A case study of Doppler-shifted inertial oscillations in the Norwegian Coastal Current

Abstract

A case study with emphasis on near-inertial oscillations with frequency below the inertial frequency ƒ has been performed based on current measurements from 40 m depth in the Norwegian Coastal Current. The measurements show evidence of transient features with periods of 15 and 130 h while the local half-pendulum day is 13.9 h. The currents related to the 15-h period show an elliptic pattern in clockwise rotation, while the currents corresponding to the 130-h period show east-west oscillations. By a modal decomposition of the velocity field it has been shown that energetically, the barotropic and first baroclinic mode dominate the field. Based on data analysis and application of a reduced gravity model with an associated uniform geostrophic current, the oscillations are considered with regard to the Doppler shift wave-current interaction mechanism. Perturbations on the basic current as plane waves propagating in the current direction, have been considered. The dispersion relationships obtained show two near-inertial and one low frequency wavemode of the Rossby type related to the sloping interface. The Doppler shift affects the dispersion properties for upstream and downstream propagating waves in different ways. For the downstream propagating waves the dispersion relationships are slightly modified compared with propagation in stationary mediums. For the upstream propagating waves the Doppler shift distorts the dispersion properties, resulting in a dispersion relation with both positive and negative group velocities and thus zero group speed for a certain wavelength with corresponding frequencies below ƒ for that waveband. In accordance with observations, the model shows zero group speed for a wavelength of 45 km with a corresponding frequency of 0.9 ƒ. The velocity components for near-inertial wavemodes show velocities in anti-cyclonic and cyclonic rotations. The pronounced observed oscillations with frequency 0.9 ƒ are explained as a result of the different dispersion properties for upstream and downstream propagating waves, where upstream propagating waves in the waveband of zero group speed are amplified due to accumulation of energy. Also the long periodic velocity field corresponds reasonably well with observations where the model shows eas-twest oscillations for symmetric disturbances.