Characteristics of wind-generated near-inertial waves in the southeast Indian Ocean

Abstract

This study presents the characteristics and spatio-temporal structure of near-inertial waves and their interaction with Leeuwin Current eddies in the eastern South Indian Ocean as observed by Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats. The floats sampled the upper ocean during July – October 2013 with a frequency of 8 profiles per day down to 1200 dbar. Near-inertial waves (NIWs) are found to be the dominant signal in the frequency spectra. Complex demodulation is used to estimate the amplitude and phase of the NIWs from the velocity profiles. The NIW energy propagated from the base of the mixed layer downward into the ocean interior, following beam characteristics of linear wave theory. We visually identified a total of 15 near-inertial internal wave packets from the wave amplitudes and phases with a mean vertical wavelength of 89±63 m, a mean horizontal wavelength of 69±85 km, a mean horizontal group velocity of 3±2 cm s⁻¹ and a mean vertical group velocity of 9±7 m day⁻¹. A strong near-inertial packet with a kinetic energy of 20 – 30 J m⁻³ found propagating below 700 m suggests that the NIWs can contribute to deep ocean mixing. A blue shift of 10 – 15% in the energy spectrum of the NIWs is observed in the upper 1200 m as the floats move toward the equator. The impacts of mesoscale eddies on the characteristics and propagation of the observed NIWs are also investigated. The elevated near-inertial shear variance in anticyclonic eddies suggests trapping of NIWs near the surface. Cyclonic eddies in contrast, were associated with weak near-inertial shear variance in the upper 400 m. This study presents the characteristics and spatio-temporal structure of near-inertial waves and their interaction with Leeuwin Current eddies in the eastern South Indian Ocean as observed by Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats. The floats sampled the upper ocean during July – October 2013 with a frequency of 8 profiles per day down to 1200 dbar. Near-inertial waves (NIWs) are found to be the dominant signal in the frequency spectra. Complex demodulation is used to estimate the amplitude and phase of the NIWs from the velocity profiles. The NIW energy propagated from the base of the mixed layer downward into the ocean interior, following beam characteristics of linear wave theory. We visually identified a total of 15 near-inertial internal wave packets from the wave amplitudes and phases with a mean vertical wavelength of 89±63 m, a mean horizontal wavelength of 69±85 km, a mean horizontal group velocity of 3±2 cm s⁻¹ and a mean vertical group velocity of 9±7 m day⁻¹. A strong near-inertial packet with a kinetic energy of 20 – 30 J m⁻³ found propagating below 700 m suggests that the NIWs can contribute to deep ocean mixing. A blue shift of 10 – 15% in the energy spectrum of the NIWs is observed in the upper 1200 m as the floats move toward the equator. The impacts of mesoscale eddies on the characteristics and propagation of the observed NIWs are also investigated. The elevated near-inertial shear variance in anticyclonic eddies suggests trapping of NIWs near the surface. Cyclonic eddies in contrast, were associated with weak near-inertial shear variance in the upper 400 m.