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

Area of Science:

• Physical Oceanography
• Fluid Dynamics
• Coastal Processes

Background:

• Wind energy input generates near-inertial velocities in the ocean.
• Coastal geometry significantly influences the behavior of these near-inertial motions.
• Internal waves are crucial for lateral energy transport and mixing in the open ocean.

Purpose of the Study:

• To analyze near-inertial velocity dynamics along a straight coastline in Lake Superior.
• To investigate the generation, propagation, and energy dissipation of wind-driven near-inertial waves in a coastal environment.
• To understand the role of coastal topography in modifying near-inertial flows.

Main Methods:

• Deployment of depth-profiles for velocity, temperature, and turbulent microstructure.
• Conducting a 96-hour repeat survey offshore (3-20 km) in August 2018.
• Utilizing a 1D momentum equation to model observed velocities and turbulent kinetic energy (TKE) shear production.

Main Results:

• Near-inertial velocities (0.2 m/s) generated by wind work (2 mW/m^2) were confined within 6 km of the coast.
• Observed TKE dissipation rate (1 mW/m^2) matched model predictions for TKE shear production.
• Offshore propagating waves carried an energy flux of 10 W/(m-coastline) and a downward flux of 1 mW/m^2.

Conclusions:

• Near-inertial wind energy is primarily lost to TKE shear production in the coastal zone.
• A portion of this energy is transferred to offshore-propagating waves.
• These waves may contribute to shear instability and mixing in areas away from the immediate coastline.