Stationary Internal Tides Observed in a Steep, Reflective, Coastal Submarine Canyon

Observations from a student-led effort in the La Jolla Canyon System in San Diego, CA are used to examine the characteristics and temporal variability of the internal wave field, internal tide dynamics, and mixing within the canyon. The data set includes 7 days of tidally-resolving spatial surveys made using a heavy towed body (SWIMS) and three multi-week moorings deployed along the canyon axis, one of which profiled for an entire year at the canyon head. We find that the internal tide is partially standing throughout the measurement period. Dissipation is enhanced at mid depths near the canyon head and occurs in concert with high strain events on the trailing edge of steep, nonlinear internal tides throughout the year despite seasonal shifts in background stratification.

Internal tide stationarity is examined by comparing isopycnal displacement (η) band-passed between 6 and 30 hours to harmonic fits computed over a range of fit windows between 3 and 90 days. At a mooring 22 km away on the open shelf, the average skill of harmonic fits to η over a 90-day fit window is 19%; within the canyon that value increases to 50%—very high when compared to most other coastal mooring records that have been similarly assessed and indicating surprisingly high internal tide stationarity in the canyon. Phase offsets between TPXO surface tides and harmonic fits at a given depth are relatively constant and consistent with arrival times expected for internal tides being generated at a steep shelf edge nearby.
Relatively stationary internal tides contribute to clear spring-neap cycles in depth-integrated energy, energy flux, stratification, and dissipation occur at times throughout the year. Depth-integrated chlorophyll fluorescence [Chl] also demonstrates spring-neap variability, and maxima in [Chl] lag behind maxima in semi-diurnal energy by 2-3 days.

Long time series within canyon systems are rare, and these results could motivate an investigation into whether the skill of internal tide predictions may be useful near canyon systems or steep topographic features more generally, and whether such regular tidal motions contribute to enhanced productivity locally.