Submesoscale Surface Tidal, Vortical, and Residual Circulations in a Semi-enclosed Bay

Abstract

We present the submesoscale surface tidal, vortical, and residual circulations in a semi-enclosed bay using (1) high-frequency radar-derived surface current map observations and (2) numerical model simulations run under realistic vertical stratification and boundary conditions at 1-km spatial and 1-hourly temporal resolutions over a two-year period (2013 to 2014). The tidal circulation is characterized by (1) the tidal ellipses and spectral contents having more baroclinic motions at the $M_{2}$ and $S_{2}$ frequencies and more barotropic motions at the $K_{1}$ frequency, and (2) the temporal and spatial variability in the tidal fronts appearing as tidal ellipses with clustered shapes and opposite signs. The vortical circulation is summarized with submesoscale eddies having similar rotational tendencies at diameters ranging from 3 to 12 km and normalized vorticity magnitudes of 0.2 to 2 for both rotations. The submesoscale eddies are primarily generated by the detachment of shoreline-following tidal currents at the boundaries, persist for less than 1.5 days, and are dissipated dominantly via vertical buoyancy fluxes associated with bottom bathymetric interactions of the tidal currents. The residual circulation is clearly shown with the non-tidal geostrophic currents associated with the pressure gradients generated by wind-driven Ekman transport against the coast and the ageostrophic low-frequency currents.