides could account for these small decreases in coherence. To the west of the Luzon Strait (away from the primary generation regions), the model suggests that diurnal internal tide energy is more coherent than semidiurnal.

M2 baroclinic tide variability modulated by the ocean circulation south of Japan

AbstractWe analyze a concurrent simulation result of the ocean circulation and tidal currents using a data‐assimilative ocean general circulation model covering the Western North Pacific with horizontal resolution of 1/36° to investigate possible interactions between them. Four sites of active M2 internal tide variability in open ocean (hot spots), such as Tokara Strait, Izu Ridge, Luzon Strait, and Ogasawara Ridge, are detected from both the satellite observation and the simulation. Energy cycle analysis of the simulated M2 baroclinic tide indicates two types of the hot spots: dissipation (Tokara Strait and Izu Ridge) and radiation (Luzon Strait and Ogasawara Ridge) dominant sites. Energy conversion from barotropic to baroclinic M2 tides at the hot spots is modulated considerably by the lower‐frequency changes in the density field. Modulation at the two spots (Tokara Strait and Izu Ridge) is affected by the Kuroshio path variation together with the seasonal variation of the shallow thermocline. At the other two sites, influence from changes in the relatively deep stratification through the Kuroshio intrusion into South China Sea (Luzon Strat) and mesoscale eddy activity (Ogasawara Ridge) is dominant in the modulation.

Characteristics and seasonal variability of internal tides in the southern South China Sea

Two sets of current mooring records at the same location are used to investigate the generation and temporal variation of internal tides (ITs) near reefs in the southern South China Sea (SCS). The total records covered more than 900 days. ITs in this region are dominated by mode-1 diurnal motions, which are mainly composed of O1, P1 and K1 constituents. The major axes of both diurnal barotropic and baroclinic tidal ellipses are oriented in the northeast-southwest direction. The ITs over the entire observation period clearly show an approximately 14-day spring-neap cycle. Meanwhile, the diurnal ITs also displayed an evident seasonal change: their energy was higher in summer and winter than in spring and autumn. The modal decomposition of diurnal motion shows that their time-averaged kinetic energy in summer and winter was about 1.2kJ/m2, which was approximately 60% higher than the values in spring and autumn. The same seasonal variation also occurred at the diurnal barotropic tides with prominent O1, P1 and K1 constituents. Because stratification does not show a significant seasonal variation in the southern SCS, the enhanced diurnal ITs in winter and summer can be attributed to the barotropic forcing with the semi-annual cycle caused by the modulation of diurnal P1 and K1.

Internal tides and turbulent mixing observed in the Bussol Strait

Repeated observations with a period of about 24h of hydrography, current velocity, and microstructures were performed at three stations surrounding a seamount in the middle of the Bussol Strait, the deepest and widest one of the Kuril Straits, to reveal spatial and temporal variability of internal tides and associated turbulent mixing. It is found that isopycnal displacements are dominated by diurnal tidal components, which show phase differences (namely, time lags) between the three stations that can be explained by a first mode topographically trapped wave (TTW) propagating clockwise around the seamount. Furthermore, at the station located near the center of the strait where energy dissipation rates are largest, diurnal variations of isopycnals and velocities are amplified toward the ocean bottom, consistent with the vertical structure of the first mode TTW. At that station, vigorous turbulent mixing with the energy dissipation rate exceeding 10-6m2s-3 and diapycnal diffusivity exceeding 10-1m2s-1 was observed in deep layers when the diurnal tidal current consisting of the first mode TTW flows from the Okhotsk Sea to the North Pacific, enhancing the mean current. These spatial and temporal variation patterns are confirmed to be reproduced by a previous numerical model successfully for the isopycnals and velocities, and partially for the turbulent mixing. The total energy dissipation rate is, however, by up to a factor of 3–10 smaller than predicted by the numerical model although the observations were performed during spring tides, suggesting that the actual diapycnal mixing is overall weaker than the previous model estimate and/or that extremely strong mixing occurs within highly localized areas.

Seasonal variability and north–south asymmetry of internal tides in the deep basin west of the Luzon Strait

Temporal and spatial variability of internal tides (ITs) in the deep basin west of the Luzon Strait were examined, based on two 9-month mooring current observations from autumn 2008 to summer 2009. The baroclinic current measurements exhibited north–south asymmetry and temporal variation. At the southern site (19.5°N), the kinetic energy of diurnal IT was dominant over that of semidiurnal IT by a factor of 3 to 4, whereas at the northern site (~21°N), the diurnal energy was comparable to or a little larger than semidiurnal energy. At both sites, the motions of semidiurnal IT were recognized as seasonally invariant, while diurnal IT showed notable seasonal variation, namely, stronger in summer and winter but weaker in spring and autumn. The seasonal variation was mainly modulated by the astronomical tides in the Luzon Strait rather than by the seasonal thermocline. Both diurnal and semidiurnal ITs contained stronger coherent signals than incoherent counterparts at two sites, but the IT at the southern site was more coherent than that at the northern site. Diurnal IT is more coherent than the semidiurnal IT at each site. The spatial–temporal variations were probably due to IT interferences from various sources within the Luzon Strait and modulation by varying background conditions.

