Semidiurnal Internal Tides in the Santa Maria Basin

Abstract

The Santa Maria Basin (SMB) extends from Point Arguello to Point Buchon along the central California coast. This region is subjected to strong wind, wave, and internal tidal forcing leading to complex three-dimensional circulation patterns, material exchange, and density evolution on synoptic and shorter time scales. Previous studies in this region indicate that during summer the semidiurnal internal tidal activity onshore of the shelf break is from internal tidal generation on the continental slope at water depths of h=1000-3000m, with no local generation occurring at the shelf break. However, the length scales over which semidiurnal internal tidal energetics vary within the SMB, along with temporal variability on seasonal and longer time scales, are not yet known. Here, we use the Regional Ocean Modeling System with a nested grid approach to simulate semidiurnal internal tidal dynamics in the SMB from July to November, 2017. Modeled sea-surface elevation, subtidal and barotropic tidal flows, temperature, and internal tidal fluxes are compared to measurements from the mid- and inner-shelf, obtained as a part of the Office of Naval Research, Inner-Shelf DRI. At 50 m water depth, modeled internal tidal energy fluxes are stronger offshore of Point Sal and Point Purisima but decrease north of Point Sal. Furthermore, internal tidal energy fluxes decay exponentially from 100 to 30m, with an e-folding length scale of 2.3-2.7 km, consistent with previous observations in the region. Alongshore variability of internal tidal energy fluxes over a length scale of 50 km at the 50 m isobath are partially attributed to differences from the internal wave field generated offshore, but also due to changing subtidal circulation, submesoscale eddy activity, and stratification. Finally, the internal wave propagation is found to be primarily a progressive wave in the outer-shelf, transitioning to a partial standing wave pattern in the inner-shelf. Funded by the National Science Foundation and the Office of Naval Research.