Semidiurnal Internal Tides and Multi-Modal Baroclinic Regime in a Narrow Deep Channel

This study analyzes the thermal structure and internal-wave dynamics of a narrow, deep channel using in situ hydrographic data and numerical simulations with the FVCOM model. A well-defined thermal structure was identified during late summer, with a warm surface layer (0–50 m), a thermocline between 50 and 200 m, and weakly stratified intermediate waters below 250 m. Oscillations of the isotherms and the uplift of the 14 °C isotherm inside the channel indicate the presence of vertical movements driven by the semidiurnal (M2) internal tide, with displacements exceeding 10 m at 120 m depth. The FVCOM model reproduced both the observed thermal structure (mean Pearson correlation of 0.986 across 11 CTD stations) and the principal barotropic tidal constituents at three coastal sites surrounding the channel. A least-squares projection of the M2-filtered vertical velocity onto the first five baroclinic modes reveals a multi-modal regime in which the gravest mode contributes only 9.8% of the resolved variance while modes 2–4 jointly account for 82.4%, in contrast to the mode-1 dominance typically reported for less-confined environments. A Hovmöller analysis revealed coherent thermal structures advancing northward at approximately 1.7 m s⁻¹, associated with variations in the northwestward velocity component. The depth-integrated M2 baroclinic energy budget yields ∼184 MW of barotropic-to-baroclinic conversion integrated over the channel, with most of the locally generated energy retained within the channel rather than radiated to the surrounding Gulf. These results underscore the role of the interaction between stratification, semidiurnal tides, and abrupt bathymetry in shaping a multi-modal internal-wave field, with significant implications for vertical mixing and the redistribution of physical properties within the channel. Read here.