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Near-Inertial Wave Propagation In Geostrophic Shear
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1985
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GeophysicsOcean Wave MechanicsOcean DynamicsEngineeringPhysical OceanographyWave PropagationOcean PhysicNegative VorticityApproximate Dispersion RelationOcean Internal WaveOceanic ForcingOceanographyGeophysical FlowEarth ScienceOceanic SystemsNear-inertial Wave PropagationClimate DynamicsVorticity ζ
Near‑inertial internal waves propagate in geostrophic shear where their horizontal scales match those of geostrophic motions, requiring retention of shear interaction terms, and vorticity shifts the lower bound of the internal waveband from the Coriolis frequency to an effective value. The authors use a ray‑tracing method to study near‑inertial wave propagation in a model geostrophic jet. The study finds that the derived dispersion relation, incorporating mean‑flow shear and vorticity‑induced effective Coriolis frequency, predicts trapping and amplification of near‑inertial waves in negative‑vorticity regions, with observed intense downward‑propagating waves at the base of upper‑ocean negative vorticity in the North Pacific Subtropical Front, warm‑core rings, a Gulf Stream cold‑core ring, and a Sargasso Sea anticyclonic eddy, while untrapped waves are focused into tight beams as they exit the jet.
An approximate dispersion relation for near-inertial internal waves propagating in geostrophic shear is formulated that includes straining by the mean flow shear. Near-inertial and geostrophic motions have similar horizontal scales in the ocean. This implies that interaction terms involving mean flow shear of the form (v·Δ)V as well as the mean flow itself [(V·Δ)v] must be retained in the equations of motion. The vorticity ζ shifts the lower bound of the internal waveband from the planetary value of the Coriolis frequency f to an effective Coriolis frequency feπ = f + ζ/2. A ray tracing approach is adopted to examine the propagation behavior of near-inertial waves in a model geostrophic jet. Trapping and amplification occur in regions of negative vorticity where near-inertial waves' intrinsic frequency &omega0 can be less than the effective Coriolis frequency of the surrounding ocean. Intense downward-propagating near-inertial waves have been observed at the base of upper ocean negative vorticity in the North Pacific Subtropical Front, warm-core rings, a Gulf Stream cold-core ring and an anticyclonic eddy in the Sargasso Sea. Waves that are not trapped are focussed into tight beams as they leave the jet.