The variability of internal tides in the Northern South China Sea

An array of three bottom-mounted ADCP moorings was deployed on the prevailing propagation path of strong internal tides for nearly 1 year across the continental slope in the northern South China Sea. These velocity measurements are used to study the intra-annual variability of diurnal and semidiurnal internal tidal energy in the region. A numerical model, the Luzon Strait Ocean Nowcast/Forecast System developed at the U.S. Naval Research Laboratory that covers the northern South China Sea and the Kuroshio, is used to interpret the observed variation of internal tidal energy on the Dongsha slope. Internal tides are generated primarily at the two submarine ridges in the Luzon Strait. At the western ridge generation site, the westward energy flux of the diurnal internal tide is sensitive to the stratification and isopycnal slope associated with the Kuroshio. The horizontal shear at the Kuroshio front does not modify the propagation path of either diurnal or semidiurnal tides because the relative vorticity of the Kuroshio in Luzon Strait is not strong enough to increase the effective inertial frequency to the intrinsic frequency of the internal tides. The variation of internal tidal energy on the continental slope and Dongsha plateau can be attributed to the variation in tidal beam propagation in the northern South China Sea.

Variability of internal tides and near‐inertial waves on the continental slope of the northwestern South China Sea

AbstractStructure and variability of internal tides (IT) and near‐inertial waves (NIW) on the continental slope of the northwestern South China Sea were investigated, based on 9‐month moored current observations from autumn to early summer in 2008 and 2009. The diurnal IT kinetic energy, dominant over that of semidiurnal tides, is found to exhibit apparent seasonal variability—strongest in summer and weakest in winter—whereas the semidiurnal variance remained nearly uniform throughout the observation period. Moreover, the diurnal IT were more coherent (i.e., phase‐locked to the astronomical forcing) than the semidiurnal constituents. Coherent diurnal variance accounts for about 40% of diurnal motions, but semidiurnal tides contain a much smaller fraction (10%) of coherent motions. Further analysis demonstrates that the diurnal IT are dominated by the first mode, whereas the semidiurnal tides show a variable multimodal structure: the second mode is dominant in summer and comparable to the first mode in spring and autumn, but the first mode predominates in winter. Multimodal semidiurnal IT are more influenced by varying stratification structures and background currents and thus exhibit highly incoherent and intermittent behavior, which may wash out seasonal variability during their long propagation from the generation source. The observed NIW are seasonally independent and comparable to the semidiurnal motions. During the passage of Typhoon Hagupit, however, the NIW became the most energetic component of the inertia‐gravity waveband motions. NIW energy and shear were significantly enhanced and exceeded tidal counterparts by a factor of 2 to 3 in the upper layer.

Temporal variability of internal tides in the northeast South China Sea

The northeast South China Sea is perhaps the largest internal tide energy source in the world. The temporal variability of internal tides was investigated on the basis of 8‐month moored acoustic Doppler current profiler observations on the continental slope at the Dongsha Plateau. The daily amplitude and phase of diurnal and semidiurnal internal tides were determined from complex demodulation, and the dominant spatial and temporal patterns were extracted with empirical orthogonal function (EOF) analysis. The EOF modal structures showed good agreement with the normal mode solution, although vertical phase propagations were apparent. The first semidiurnal EOF mode corresponded to the first normal mode, and the first two diurnal EOF modes corresponded to the second and third normal modes, respectively. The modal structure and energy flux also were consistent with previous observations near the shelf break from the Asian Seas International Acoustic Experiment. On the other hand, the amplitudes of diurnal and semidiurnal EOF modes both indicated large irregular fortnightly variations that were not phase locked (incoherent) with astronomical forcing. The study highlighted the importance of incoherent internal tidal motion, which accounted for about three fourths of the observed tidal energy.

Modeling Semidiurnal Internal Tide Variability in the Southern California Bight

Abstract The Regional Oceanic Modeling System (ROMS) is applied in a nested configuration with realistic forcing to the Southern California Bight (SCB) to analyze the variability in semidiurnal internal wave generation and propagation. The SCB has a complex topography with supercritical slopes that generate linear internal waves at the forcing frequency. The model predicts the observed barotropic and baroclinic tides reasonably well, although the observed baroclinic tides feature slightly larger amplitudes. The strongest semidiurnal barotropic to baroclinic energy conversion occurs on a steep sill slope of the 1900-m-deep Santa Cruz Basin. This causes a forced, near-resonant, semidiurnal Poincaré wave that rotates clockwise in the basin and is of the first mode along the radial, azimuthal, and vertical directions. The associated tidal-mean, depth-integrated energy fluxes and isotherm oscillation amplitudes in the basin reach maximum values of about 5 kW m−1 and 100 m and are strongly modulated by the spring–neap cycle. Most energy is locally dissipated, and only 10% escapes the basin. The baroclinic energy in the remaining basins is orders of magnitudes smaller. High-resolution coastal models are important in locating overlooked mixing hotspots such as the Santa Cruz Basin. These mixing hotspots may be important for ocean mixing and the overturning circulation.