UC San Diego

AbstractHere, we explore the kinematics and dynamics of coastal density fronts (within 10 km from shore and <30-m depth), identified using an edge detection algorithm, in a realistic high-resolution model of the San Diego Bight with relatively weak winds and small freshwater input. The density fronts have lengths spanning 4–10 km and surface density gradients spanning 2–20 × 10−4 kg m−4. Cross-shore-oriented fronts are more likely with northward subtidal flow and are 1/3 as numerous as alongshore-oriented fronts, which are more likely with onshore surface baroclinic diurnal flow. Using a subset of the cross-shore fronts, decomposed into cross-front mean and perturbation components, an ensemble front is created. The ensemble cross-front mean flow is largely geostrophic in the cross- and alongfront directions. The ensemble cross-shore front extends several kilometers from shore, with a distinct linear front axis and downwelling (upwelling) on the dense (light) side of the front, convergent perturbation cross-front flow within the upper 5 m, strengthening the ensemble front. Vertical mixing of momentum is weak, counter to the turbulent thermal wind mechanism. The ensemble cross-shore front resembles a gravity current and is generated by a convergent strain field acting on the large-scale density field. The ensemble front is bounded by the shoreline and is alongfront geostrophic and cross-front ageostrophic. This contrasts with the cross-front geostrophic and alongfront ageostrophic balances of classic deformation frontogenesis, but is consistent with semigeostrophic coastal circulation.