Radiation of Mixed Layer Near-Inertial Oscillations into the Ocean Interior (original) (raw)

Near-inertial waves in the ocean: beyond the

J Fluid Mech, 2005

The dynamics of linear internal waves in the ocean is analysed without adopting the 'traditional approximation', i.e. the horizontal component of the Earth's rotation is taken into account. It is shown that non-traditional effects profoundly change the dynamics of near-inertial waves in a vertically confined ocean. The partial differential equation describing linear internal-wave propagation can no longer be solved by separation of spatial variables; it was however pointed out earlier in the literature that a reduction to a Sturm-Liouville problem is still possible, a line that is pursued here. In its formal structure the Sturm-Liouville problem is the same as under the traditional approximation, but its eigenfunctions are no longer normal vertical modes of the full problem. The question is addressed of whether the solution found through this reduction is the general one: a set of eigenfunctions to the full problem is constructed, which depend in a non-separable way on the two spatial variables; these functions are orthogonal and form, under mild assumptions, a complete basis.

Near-inertial waves in the ocean: beyond the ‘traditional approximation’

Journal of Fluid Mechanics, 2005

Decay Mechanisms of Near-Inertial Mixed Layer Oscillations in the Bay of Bengal

Oceanography, 2016

Winds generate inertial and near-inertial currents in the upper ocean. These currents dominate the kinetic energy and contain most of the vertical shear in horizontal currents. Subsequent shear instabilities lead to mixing. In the Bay of Bengal, the annual mean wind energy input and near-inertial mixed layer energy is almost as large as in the mid-latitude storm tracks. Also, mixing associated with these waves is known to affect mixed layer heat content, sea surface temperature, and, thus, precipitation in coupled global models. Therefore, the mechanisms leading to the decay of these currents in the mixed layer and below are of considerable importance. Two such decay mechanisms are examined here. One mechanism is the downward propagation of near-inertial internal waves, which is aided by the mesoscale circulation and is observed with a rapidly profiling float. In a few days (faster than at mid-latitudes), the near-inertial wave group propagated from the base of the mixed layer to 250 m depth in the stratified interior. Another decay mechanism is enhanced shear generation at the mixed layer base from periodic alignment of rotating, near-inertial current shear and winds, which is observed with a mooring and analyzed with a simple two-layer model.

Near-inertial motions in the coastal ocean

1995

Internal-inertial waves are frequently observed in the upper ocean and below the thermocline. A three-dimensional general circulation model with turbulence-closure mixed layer is used to study the generation and propagation of near-inertial motion below the mixed layer. In particular, the problem of effect of a coastal wall on the wind induced inertial-internal wave field is re-examined using a fully nonlinear model. Responding to a wind pulse, a sharp wavefront propagates offshore. After the wavefront passage, strong near-inertial internal waves, marked by the tilting velocity isolines and the interface oscillations, are generated. The predicted near-inertial motion is consistent with the wave dispersion relation. Downward energy propagation occurs after the wavefront passage, and both kinetic and potential energy are strongly modified. After several inertial periods, the kinetic energy in the upper layer can be completely removed. The theoretical results, which are supported by observations, indicate that internal-inertial wave are important for mixing in the upper coastal ocean. 0924-7963/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO924-7963(94)00030-l

Near-inertial waves and deep ocean mixing

Physica Scripta, 2013

For the existing pattern of global oceanic circulation to exist, there should be sufficiently strong turbulent mixing in the abyssal ocean, the mechanisms of which are not well understood as yet. The review discusses a plausible mechanism of deep ocean mixing caused by near-inertial waves in the abyssal ocean. It is well known how winds in the atmosphere generate near-inertial waves in the upper ocean, which then propagate downwards losing their energy in the process; only a fraction of the energy at the surface reaches the abyssal ocean. An open question is whether and, if yes, how these weakened inertial motions could cause mixing in the deep. We review the progress in the mathematical description of a mechanism that results in an intense breaking of near-inertial waves near the bottom of the ocean and thus enhances the mixing. We give an overview of the present state of understanding of the problem covering both the published and the unpublished results; we also outline the key open questions. For typical ocean stratification, the account of the horizontal component of the Earth's rotation leads to the existence of near-bottom wide waveguides for near-inertial waves. Due to the β-effect these waveguides are narrowing in the poleward direction. Near-inertial waves propagating poleward get trapped in the waveguides; we describe how in the process these waves are focusing more and more in the vertical direction, while simultaneously their group velocity tends to zero and wave-induced vertical shear significantly increases. This causes the development of shear instability, which is interpreted as wave breaking. Remarkably, this mechanism of local intensification of turbulent mixing in the abyssal ocean can be adequately described within the framework of linear theory. The qualitative picture is similar to wind wave breaking on a beach: the abyssal ocean always acts as a surf zone for near-inertial waves.

