Sub-kilometer current divergence and vertical velocities in the macrotidal Iroise Sea from airborne radar interferometry and models

Abstract

Fine-scale ocean dynamics strongly influence coastal mixing, sediment transport, and vertical exchange, yet remain difficult to observe at the spatial scales at which they occur. Here, we use total surface current vectors measured by the airborne Ocean Surface Current Airborne Radar (OSCAR) to investigate sub-kilometer divergence, vorticity, shear rate, and vertical velocities in the tidally dominated Iroise Sea around Ushant Island. Observations acquired during ebb and flood tides in May 2022 reveal two distinct mechanisms generating intense fine-scale dynamics. North and west of Ushant Island, interactions between strong tidal currents and complex bathymetry produce alternating zones of divergence and convergence associated with flow acceleration and deceleration over bathymetric gradients. South of the island, a tidal jet flowing alongside comparatively calm waters generates strong lateral shear, producing divergence of up to O(20 f ) and vertical velocities approaching 0.2 m.s −1 . Comparison with the high-resolution MARS2D model shows that currents and bathymetry-driven divergence are reproduced reasonably well, whereas the shear-driven divergence associated with the southern tidal jet is underestimated by approximately an order of magnitude. Coarsening the OSCAR observations from 200 m to 1 km reduces divergence, vorticity, and shear rate from O(10 f ) to O(1 f ), demonstrating the strong sensitivity of derivative quantities to spatial resolution. These results highlight the importance of sub-kilometer observations for resolving fine-scale coastal dynamics, provide a unique observational benchmark for evaluating high-resolution numerical models, and illustrate the complementary roles of airborne, in situ and satellite measurements in characterizing coastal and submesoscale processes across a wide range of spatial and temporal scales.