The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Coastal ocean experiences abundant eddy activities and external strain field can induce eddy distortion and even vortex breakup. Here we investigate how coastal eddy distortion affects the cross-shore and vertical tracer transports using a high-resolution, wave-current coupled model in the San Diego Bight region during a one-month winter season. The coupled model receives realistic air-sea forcing, tides, waves and offshore boundary conditions inherited from a larger-scale data assimilated model. Model dye is released from three small rivers with episodic discharges. The study period is characterized by weak winds and fluctuating alongshore flow. Analyses focus
primarily on an eddy-induced offshore dye transport event and the cross-shore transport is examined along a mid-shelf boundary chosen as a streamline of the time mean and depth-averaged velocity. During the event, the targeted coastal cyclonic eddy is squeezed in the alongshore direction and extends in the cross-shore direction, prior to vortex breakup. Along the mid-shelf boundary, the cross-shore dye flux is decomposed into three components that represent contributions from the bathymetrically-induced flow, the along-boundary averaged baroclinic flow and the along-boundary perturbation flow. The results show that the bathymetrically-induced flow makes negligible contribution. The remaining two components are much bigger and have comparable instant values.
However, time cumulative values of these two components reveal that the total cross-shore dye transport is dominated by the along-boundary perturbation flow, which is linked to the eddy distortion. In addition, this coastal eddy also possesses rigorous vertical motions and the vertical velocity is more negative on the northern side. As a result, the dye on the northern side shows notable subduction if compared with that on the southern side. A possible explanation is that, this N-S vertical velocity asymmetry is primarily induced by topographic

eddy distortion, tracer transport