We investigate the interplay between submesoscale instabilities, bathymetry, and internal tides and waves in a high-resolution, realistic simulation of the flow of the California undercurrent past the Santa Rosa Ridge in the Southern California bight. Our tool is the UCLA ROMS code that is equipped with new open boundary conditions that allow for the remote forcing of high frequency signals such as internal tides and waves, without undue reflections at those open boundaries.

The Santa Rosa ridge, with minimum depths of around 100 m forms the western boundary of the Santa Cruz basin The California undercurrent flows in a generally West- and North-ward direction along the California shelf break. In this area, the undercurrent flows along the break south of Santa Cruz Island, is diverted south along the SR ridge, and then flows through the gap in the ridge where the depth is as much as much as 360 m. Past the ridge, the undercurrent generally separates from the bathymetry. This is accompanied by instabilities, strong mixing, and the regular formation of sub surface anti-cyclones. The mean flow is generally in geostrophic balance, but has a turbulent boundary layer near the bottom which imparts stress and vertical shear on the flow; on a sloping boundary, this results in horizontal shear as well. As a result, the interaction of the flow with the boundary is a source for negative potential vorticity on the northern side of the ridge. Negative values of potential vorticity (PV) are generated on the northern side of the gap, which lead to inertial (mostly centrifugal) instabilities and subsequent turbulence and mixing. As the flow progresses westward, the negative PV values are mixed away towards a state of neutral stability (PV= 0). On the other side of the gap, positive PV is generated and these positive values are advected westward in the form of filaments. The filaments are also susceptible to shear instabilities and may lead to roll up of the filaments.

Using multiple nested numerical simulations that start at the basin scale and that end at resolutions that reach into the LES simulations, we investigate the physics of the various instabilities and subsequent mixing and energy dissipation. This work will be accompanied by an intensive observational campaign in the summer of 2026, using a multitude of underwater autonomous vehicles that are in connected through acoustic communication.