Near-Inertial waves

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Overview: The wind over the ocean generates oscillations called near-inertial waves (NIW). The subsequent downwards and equator-ward propagation of these NIWs is a main path for mechanical energy from the wind to travel from the surface into the deep ocean. Via wave-wave interactions and scattering, NIWs and internal tides energize the remainder of the internal wave band. Moreover, NIWs control the local stability of the water column that is relevant for breaking and mixing throughout the ocean's depths. A main dynamical reason for the dominance of NIWs which contain about half of the kinetic energy in the ocean is that the inertial frequency is the lowest available to the internal wave band, and the quasi-horizontal oscillatory motion of NIWs offers little impedance to surface wind stress and stratification. Because of their volatile response to atmospheric (wind) forcing, NIWs cannot be described as part of a "universal" wave spectrum of internal gravity waves and different approaches and theories are needed. Intellectual merit: A theoretical and computational effort will be directed at three problems related to the generation and propagation of ocean near-inertial waves (NIWs): (a) the interaction of NIWs with geostrophically balanced eddies, particularly the exchange of energy between eddies and NIWs; (b) the windy generation of NIWs, the role of the beta-effect versus eddy vorticity gradients in accelerating vertical radiation, and the reflection of NIWs from seafloor topography with bottom slopes that are comparable to, or larger than, the wave slope; (c) the interaction of surface gravity waves with NIWs and the partitioning of wind stress between the two modes during the generation process. The processes above control the rate at which the wind works on the ocean internal wave field, the damping of mixed-layer near-inertial oscillations and the downward and equatorward radiation of NIWs. These mechanisms act on time scales ranging from a few seconds (gravity wave frequencies) to weeks (surface-intensified geostrophic eddies). Theoretical developments involve multi-scale analysis, stochastic modeling and averaging. Although parameterization is not an immediate goal of this project, the process-oriented work proposed here is essential to the incorporation of internal-wave physics into comprehensive circulation models. Broader impacts: Ocean mixing rates cannot be characterized by a single universal diffusivity and thus it is essential to understand how spatial, temporal and environmental factors affect the supply of energy to the near-inertial peak. This peak contains most of the shear variance that determines Richardson numbers relevant for ocean mixing. Thus this problem is central to modeling and diagnosing the ocean's role in climate, the ocean carbon cycle, the nutrient supply to the euphotoic zone and the intentional deep-water disposal of carbon and other industrial waste products. Understanding the role of internal gravity waves in this context is a long-term goal of the proposal. The project involves both theory and numerical modeling on important current problems in physical oceanography suitable for a doctoral thesis for a physical oceanographer or an engineer. The project involves an international collaboration and will contribute to an effort to attach a phase-averaged NIW model to the Regional Ocean Model System. The project will improve the Wikipedia article "Stokes drift" and expand the stub "Coriolis-Stokes force" into a full entry.