Submesoscale Surface Tidal, Vortical, and Residual Circulations in a Semienclosed Bay

Abstract

We scrutinize three different components of submesoscale surface tidal, vortical, and residual circulations in a semienclosed bay using (1) observations of high-frequency radar-derived surface current maps and (2) numerical model simulations run under realistic vertical stratification and boundary conditions at O(1)-km spatial and 1-hourly temporal resolutions over a 2-year period (2013-2014). 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 rotational directions. The regional vortical circulation is presented with the submesoscale eddies at diameters ranging from 3 to 12km and normalized vorticity magnitudes of 0.2 to 2 for both clockwise and counterclockwise rotations. Based on the spatial statistics of the identified submesoscale eddies and eddy kinetic energy budget analysis, the submesoscale eddies are primarily generated by the detachment of shoreline-following tidal currents at the coastal 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 nontidal geostrophic currents associated with the pressure gradients generated by wind-driven Ekman transports against the coast and the ageostrophic low-frequency currents.