Internal wave propagation in the Arctic Ocean

Abstract

The propagation of internal gravity waves in the Arctic Ocean is studied using direct and indirect observations, reanalysis data and two numerical models. I focus on how stratification modifies wave propagation and its connection to the pathways of internal wave energy from the surface to the deep ocean. Understanding internal wave propagation in the Arctic Ocean is crucial for better understanding wave-driven mixing and its implications for climate projections. Therefore, three specific aspects of internal wave propagation have been addressed in this thesis. First, the transient transmission of a wave packet across a density staircase is studied using a 2D Boussinesq model. A series of simulations with fixed wave frequency and varying horizontal wave number are carried out. The results show that the incident wave excites trapped modes by a near resonance mechanism, which slowly transfer energy above and below the staircase. A theoretical prediction was made using typical values for thermohaline staircases and internal waves in the Arctic Ocean. Surface-generated near-inertial internal waves that excite trapped modes should have a critical horizontal wavelength of ∼ 400 m. For higher-frequency non-hydrostatic waves, this critical horizontal wavelength decreases to ∼ 80 m. Such waves are likely to be generated by wind-driven ice floes. Secondly, the existence of turning depths for near-inertial internal waves and their effect on wind-driven deep mixing is assessed using 10 years of temperature and salinity profiles in the Canadian Basin. It is found that turning depths exist in the deep Canadian Basin at ∼ 2750 m, but with decreasing distance from the bottom towards the slope. A subsequent discussion focuses on the possible topographic interaction of internal wave reflection and dissipation, especially where turning depths are shallow and above the slope, where the evanescent perturbation of the internal waves can still interact with the topography. Finally, direct observations of near-inertial internal waves are investigated using current observations from a mooring on the Gakkel Ridge, and their surface generation is addressed using a wind and ice drift speed and ice concentration dataset. Cross correlation analysis shows that there is a correlation between ice drift speed and near inertial wave energy with a lag of < 26 days. In addition, a correlation of ∼ 15 days is observed between the wind factor and the near-inertial wave energy. This result suggests that near-inertial internal waves may be generated at the surface by an interplay of wind and ice properties and propagate from the surface to the seafloor. Evidence for wave reflection is also found, and 2D numerical simulations of waves reflected at a turning depth are performed and compared with observations, showing qualitative agreement.