Characteristics and Generation Mechanisms of Submesoscale Fronts over the Inner to Mid-shelf

A recent modeling study of shoreline-released tracer transport in the San Diego Bight region has revealed that offshore tracer transport in 20-30 m water depth (mid-shelf) is primarily induced by submesoscale flows associated with enhanced alongshore density gradients and shorter alongshore tracer length scales. Several generation mechanisms of the density fronts have been identified in previous work, including deformation flow induced frontogenesis and turbulent thermal wind (TTW) balance. Associated with frontogenesis, a lateral secondary circulation develops with downwelling on the heavy-side. In this work, we investigate the structure of these density fronts that develop in shallow waters (<30 m) and diagnose the possible generation mechanisms. Some of our case studies of front development show that, these fronts are squeezed in the cross-front direction and stretched in the along-front direction with downwelling developing on the heavy side, consistent with previous work. Analysis of the frontogenesis tendency finds that the alongshore density advection is the dominant frontogenetic term and intensifies the density gradient. Moreover, an ensemble density front is created by averaging 256 individual density fronts that are manually detected during the three-month study period. This ensemble density front shows elevated offshore flow along the longitudinal axis and the maximum offshore flow develops on the light-side of the front. This is different from the classic frontal structure which has maximum along-front flow coincident with the maximum cross-front pressure gradient. One possible explanation for this inconsistency is that horizontal advection becomes non-negligible and acts to partially balance the cross-front pressure gradient and thus affects the along-front flow. This and other explanations will be explored further. To diagnose the front generation mechanisms, the magnitude of the cross-front flow associated with each detected front will be correlated with the scaling of cross-front flow generated by TTW or a background deformation flow. The former scaling is proportional to the eddy viscosity, while the latter is proportional to the deformation rate. The correlation coefficient will help explain whether these two mechanisms play a role in the frontogenesis.