Jay F. Shriver

Vertical motion in the East China Sea is examined using a 1/12.5° regional HYbrid Coordinate Ocean Model simulation from 2018 to 2022. Here we present an area of persistent downwelling ∼200 km East‐Northeast of Taiwan where the Kuroshio flows along the continental slope. Analysis of daily mean vertical velocity () shows a 5,000 km ² zone of strong and persistent downwelling where m/day is observed over 95% of the time in a 5‐year period at depths around 100 m. XBT data from the World Ocean Database corroborate the authenticity of this feature. The fixed location of two downwelling cores relative to sub‐surface ridges in along‐slope isobaths indicate that the permanent downwelling zone (PDZ) is initiated by inertial flow over undulating topography. As the current transitions from cross‐slope to along‐slope orientation over steep downslope gradients, isopycnals are depressed and thermal‐wind balance is established. The resulting clockwise‐offshore rotation of the horizontal velocity—a localized topographic beta spiral—reinforces the downwelling while the consequent Ekman suction at the bottom boundary layer sustains it and extends it upward into the ocean interior. A seasonal variation reduces the intensity of the downwelling by 27% during the winter monsoon but does not trigger upwelling despite the winds becoming upwelling‐favorable. Comparison of wind stress curl to downwelling strength shows that local winds only modulate (not drive) the PDZ; the primary seasonal control is the cyclic on‐shelf/off‐shelf shifting of the Kuroshio's core.

Accurately predicting surface and internal tides in 3D global ocean simulations is crucial for understanding ocean mixing, acoustics, and tracer dispersal. Despite advances, numerical model-derived tidal estimates and energy budgets still fall short of those obtained from altimetry-constrained tidal models. This study investigates the effects of nudging barotropic sea surface height towards the Ocean Topography Experiment/Poseidon (TPXO) tidal inversion model on tidal sea level and energetics in 1/12.5◦ global Hybrid Coordinate Ocean Model (HYCOM) simulations. We explore two nudging scenarios: (1) Deep Nudging below 500 m seafloor depth and (2) Shelf-toe Nudging along a 3◦ wide strip offshore the continental shelf. Our objectives are to (1) reduce barotropic tidal errors and (2) improve internal tide (IT) predictability and energetics, while accurately representing the subtidal and mesoscale flows. The Deep Nudging simulation shows significant improvements in surface tides, particularly for M2, with a root-mean-square error of 0.6 cm, representing a 77% reduction relative to our best non-assimilative simulation. A 48% reduction in error (to 0.4 cm) is also observed for K1. However, this method adversely affects barotropic subtidal waves and mesoscale flows. In contrast, the Shelf-toe Nudging method preserves subtidal flows while yielding smaller surface-tide error reductions. The largest regional improvements in the M2 tide occur in the Atlantic Ocean relative to historical HYCOM simulations. Area-averaged IT amplitudes remain broadly consistent with estimates from the empirical internal tide model HRET22 in some hotspot regions. However, an increase in M2 surface tidal amplitude in the northwest Atlantic increases barotropic-to-baroclinic energy conversion, thereby increasing IT amplitudes in this region. While nudging causes small regional changes in M2 internal tide variance, the global, depth-integrated modal energy estimates remain consistent with historical HYCOM simulation. Finally, we implement a correction for thermobaric instabilities, which have adversely affected historical HYCOM simulations.

While geostrophy remains the simplest and most practical balance to extract velocity information from sea‐surface height anomaly (SSHa), confusions remain within the oceanographic community to what extent this balance can be applied to altimetric observations with the launch of the Surface Water and Ocean Topography (SWOT) satellite. Given the limited temporal resolution of SWOT, many studies have resorted to claiming that the spatially filtered SSHa fields correspond to the geostrophic component. This introduces the ambiguity of which spatial scale to choose. Here, we build upon the recent developments in internal tide (IT) corrections (Yadidya, Arbic, et al., 2025, https://doi.org/10.1029/2025ea004511 ) and apply a dynamic mode decomposition (DMD)‐based method introduced by Lapo, Ichinaga, and Kutz (2025, https://doi.org/10.1073/pnas.2415786122 ) to robustly extract the geostrophic component associated with sub‐inertial frequencies from the SWOT one‐day‐repeat orbit; we distribute the global data set as a public good. We provide the joint probability density function (PDF) of vorticity and strain, and spectra of SSHa at a few cross‐over regions.

