Summertime M2 Internal Tides in the Northern Yellow Sea

The summertime M2 internal tide in the northern Yellow Sea is investigated with moored current meter observations and numerical current model results. The hydrodynamic model, which is implemented from the Regional Ocean Model System (ROMS) with 1 km horizontal resolution, is capable of resolving the internal tidal dynamics and the results are validated in a comparison with observations. The vertical pattern of a mode-1, semi-diurnal internal tide is clearly captured by the moored ADCP as well as in the simulation results. Spectral analysis of the current results shows that the M2 internal tide is dominant in the northern Yellow Sea. Analysis of the major M2 internal tide energetics demonstrated a complex spatial pattern. The tidal mixing front along the Korean coast and on the northern shelf provided proper conditions for the generation and propagation of the internal tides. Near the Changshan islands, the M2 internal tide is mainly generated near the local topography anomalies with relatively strong current magnitude, equal to about 30% of the barotropic component, thus modifying the local current field. These local internal tides are short-lived phenomena rapidly being dissipated along the propagation pathway, restricting their influence within a few kilometers around the islands.

Effects of an along-shelf current on the generation of internal tides near the critical latitude

The effects of along-shelf barotropic geostrophic currents on internal wave generation by the $K_1$ tide interacting with a shelf at near-critical latitudes are investigated. The horizontal shear of the background current results in a spatially varying effective Coriolis frequency which modifies the slope criticality and potentially creates blocking regions where freely propagating internal tides cannot exist. This paper is focused on the barotropic to baroclinic energy conversion rate, which is affected by a combination of three factors: slope criticality, size and location of the blocking region where the conversion rate is extremely small and the internal tide (IT) beam patterns. All of these are sensitive to the current parameters. In our parameter space, the current can increase the conversion rate up to 10 times.

Background stratification impacts on internal tide generation and abyssal propagation in the western equatorial Atlantic and the Bay of Biscay

The forthcoming SWOT altimetric missions aim to resolve the mesoscale with an unprecedented spatial resolution and swath. However, high-frequency processes, such as tides, are undersampled in time and aliased onto lower frequencies, so they need to be corrected properly. Unlike barotropic tides, internal tides (ITs) are not completely stationary and have significant temporal variability due to their interactions with the ocean circulation and the stratification variability. Stratification changes impact both the generation and the propagation of ITs. The present study proposes a methodology to quantify the impacts of background stratification using a clustering method for the classification of a broad range of stratification and idealized modeling of ITs in the frequency domain. The methodology is successfully tested in the western equatorial Atlantic and in the Bay of Biscay. For the western equatorial Atlantic, a single pycnocline is observed and only the two first vertical modes of ITs have significant amplitudes. With no variation in the stratification intensity, the variation in the depth of this single pycnocline linearly impacts the elevation amplitude, energy fluxes and surface wavelength of the two modes. In the Bay of Biscay, there is a permanent deep pycnocline and secondary seasonal pycnoclines near the surface. No proxy have been found to describe the changes in ITs, so a seasonal climatology is explored. The seasonality of the stratification strongly affects the elevation amplitudes as well as the energy fluxes of modes 1, 2 and 3. The distribution of the modes vary with the background stratification, changing the horizontal scales of the ITs.

Internal Solitary Waves in the Andaman Sea Revealed by Long-term Mooring Observations

The complex behaviors of internal solitary waves (ISWs) in the Andaman Sea were revealed using data collected over nearly 22-month-long observation period completed by two moorings. Emanating from the submarine ridges northwest of Sumatra Island and south of Car Nicobar, two types of ISWs, referred to as S- and C-ISWs, respectively, were identified in the measurements, and S-ISWs were generally found to be stronger than C-ISWs. The observed S- and C-ISWs frequently appeared as multi-wave packets, accounting for 87% and 43% of their observed episodes, respectively. The simultaneous measurements collected by the two moorings featured evident variability along the S-ISW crests, with the average wave amplitude in the northern portion being 36% larger than that in the southern portion. The analyses of the arrival times revealed that the S-ISWs in the northern portion occurred more frequently and arrived more irregularly than those in the southern portion. Moreover, the temporal variability of ISWs drastically differed on monthly and seasonal time scales, characterized by relatively stronger S-ISWs in spring and autumn. Over interannual time scale, the temporal variations in ISWs were generally subtle. The monthly-to-annual variations of ISWs could be mostly explained by the variability in stratification, which could be modulated by the monsoons, the winds in equatorial Indian Ocean and the mesoscale eddies in Andaman Sea. From careful analyses preformed based on the long-term measurements, we argued that the observed ISWs were likely generated via internal tide release mechanism and their generation processes were obviously modulated by background circulations.

