Observations of High‐Frequency Internal Waves Near Submesoscale Fronts

Abstract

Nonlinear high‐frequency internal waves (HIWs) can occur at oceanic frontal interfaces but are rarely observed directly. This study presents field observations of HIWs near submesoscale fronts (SFs) in the northern Gulf of Mexico. A marine X‐band radar detected a series of organized bands, with a spacing of 50 ∼ 300 m, near SFs. The bands propagate toward SFs at speeds of 0.10–0.40 m/s, derived from consecutive radar backscatter intensity images. These fortuitous bands were identified as the surface expression of HIWs. This identification is supported by corresponding temperature undulations from a thermistor chain below a wave glider. The ship‐based observations provided vertical density and current (buoyancy and shear) profiles at a large frontal scale. These profiles, taken beneath and upstream of the HIW signature, are employed in the Taylor‐Goldstein equation using the mixed layer depth as a surrogate bottom, which predicts the shear instability growth rate at different wavelengths and corresponding phase speeds. The theoretically predicted fastest‐growing mode wavelength and its phase speed closely align with the observed HIWs after adjustments to the background shear and buoyancy uncertainty ranges. These findings support a shear–stratification instability framework for HIW generation along the shoaling thermohaline front. As HIWs influence local mixing and energy dissipation, they may bear the significance of the frontal energy budget and forward energy cascade from the submesoscale currents to isotropic turbulence in the upper ocean.