Madeleine Hamann, Scripps Institution of Oceanography, US
Matthew Alford, Scripps Institution of Oceanography, US
Andrew Lucas, Scripps Institution of Oceanography, US
Veronica Tamsitt, Scripps Institution of Oceanography, US
Celia Ou, Scripps Institution of Oceanography, US
Sam Billheimer, Scripps Institution of Oceanography, US
Marion Alberty, Scripps Institution of Oceanography, US
Observations of nonlinear internal tides and turbulence in a steep canyon
Submarine canyons are common features of the coastal ocean. Their complicated topographies host dynamical processes with a wide range of temporal and spatial scales, many of which give rise to small-scale turbulence. Previous research has shown that canyons can focus internal wave energy propagating from the open ocean and dissipate that energy very efficiently. However, previous research has been limited to a few large canyons with slopes that allowed the propagation and/or breaking of linear internal waves.
In order to investigate mixing dynamics in canyon smaller and steeper (arguably more "generic") than those studied to date, a process study was conducted in the La Jolla Canyon (LJC) off the coast of La Jolla, CA. CTD, velocity, and temperature microstructure data were taken at two 24-hour shipboard stations and a 54-hour profiling mooring along the canyon axis. KE/PE ratios at all stations indicate that the steep canyon axis was indeed reflective to the M2 internal tide giving rise to a partially standing wave. Velocity and isopycnal displacements were dominated by the M2 frequency, but the variance explained by the M2 signal decreased up-canyon, suggesting that this reflection also caused scattering into higher harmonics and a visible steepening of the internal tide waveform.
Baroclinic energy flux was oriented up-canyon and decreased from 182 W/m at the canyon mouth to 46 W/m near the head, and the convergence of up-canyon baroclinic energy flux roughly balanced the observed dissipation rates. Two maxima were apparent in the vertical distribution of energy flux averaged over an integral number of M2 periods. The depths of these maxima corresponded to the depths at which oscillations of the steep, nonlinear waveform gave rise to periods of high shear and strain. These events at mid-depth coincided with observations of enhanced dissipation rates O(10-7 -- 10-5 W/kg). While previous canyon studies observed enhanced dissipation near the bottom primarily due to bottom-trapped bores or topographically controlled hydraulic flows, here we observed elevated dissipation and periods of weak stratification at mid-depths. This suggests a different breaking mechanism where the superposition of incident and reflected waves gives rise to mid-column rather than near-bottom mixing.
Our results provide evidence that narrow, steep canyons that are expected to reflect that energy back out to the open ocean are able to focus and dissipate internal wave energy locally through nonlinear processes. Despite their size, they produce significant energy dissipation that is underrepresented or inappropriately distributed in ocean models on a variety of scales. Because such canyons are much smaller in scale than the current resolution of climate models, it is vital to parameterize this mixing for global simulations of climate, circulation, and biological productivity. Regionally, this dissipation is critical for mixing of tracers including heat, carbon and nutrients. In particular, mid-depth mixing near the nutricline may significantly alter the local nutrient distribution, with important but understudied impacts on local ecology.
Theme 1: Canyon processes in the space-time continuum (formation, evolution, circulation)
turbulence, mixing, internal waves, internal tide, nonlinear, steep