Simultaneous estimation of Ocean mesoscale and coherent internal tide Sea Surface Height signatures from the global Altimetry record

This study proposes an approach to estimate the Ocean Sea Surface Height signature of coherent internal tidesfrom 25 years of along-track altimetry record, with a single inversion over time, resolving both internal tide contributions andmesoscale eddy variability. The inversion is performed through reduced-order basis with conjugate gradient resolution. Theparticularity of this approach is to mitigate the potential aliasing effects between mesoscales and internal tide estimation fromthe uneven altimetry sampling (observing the sum of these components) by accounting of their statistics simultaneously, while other methods generally use a prior for mesoscales. The four major tidal components are considered (M2,K1,S2,O1) over theperiod 1992–2017 on a global configuration. From the solution, we use altimetry data after 2017 for an independent validation,to evaluate the benefits of the simultaneous inversion, and also to compare the skills with an existing model.

Effects of Stratification on Shoaling Internal Tidal Bores

Idealized simulations of a shoaling internal tide on a gently sloping, linear shelf provide a tool to investigate systematically the effects of stratification strength, vertical structure, and internal wave amplitude on internal tidal bores. Simulations that prescribe a range of uniform or variable stratifications and wave amplitudes demonstrate a variety of internal tidal bores characterized by shoreward propagating horizontal density fronts with associated overturning circulations. Qualitatively, we observe three classes of solution: 1) bores, 2) bores with trailing wave trains, and 3) no bores. Very strong stratification (small wave) or very weak stratification (large wave) inhibits bore formation. Bores exist in an intermediate zone of stratification strength and wave amplitude. Within this intermediate zone, wave trains can trail bores if the stratification is relatively weak or wave amplitude large. We observe three types of bore that arise dependent on the vertical structure of stratification and wave amplitude: 1) a ‘backward’ downwelling front (near uniform stratification, small to intermediate waves), 2) a ‘forward’ upwelling front (strong pycnocline, small to large waves), and 3) a ‘double’ bore with leading up and trailing downwelling front (intermediate pycnocline, intermediate to large waves). Visualization of local flow structures explores the evolution of each of these bore-types. A frontogenetic diagnostic framework elucidates the previously undiscussed, yet, universal role of vertical straining of a stratified fluid that initiates formation of bores. Bores with wave trains exhibit strong non-hydrostatic dynamics. The results of this study suggest that mid-to-outer shelf measurements of stratification and cross-shore flow can serve as proxies to indicate the class of bore further inshore.

Seasonal mode-1 M2 internal tides from satellite altimetry

The seasonal variability of mode-1 M2 internal tides is investigated using 25 years of multi-satellite altimeter data from 1992–2017. Four seasonal internal tide models are constructed using seasonally-subsetted altimeter data and World Ocean Atlas seasonal climatologies. This work is made possible by a newly-developed mapping procedure that can significantly suppress model errors. Seasonal-mean and seasonally-variable internal tide models are derived from the four seasonal models. All the models are inter-compared and evaluated using independent CryoSat-2 data. The seasonal-mean model is overall the best model because averaging the four seasonal models further reduces model errors. The seasonally-variable models are better in the tropical zone, where large seasonal signals may overcome model errors. Each seasonal model works best in its own season and worst in its opposite season. These internal tide models reveal that mode-1 M2 internal tides are subject to significant seasonal variability and their seasonal variations are a function of location. Large seasonal variations dominantly occur in the tropical zone, where the World Ocean Atlas climatology shows strong seasonal variations in ocean stratification. Seasonal phase variations are obtained from the directionally-decomposed internal tide components. They are dominantly ±60° at the equator and up to ±120° in the central Arabian Sea. Incoherence caused by seasonal phase variations is usually <10%, but may be up to 40–50% in the tropical zone.