Transition from partly standing to progressive internal tides in Monterey Submarine Canyon

Abstract

Monterey Submarine Canyon is a large, sinuous canyon off the coast of California, the upper reaches of which were the subject of an internal tide observational program using moored profilers and upward-looking moored ADCPs. The mooring observations measured a near-surface stratification change in the upper canyon, likely caused by a seasonal shift in the prevailing wind that favoured coastal upwelling. This change in near-surface stratification caused a transition in the behaviour of the internal tide in the upper canyon from a partly standing wave during pre-upwelling conditions to a progressive wave during upwelling conditions. Using a numerical model, we present evidence that either a partly standing or a progressive internal tide can be simulated in the canyon, simply by changing the initial stratification conditions in accordance with the observations. The mechanism driving the transition is a dependence of down-canyon (supercritical) internal tide reflection from the canyon floor and walls on the depth of maximum stratification. During pre-upwelling conditions, the main pycnocline extends down to 200 m (below the canyon rim) resulting in increased supercritical reflection of the up-canyon propagating internal tide back down the canyon. The large up-canyon and smaller down-canyon progressive waves are the two components of the partly standing wave. During upwelling conditions, the pycnocline shallows to the upper 50 m of the watercolumn (above the canyon rim) resulting in decreased supercritical reflection and allowing the up-canyon progressive wave to dominate.

Generation and Propagation of Nonlinear Internal Waves in Sheared Currents Over the Washington Continental Shelf

“…Results of the mode‐by‐mode calculation suggested the same: offshore, the ratio of to was for all modes much less than one; onshore, the ratio was 1 ± 0.2 for all modes (not shown) except for mode 2 whose ratio was 0.7 ± 0.3. In addition, the ratio of HKE to APE at each offshore station was well below the theoretical ratio (3.88) for progressive linear D 2 internal waves at this latitude, but the ratio increased to closer to the expected value beyond the shelf break (Figure c) suggesting progressive waveforms there (Hall et al, ). Taken together, these results provide the first clear evidence that the incident internal tide is partially reflected, leading to lower net flux and partly standing waveforms offshore and progressive tidal waveforms over the shelf.…”

Upwelling rebound, ephemeral secondary pycnoclines, and the creation of a near‐bottom wave guide over the Monterey Bay continental shelf

“…The correlation between low‐frequency wind forcing and internal wave propagation also holds for the nearshore region of Monterey Bay, in that the structure and the strength of the outer shelf to midshelf stratification modulates the structure and strength of internal bores propagating in the nearshore region [ Walters et al, ]. In addition, observations from within the Monterey Submarine Canyon also indicate that different stratification regimes can result in different manifestations of the internal tide (e.g., standing or progressive wave) [ Petruncio et al, ; Zhao et al, ; Hall et al, ]. Our observations of the IWOE field associated with a near‐bottom pycnocline also illustrate the connection between internal wave propagation and upwelling‐driven stratification.…”

Propagation and dissipation of the internal tide in upper Monterey Canyon

“…This question of the appropriate slope is explored more thoroughly in Hall et al . []. Landward of Group 7 (km‐10), critical reflection of the internal tide is likely the turbulence generating mechanism, although the convergence of the canyon walls may also play a role as the head of the canyon is approached.…”