Coastal Ocean Response to Near-resonant Sea Breeze/Land Breeze Near the Critical Latitude in the Georgia Bight

On the mid- to outer shelf of the Georgia Bight, surface-intensified non-tidal diurnal currents can exceed 25 cm/s more than 120 km offshore, with currents in a lower layer 180 deg out of phase upper layer currents. Persistent from April through October, these diurnal motions appear to be inertial oscillations and near-inertial internal waves, forced by sea breeze/land breeze (SBLB) and other diurnal winds, resonant with the inertial frequency at 30 degrees N or S. Observational wind and current data from 1999-2007 are analyzed from a moored array in the Georgia Bight, between 29-32 degrees N, where linear theory predicts maximum SBLB magnitude and offshore extent. Complex empirical orthogonal function analysis is used to separate non-tidal diurnal/inertial currents from the tidal currents at frequencies that cannot be simultaneously resolved at relevant time scales. The spatial structure, variability, and phase of diurnal/inertial currents are described and compared to those of SBLB as both the atmospheric forcing and ocean response pass through the critical latitude for diurnal/inertial resonance. Diurnal variance of observed and modeledwinds indicate SBLB winds on the order of 1-2 m/s at least 250 km offshore, nearly an order of magnitude greater than the anticipated offshore scale. The magnitude of coastal ocean response is strongly controlled by the interaction of bottom friction and stratification, and increases with distance offshore, producing diurnal/inertial divergence/convergence over the inner to mid-shelf. Examination of shear and stratification on the mid- to outer shelf reveals that the pycnocline partially decouples the water column. The level of maximum shear bounds rather than coincides with the pycnocline, which contains a sub-surface jet that rotates anticyclonically but is 180 deg out of phase with the directly-forced surface currents. The vertical structure of the currents is not-well represented by models that describe the vertical structure observed elsewhere, and more closely resembles a three-layer structure: a wind-forced surface layer overlying a stratified inertial jet layer and a well-mixed quiescent bottom layer. The vertical structure and its variability has significant implications for mixing, as the shelf of the Georgia Bight appears to trap near-inertial energy input from the wind, enhanced near the critical latitude for diurnal/inertial resonance