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

Abstract

Here, 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 along-front 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 geostropic and cross-front ageostrophic. This contrasts with the cross-front geostrophic and along-front ageostrophic balances of classic deformation frontogenesis, but is consistent with semi-geostrophic coastal circulation.

Measuring Estuarine Total Exchange Flow From Discrete Observations

“…The San Diego Bay (SDB) and adjacent coastal dynamics were simulated using the Coupled Ocean‐Atmosphere‐Wave‐Sediment‐Transport (COAWST) model system (Kumar et al., 2012 ; Warner et al., 2010 ) to represent the surfzone and shelf circulation (Wu et al., 2020 , 2021 ). This model grid sits within three one‐way nested parent models using ROMS (Shchepetkin & McWilliams, 2005 ) and is coupled with the Simulating Waves Nearshore (SWAN) model to include surface gravity waves (Booij et al., 1999 ).…”

Modeling Untreated Wastewater Evolution and Swimmer Illness for Four Wastewater Infrastructure Scenarios in the San Diego‐Tijuana (US/MX) Border Region

“…A full model description is found in Wu et al. (2020) and application is found elsewhere (Wu et al., 2021a, 2021b). The regional shoreline is relatively straight, except for curvature to the north around Coronado near the San Diego Bay entrance and a broad 15 m depth shoal offshore of the TJRE mouth.…”

Wintertime Submesoscale Offshore Events Overcoming Wind‐Driven Onshore Currents in the East China Sea

“…Wu et al. (2021) pointed out that the cross‐shore fronts can be a conduit for cross‐shore transport. The frontogenesis is induced by the strong strain rate of the background horizontal deformation flow with a favorably aligned surface lateral buoyancy gradient.…”

“…The MLIs occurs in a weakly stratified layer with a lateral buoyancy gradient along the surface boundary and a more strongly stratified interior below. Wu et al (2021) pointed out that the cross-shore fronts can be a conduit for cross-shore transport. The frontogenesis is induced by the strong strain rate of the background horizontal deformation flow with a favorably aligned surface lateral buoyancy gradient.…”

“…The generation mechanisms of submesoscale WCCs in coastal areas are often considered to be mixed layer instabilities (MLIs), frontogenesis, turbulent thermal wind and topographic wakes (Dauhajre et al, 2017(Dauhajre et al, , 2019McWilliams, 2016;Ohlmann et al, 2017;Romero et al, 2013;Wu et al, 2020Wu et al, , 2021. The MLIs occurs in a weakly stratified layer with a lateral buoyancy gradient along the surface boundary and a more strongly stratified interior below.…”