Experimental Investigation of Tidally-Forced Internal Wave Turbulence at High Reynolds Number*

Through basin-scale circulations, the ocean regulates global distributions of heat, nutrients, and greenhouse gases. To properly predict the future of the ocean under climate change, we need to develop a thorough understanding of the underlying mechanisms that drive global circulations. An estimated 2 TW of power is required to support interior mixing that balances deep-water formation. Roughly half of this power is believed to come from tidal flow over topography, producing internal gravity waves (IGW's), which can radiate energy throughout the ocean interior. But it is difficult to track the subsequent journey from tidal injection to dissipation, as the energy cascade spans an enormous range of spatio-temporal scales and multiple different nonlinear transfer mechanisms. 

To investigate the full energy pathway from topographic forcing to irreversible mixing, we built a model ocean in a large-scale laboratory wavetank (9 m x 2.9 m x 0.75 m) allowing Reynolds numbers up to O(10^5). We replicate the tidal forcing by oscillating an idealized ocean ridge, mounted via strain gauges to quantify the energy input. We track energy transfer across the first cascade, driven by wave turbulence, using Background Oriented Schlieren (BOS) over the full tank. Lastly, we take continuous localized measurements of salinity and vertical casts between experiments to quantify energy conversion into altering the initial linear stratification. 

This presentation will focus on the initial large-scale energy transfer through wave turbulence. Through the BOS we observe the formation of various sets of subharmonics, driven by Triadic Resonant Instabilities (TRI). We validate the three-wave resonant conditions through a Fourier decomposition and confirm a backward cascade in frequency but a forward cascade in vertical wavenumber. Through our spatial analysis, we identify relevant three-wave interactions and show the significance of elastic scattering, a nonlocal interaction, in our system.