Small-scale oceanic eddies and filaments mediate the vertical exchange of heat and carbon within the global ocean. The Surface Water and Ocean Topography (SWOT) mission resolves these features through wide-swath interferometry, but internal tides often mask these observations. Non–phase-locked internal tides present special difficulty because they vary with the evolving ocean background. We show that this chaotic variability can be predicted. We use a data-assimilative ocean forecast model to resolve the mesoscale environment and separate tidal signals from the broader circulation. The model captures the organized structure of these incoherent waves in the independent SWOT measurements. Correcting for the total (phase-locked plus non–phase-locked) internal tide signal reduces the error by 59% relative to current empirical methods. These findings show that non–phase-locked internal tides become predictable when the evolving ocean state is explicitly modeled. Our framework allows wide-swath altimetry to more accurately map climatically important fine-scale dynamics in the ocean.

Internal tides are sub‐surface inertia‐gravity waves that generate significant sea surface height signals detectable with satellite altimetry. The Surface Water and Ocean Topography (SWOT) mission provides an exciting opportunity to characterize these signals with unprecedented spatial detail. Separating tidal and non‐tidal oceanic signals is necessary for achieving the SWOT mission's objective of advancing our understanding of mesoscale and submesoscale processes. In this study, we evaluate the performance of a data‐assimilative HYbrid Coordinate Ocean Model (HYCOM) forecast system in resolving both phase‐locked and non‐phase‐locked internal tides during the SWOT Cal/Val period. We compare HYCOM's effectiveness to the High‐Resolution Empirical Tide model (HRET22), which is currently used for internal tide corrections but only accounts for the phase‐locked component. HYCOM achieves an average of 5% greater reduction in phase‐locked internal tide variance and a 24.6% greater total variance reduction compared to HRET22 by also accounting for non‐phase‐locked internal tides. At the frequency, HYCOM reduces up to 73% of total internal tide variance in SWOT observations, including substantial contributions from the non‐phase‐locked component. Despite these advances, persistent residuals remain in energetic, topographically complex areas, pointing to the continued need for improved modeling and data assimilation. These findings demonstrate the crucial role of forecast models in advancing internal tide mapping and significantly improving altimetric data correction in the era of SWOT oceanography.

Internal tides are sub-surface inertia-gravity waves that generate significant sea surface height signals detectable with satellite altimetry. The Surface Water and Ocean Topography (SWOT) mission provides an exciting opportunity to characterize these signals with unprecedented spatial detail. Separating tidal and non-tidal oceanic signals is necessary for achieving the SWOT mission’s objective of advancing our understanding of mesoscale and submesoscale processes. In this study, we evaluate the performance of a data-assimilative HYbrid Coordinate Ocean Model (HYCOM) forecast system in resolving both phase-locked and non-phase-locked internal tides during the SWOT Cal/Val period. We compare HYCOM's effectiveness to the High-Resolution Empirical Tide model (HRET22), which is currently used for internal tide corrections but only accounts for the phase-locked component. HYCOM achieves an average of 5% greater reduction in phase-locked internal tide variance and a 24.6% greater total variance reduction compared to HRET22 by also accounting for non-phase-locked internal tides. At the M2 frequency, HYCOM reduces up to 73% of total internal tide variance in SWOT observations, including substantial contributions from the non-phase-locked component. Despite these advances, persistent residuals remain in energetic, topographically complex areas, pointing to the continued need for improved modeling and data assimilation. These findings demonstrate the crucial role of forecast models in advancing internal tide mapping and significantly improving altimetric data correction in the era of SWOT oceanography.

The decomposition of oceanic flow into its geostrophically balanced and unbalanced motions carries theoretical and practical significance for the oceanographic community. These two motions have distinct dynamical characteristics and affect the transport of tracers differently from one another. The launch of the Surface Water and Ocean Topography (SWOT) satellite provides a prime opportunity to diagnose the surface balanced and unbalanced motions on a global scale at an unprecedented spatial resolution. Here, we apply dynamic‐mode decomposition (DMD), a linear‐algebraic data‐driven method, to tidally‐forced idealized and realistic numerical simulations at submesoscale‐permitting resolution and one‐day‐repeat SWOT observations of sea‐surface height (SSH) in the Gulf Stream downstream of Cape Hatteras, a region commonly referred to as the separated Gulf Stream. DMD is able to separate out the spatial modes associated with sub‐inertial periods from super‐inertial periods. The sub‐inertial modes of DMD can be used to extract geostrophically balanced motions from SSH fields, which have an imprint of internal gravity waves, so long as the data extends long enough in time. We utilize the statistical relation between relative vorticity and strain rate as the metric to gauge the extraction of geostrophy.

