Instabilities of internal gravity waves have been studied widely in an effort to shed light on how internal tidal energy is transferred to smaller-scale disturbances and eventually gets dissipated in oceans.  Among various mechanisms, considerable attention has been paid to triad resonance instabilities, particularly  parametric subharmonic instability (PSI), where the two unstable perturbations that form resonant triads with the basic wave state are fine-scale disturbances at half the basic-wave frequency.  Another approach to possible small-scale instabilities is via the so-called local stability analysis, which assumes disturbances in the form of WKB modes.  In this presentation, rather than postulating a particular instability mechanism, we use formal stability analysis based on Floquet theory.  Studying the Floquet stability eigenvalue problem asymptotically in the limit of short-scale disturbances reveals a novel, broadband instability mechanism, where instability modes comprise multiple frequency components, due to the advection of the perturbation by the underlying wave.  Applications to finite-width wave beams in an unbounded fluid and to propagating wave modes in a layer of finite depth are discussed.

Planning and adapting to future coastal ocean conditions requires accurate coastal ocean predictions of the nutrient, pollutant, heat and sediment transport. In regions with an energetic internal wave field, turbulent mixing can be dominated by nonlinear internal waves which can steepen and break in the coastal ocean, generating bursts of intense turbulence that can drive nutrients into the euphotic zone. Here, I will present data from both process-based field campaigns and long-term monitoring efforts to demonstrate the role of internal waves in mixing the coastal ocean.  Our observations demonstrate that overall the diapycnal mixing is dominated by relatively rare but energetic mixing events, driven by high-frequency internal waves and steepened internal bores. Long-term records reveal that the semi-diurnal barotropic tide, the spring-neap tidal variability, and the seasonal variability in stratification all affect the magnitude of diapycnal mixing and its vertical distribution. Finally, I will share detailed near-seabed observations that reveal the role of internal waves and boundary layer turbulence in resuspending and transporting sediment.

We examine the life cycle of wind-generated, near-inertial internal waves with a suite of nested high resolution simulations spanning a range of scales from hundreds of kilometers  to a few meters. The outer domain encompasses a region that includes the  Sidra Gyre, a semi-permanent anticyclone located off the coast of Libya, while the innermost domain moves with the eddy core and is limited to the region at the base of Sidra.

The coordinated nested simulations produce a dynamically-coupled flow field consisting of mesoscale eddies, submesoscale filaments, near-inertial and higher frequency internal waves, shear-driven instabilities, and nearly isotropic turbulent motions down to the Ozmidov scale, allowing for a comprehensive detailed study of dynamical processes from the meso- to turbulent scales in a realistic ocean setting. Our one-way nesting approach reproduces energy cascades tracked from wind-forced near-inertial waves in the surface mixed layer, through refraction and trapping in a baroclinic anticyclone, downward propagation into the pycnocline, critical-layer amplification and ultimately to shear instability, yielding insight unattainable with stand-alone simulations.

In this talk, we will address the fundamental question of how near-inertial internal waves are connected to diapycnal mixing: Do near-inertial waves break and directly produce small scale,  isotropic turbulence or is there an intermediate transition to large-scale anisotropic turbulence on the path to isotropy and dissipation?

The decay of the internal tide contributes to watermass mixing in the global ocean, which is relevant for the overturning circulation and the dispersal of biogeochemial tracers. In this study, we report on an understudied decay mechanism due to near-resonant interactions between low-mode internal tides. We diagnose 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. We decompose the 3D fields into tidal and supertidal (>2.5 cycles per day) vertical modes and quantify their energetics. 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 mode-mode interactions, which are enhanced for low f. These interactions drive cross-scale energy transfers, which we quantify with a coarse-graining method. The transfers agree with the supertidal flux divergence in spatial patterns and magnitude. 

Tides are a key source of energy for ocean mixing, particularly in the quiescent deep ocean where alternative energy sources are limited. Though much is qualitatively understood about the mechanisms by which tidal energy is dissipated, knowledge gaps remain in our ability to quantitatively predict when and where tidal mixing will occur, hindering our ability to accurately model ocean circulation on global scales. This is especially true for low-mode internal tides that can transfer energy across ocean basins away from their generation sites.

In this talk, I will discuss the role of coastally trapped waves (CTWs) in tidal energy pathways, with a particular focus on their superinertial variants. Unlike subinertial CTWs—which are well documented—superinertial CTWs have received comparatively little attention. These waves travel along coastlines while radiating energy into the ocean interior as freely propagating internal tides. Recent work has shown that superinertial CTWs can be generated through scattering processes when along-shelf topographic corrugations satisfy a triad resonance condition with incoming waves. In this way, superinertial CTWs may serve as a key mechanism linking low-mode internal tides to turbulent dissipation near coastal boundaries.