Observations and modeling of coastal internal waves driven by a diurnal sea breeze

Authors

  • J. A. Lerczak,

  • M. C. Hendershott,

  • C. D. Winant


Abstract

During the Internal Waves on the Continental Margin (IWAVES) field experiments of 1996 and 1997 off of Mission Beach, California (32.75°N), we observed energetic, diurnal-band motions across the entire study site in water depths ranging from 15 to 500 m and spanning a cross-shore distance of 15 km. The spectral peak of the currents was at the diurnal frequency (σDi = 1 cpd) and was sufficiently well resolved to be clearly separated from the slightly higher local inertial frequency (ƒ = 1.08 cpd). These motions were surface enhanced and clockwise circularly polarized and had an upward phase propagation speed of ∼68 m d−1, suggesting that the motions were driven predominantly by the diurnal sea breeze. However, the downward energy (upward phase) propagation seems irreconcilable with the subinertial diurnal period, and moreover, the intermittent diurnal current events were not obviously associated with diurnal sea breeze events. We rationalize these features using a flat-bottomed linear modal sum internal wave model that includes advection and refraction due to subtidal alongshore flow, V(x, t). Fluctuations in V at the observing site can change the “effective” local Coriolis parameter ƒ + Vx by as much as 50%, thus making the diurnal motions at different times effectively either subinertial or superinertial. The model is integrated numerically for 200 days at a latitude of 32.75°N under different wind and subtidal flow conditions: purely diurnal winds and no V, purely diurnal winds and a time-independent V, narrow-band diurnal winds and no V, and narrow-band diurnal winds and subtidal, time-dependent V. Model diurnal currents forced by narrow-band diurnal winds and subtidal V show complex offshore structure with realistic intermittency and spectral broadening. This study suggests that continental margins in the vicinity of the 30° latitude (where σDi = ƒ) are regions that could potentially produce energetic, sea breeze-driven baroclinic motions and that these motions could be regulated by the vorticity of the local subtidal currents.

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