Coastal Semidiurnal Internal Tidal Incoherence in the Santa Maria Basin, California: Observations and Model Simulations

Abstract

A series of five realistic, nested, hydrostatic numerical ocean model simulations are used to study semidiurnal internal tide generation and propagation from the continental slope, through the shelf break and to the midshelf adjacent to Point Sal, CA. The statistics of modeled temperature and horizontal velocity fluctuations are compared to midshelf observations (30‐ to 50‐m water depth). Time‐ and frequency‐domain methods are used to decompose internal tides into components that are coherent and incoherent with the barotropic tide, and the incoherence fraction is 0.5–0.7 at the midshelf locations in both the realistic model and observations. In contrast, the incoherence fraction is at the most 0.45 for a simulation with idealized stratification, and neither atmospheric forcing nor mesoscale currents. Negligible conversion from barotropic to baroclinic energy occurs at the local shelf break. Instead, the dominant internal tide energy sources are regions of small‐scale near‐critical to supercritical bathymetry on the Santa Lucia escarpment (1,000–3,000 m), 70–80 km from the continental shelf. Near the generation region, semidiurnal baroclinic energy is primarily coherent and rapidly decays adjacent to the shelf break. In the realistically forced model, incoherent energy is less than 10% in the generation region, with a steady increase in incoherence fraction from the continental slope to the midshelf. Backward ray tracing from the midshelf to the Santa Lucia escarpment identifies multiple energy pathways potentially leading to spatial interference. As internal tides shoal on the predominantly subcritical slope/shelf system, temporally variable stratification and Doppler shifting from mesoscale and submesoscale features appear equally important in leading to the loss of coherence.

Inner‐Shelf Vertical and Alongshore temperature Variability in the Subtidal, Diurnal, and Semidiurnal Bands Along the central California coastline with headlands

“…In 50‐m depth, the SD internal tide was relatively alongshore uniform north and south of Pt. Sal and was approximately one half coherent with the barotropic tide (Kumar et al, ). Modeling suggests the SD internal tide is generated at least a few 100 km offshore (Kumar et al, ).…”

Characteristics and Dynamics of Density Fronts over the Inner to Midshelf under Weak Wind Conditions

“…San Diego Bay (SDB), Point Loma, and the U.S.-Mexico border are also labeled. model system (Warner et al 2010;Kumar et al 2012). A full description of the model setup is found in W20.…”

“…25 m and for 210 , z , 25, the average vertical resolution is Dz 5 2 m. The initial and boundary conditions, nested from the parent LV3 solution, include both barotropic and baroclinic tides. Barotropic tides are prescribed on the outmost LV1 grid, allowing for the generation of baroclinic tides within all model domains (e.g., Kumar et al 2015;Suanda et al 2017;Kumar et al 2019). LV4 receives realistic freshwater discharge from PB, TJRE, and the Sweetwater River within SDB.…”

Vorticity Recirculation and Asymmetric Generation at a Small Headland With Broadband Currents

“…Furthermore, semidiurnal nonlinear internal waves (NLIWs) regularly propagate into Pt. Sal (Colosi et al, 2018;Feddersen et al, 2020;Kumar et al, 2019), adding complexity.…”