Direct Observations of Coastally Generated Near-Inertial Waves During a Wind Event

Abstract

Wind over the ocean generates near-inertial velocities. In the open ocean, horizontal variability in the inertial frequency and mesoscale vorticity generate internal waves that transport energy laterally and drive diapcynal mixing in remote locations. In the coastal ocean, horizontal variability is produced by the coastline. This study analyzes observations along a straight coastline in Lake Superior, which acts as a “natural laboratory” for the coastal ocean. Depth-profiles of velocity, temperature, and turbulent miscrostructure were collected during a 96 hr repeat survey from 3 to 20 km offshore in Aug 2018. Wind work was 2 mW $m^{-2}$ and generated 0.2 m $s^{-1}$ near-inertial velocities that were inhibited within two internal Rossby radii (6 km) of the coast. The velocities are interpreted as a superposition of a “forced flow”, which is horizontally uniform, and a “wave flow”, associated with offshore propagating near-inertial waves. A 1D momentum equation skillfully predicts $\left(r^2=0.82\right)$ the horizontally averaged near-inertial velocities and the TKE shear production, which matches the 1 mW $m^{-2}$ observed TKE dissipation rate. The offshore propagating wave has an energy flux of 10 W (m-coastline)⁻¹ and a downward energy flux of 1 mW $m^{-2}$. These results suggest that most near-inertial wind work is lost directly to TKE shear production, but some energy is transferred to offshore propagating waves that may help catalyze shear instability away from the coast.