In this study, we diagnose the spatial variability in the energetics of tidally generated diurnal, semidiurnal, and supertidal ( cycles per day) internal wave vertical modes (up to mode 6) in a 30‐day forward global ocean model simulation with a 4‐km grid spacing and 41 layers. The simulation is forced with realistic tides and atmospheric fields. Diurnal modes are resolved beyond mode 6, semidiurnal modes are resolved up to mode 4, and supertidal modes are resolved up to mode 2, in agreement with a canonical horizontal resolution criterion. The meridional trends in the kinetic to available potential energy ratios of these resolved modes agree with an internal wave consistency relation. The supertidal band is dominated by the higher harmonics of the diurnal and semidiurnal tides. Its higher harmonic energy projects on the internal wave dispersion curves in frequency‐wavenumber spectra and is captured mostly by the terdiurnal and quarterdiurnal mode‐1 waves. Terdiurnal modes are mostly generated in the west Pacific, where diurnal internal tides are strong. In contrast, quarterdiurnal modes occur at all longitudes near strong semidiurnal generation sites. The globally integrated energy in the supertidal band is about one order of magnitude smaller than the energy in the tidal band. The supertidal energy as a fraction of the tidal energy is elevated along semidiurnal internal wave beams in the tropics. We attribute this to near‐resonant interactions between tidal modes of the same mode number.

In this study, we diagnose the spatial variability in the energetics of tidally generated diurnal, semidiurnal, and supertidal internal wave vertical modes (up to mode 6) in a 30-day forward global ocean model simulation with a 4-km grid spacing and 41 layers. This simulation is forced with realistic tides and atmospheric fields. While diurnal modes are mostly generated in the western Pacific, semidiurnal modes are more ubiquitous in the global ocean. Supertidal modes are mostly generated at low latitudes. For all frequency bands, most of the energy is in mode 1. Diurnal modes are fully resolved beyond mode 6, semidiurnal modes are fully resolved up to mode 4, and supertidal modes are fully resolved up to mode 2, in agreement with a canonical horizontal resolution criterion. The meridional trends in the kinetic to available potential energy ratios of these resolved modes agree with an internal wave consistency relation. The supertidal band is dominated by the higher harmonics of the diurnal and semdiurnal tides, of which the terdiurnal and quarterdiurnal mode-1 waves have the most energy. Terdiurnal modes are mostly generated in the west Pacific, where diurnal internal tides are strong. In contrast, quarterdiurnal modes occur at all longitudes near strong semidiurnal generation sites. The consistency relation and frequency-wavenumber spectra show that the resolved supertidal modes are dispersive internal waves. These waves are likely generated by near-resonant interactions between tidal modes of the same mode number, which are enhanced in the tropics.

This paper examines spectra of horizontal kinetic energy (HKE) in the surface and sub‐surface ocean, with an emphasis on internal gravity wave (IGW) motions, in global high‐resolution ocean simulations. Horizontal wavenumber‐frequency spectra of surface HKE are computed over seven oceanic regions from two global simulations of the HYbrid Coordinate Ocean Model (HYCOM) and three global simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). In regions with high IGW activity, high surface HKE variance in the horizontal wavenumber‐frequency spectra is aligned along IGW linear dispersion curves. For both HYCOM and MITgcm, and in almost all regions, finer horizontal resolution yields more energetic supertidal IGW continuum spectra. The ratio of high‐horizontal‐wavenumber variance in semi‐diurnal and supertidal motions relative to lower‐frequency motions, a quantity of great interest for swath altimetry, depends on the model employed and the horizontal resolution within the model, implying that quantitative predictions of the partition between low‐ and high‐frequency motions taken from particular simulations should be treated with care. The frequency‐vertical wavenumber spectra, frequency spectra, and vertical wavenumber spectra from the models are compared to spectra computed from McLane profilers at nine locations. In general, MITgcm spectra match the McLane profiler spectra more closely at high frequencies (|ω| > 4.5 cpd). In both models, vertical wavenumber spectra roll off more steeply than observations at high vertical wavenumbers (m > 10⁻² cpm). The vertical wavenumber spectra in such models is an important target for improvement, due to turbulence production and dissipation that takes place at high vertical wavenumbers.