Near-inertial waves on the “nontraditional” β plane

Journal of Geophysical Research, 2005

Propagation of linear near-inertial waves on the b plane is considered, taking into account the horizontal component of the Earth's rotation,f. (Terms, effects etc., due to this component will be referred to as ''nontraditional,'' for brevity.) It is shown that the combined effect of b andf changes the dynamics in a fundamental way. For a vertically unbounded domain, an exact solution shows that near-inertial waves can pass through the inertial latitude, unlike under the traditional approximation. For parameter values typical of the ocean, the subinertial domain extends several hundreds of kilometers poleward of the inertial latitude. The solution undergoes a profound change if a vertically bounded, instead of unbounded, domain is considered. Part of the wave energy then accumulates at the poleward end of the subinertial domain, which involves an evolution toward infinitesimal horizontal and vertical scales. For vertically nonuniform stratification, examined here using the Garrett-Munk exponential profile, one finds a wedge-like waveguide, which becomes increasingly narrow in the poleward direction, and drives subinertial waves into the region of the weakest stratification, i.e., the abyss. For typical parameters, the relative amount of poleward traveling energy that gets trapped is estimated to lie between 10 and 30%; its dependence on latitude and stratification is also outlined. The observational evidence and possible implications for abyssal mixing are discussed.

Generation, propagation and dissipation of near-inertial waves in the tropical ocean

2015

Several aspects of the generation, propagation and dissipation of near-inertial waves were examined for tropical regions using a suite of numerical models and observational data. The primary goals were to investigate how the wind input of inertial kinetic energy partitions between loss by turbulent dissipation at the base of the mixed layer versus downward radiation of near-inertial waves, how deep the near-inertial energy penetrates into the interior, and how the background circulation and stratification impact the radiation of near-inertial waves. Results from a 1D model for upper ocean turbulence indicate that, in the eastern tropical Pacific, approximately 50% of the energy input by the wind is radiated downwards into the thermocline as near-inertial waves, despite displaying significant variability between forcing events. Estimates of the vertical energy flux for near-inertial wave packets observed in data collected in the tropical Indian Ocean are in general agreement with the...

Propagating and evanescent internal waves in a deep ocean model

Journal of Fluid Mechanics, 2012

We present experimental and computational studies of the propagation of internal waves in a stratified fluid with an exponential density profile that models the deep ocean. The buoyancy frequency profile N(z) (proportional to the square root of the density gradient) varies smoothly by more than an order of magnitude over the fluid depth, as is common in the deep ocean. The non-uniform stratification is characterized by a turning depth z c , where N(z c ) is equal to the wave frequency ω and N(z < z c ) < ω. Internal waves reflect from the turning depth and become evanescent below the turning depth. The energy flux below the turning depth is shown to decay exponentially with a decay constant given by k c , which is the horizontal wavenumber at the turning depth. The viscous decay of the vertical velocity amplitude of the incoming and reflected waves above the turning depth agree within a few per cent with a previously untested theory for a fluid of arbitrary stratification (Kistovich and Chashechkin,

Resonant excitation of coastal Kelvin waves in the two-layer rotating shallow water model

Nonlinear Processes in Geophysics, 2013

Resonant excitation of coastal Kelvin waves by free inertia-gravity waves impinging on the coast is studied in the framework of the simplest baroclinic model: two-layer rotating shallow water with an idealized straight coast. It is shown that, with respect to the previous results obtained with the one-layer model, new resonances leading to a possible excitation of Kelvin waves appear. The most interesting ones, described in the paper, are resonances of a baroclinic inertiagravity wave with either another wave of this kind, or with a coastal current, leading to generation of a barotropic Kelvin wave. A forced Hopf equation results in any case for the evolution of the Kelvin wave amplitude. U f L , which will be supposed to be small in what follows, and assuming that typical perturbations η 1 , η 2 of the thicknesses Published by Copernicus Publications on behalf of the European Geosciences Union & the American Geophysical Union.

Radiation of Mixed Layer Near-Inertial Oscillations into the Ocean Interior (original) (raw)

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