| Abstrakt: |
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 ${\mathrm{m}}^{-2}$ and generated 0.2 m s−1 ${\mathrm{s}}^{-1}$ 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 r2=0.82 $\left({r}^{2}=0.82\right)$ the horizontally averaged near‐inertial velocities and the TKE shear production, which matches the 1 mW m−2 ${\mathrm{m}}^{-2}$ 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 ${\mathrm{m}}^{-2}$. 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. Plain Language Summary: Wind blowing over the ocean sets the surface layer in motion. Once in motion, the water continues moving under its inertia, slowly curving to the right (in the Northern Hemisphere) and eventually completing circles due to the Earth's rotation. These motions, known as inertial oscillations, are ubiquitous in the ocean and large lakes, but the mechanisms that eventually cause them to die out are not fully understood. Here, we examine observations of inertial oscillations in Lake Superior. As the currents run up against a coastal wall the incoming or outgoing currents push the base of the warm surface layer down or up, respectively. This pumping generates waves on the interface between the warm surface water and cold deep water, which propagate offshore. We examine observations of currents, temperature profiles, and small‐scale turbulence; and we find that most inertial currents are damped by turbulence, so that relatively little energy is transferred to the offshore propagating sub‐surface waves. Key Points: Observed near‐inertial velocities in Lake Superior were 0.2 m/s and inhibited within two internal Rossby radii (6 km) of the coastMost TKE shear production (1 mW/m2) was driven by direct wind forcing and predictable from a horizontally averaged modelCoastally generated near‐inertial waves with an energy flux of 10 W (m‐coastline)−1 radiated offshore after a wind event [ABSTRACT FROM AUTHOR] |
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