Interactions Between Internal Tidal Bores and Submesoscale Currents on the Continental Shelf.

A realistic simulation of Central California reveals interactions between shoaling internal tidal bores and submesoscale currents on the inner‐shelf (approximately 30–60 m depth). These interactions comprise collisions between internal tidal upwelling, "forward" bores (FBs) with submesoscale currents (SMCs) in the form of surface layer density fronts or filaments with downwelling secondary circulation. Along‐shore oriented FBs collide with both cross‐shore (perpendicular interaction) or along‐shore (parallel interaction) oriented SMCs. In perpendicular interaction, FBs colliding into cross‐shore oriented SMCs refract around the offshore tip of the downwelling front or filament. SMCs generally survive perpendicular interaction, despite partial disruption of downwelling secondary circulation by FBs. An example of parallel interaction demonstrates regional wind regulating the competition between FBs and SMCs to set cross‐shore stratification structure. This example comprises (a) blocking of FB propagation by elevated mixing (strong wind) and dense filament formation on the inner‐shelf and (b) the subsequent destruction of the dense filament that is initiated by a decrease in vertical mixing (weakening wind) and completed by renewed FB propagation underneath it. For both perpendicular and parallel interaction, FB propagation is modulated by a varying medium introduced by SMC density and current structure. The computational evidence of these interactions corroborates recent observations of interactions between small‐scale, nearshore currents in the real ocean. This study motivates further exploration of interactions between fronts, filaments, internal tidal bores, and vortices in the nearshore. Plain Language Summary: The coastal ocean hosts a collection of flow patterns that come in the form of elongated regions of large, horizontal density and velocity differences across 0.01–1 km scales. Tidally driven, "internal" waves ride along interior density layers and eventually shoal and steepen (analogous to surface waves at the beach) into a shoreward propagating density front, known as an "internal tidal bore." When the water‐column is vertically homogenous in density, horizontally sheared currents can elongate and amplify regions of lateral density differences to generate "submesoscale fronts or filaments." Bores and submesoscale fronts and filaments exhibit strong vertical velocity relative to other shelf currents, and can individually modulate nearshore material and heat transport. Past investigation treats these flow structures separately. Utilizing a realistic computer simulation, this study demonstrates a previously unobserved interaction between these two phenomena on the Central California shelf. These interactions comprise competitive collisions between shoreward traveling bores that flux material up and spontaneously arising submesoscale filaments further inshore that flux material down. We observe that submesoscale structures can block the shoreward transit of bores and bores can disrupt the circulation of submesoscale structures. The results of this study contextualize recent, nearshore hydrodynamic observations in Central California. Key Points: A realistic simulation reveals interactions between internal tidal bores and submesoscale currentsSubmesoscale stratification and velocity gradients modulate bore shoalingBores disrupt submesoscale secondary circulation [ABSTRACT FROM AUTHOR]