Interaction of high-frequency internal waves with the wind-driven river plume

• DOI: 10.5194/egusphere-egu25-14148
• Publication Date: 2025-03-15T02:16:55Z
• Authors: Alexander Yankovsky; George Voulgaris; Christopher Papag...; Diane Fribance

We present shipboard observations of high-frequency internal waves (IW) propagating through the Winyah Bay plume on the South Atlantic Bight shelf (the US East Coast). Two surveys are analyzed: 6-7 June, 2023, and 23-24 May, 2024. On both occasions, the plume was affected by a moderate upwelling-favorable wind resulting in a significant offshore spreading of the Winyah Bay plume, well beyond its natural (unforced) offshore limit. While stratification conditions were roughly comparable in both years, IWs exhibited very different behavior, which is attributed to the properties of mean (averaged over multiple IW periods) currents. Specifically, IWs in May 2024 propagated as a dispersive train with a near-zero depth-averaged velocity component and highly polarized velocity fluctuations. The TKE dissipation values in the pycnocline were unaffected by the IW train passage. The mean current did not reverse with depth and its maximum magnitude in the direction of wave propagation (inferred from the wave velocity vector orientation) was less than 0.3 m/s. In contrast, in the 2023 observations there was a significant depth-averaged velocity component corresponding to the IW frequency band at a close to normal angle with depth-dependent velocity fluctuations. There were elevated values of TKE dissipation in the pycnocline exceeding corresponding values in the surface boundary layer. In addition to high TKE dissipation, salinity profiles exhibited clearly visible overturning events. The mean current velocity profile in the direction of the IW propagation had a distinctive two-layer structure reaching 0.4 m/s at the surface (seaward) and -0.2 m/s below the plume layer (shoreward). In both years, IW velocity structure closely resembled theoretical velocity profiles obtained from a numerical solution of the Taylor–Goldstein equation for the observed buoyancy and mean current profiles. We conclude that IW breaking with enhanced TKE dissipation occurs when IWs approach critical layers, where the wave phase speed matches the mean current. Critical layers can be readily encountered when the mean current reverses with depth such that its Doppler effect on the wave dispersion curve is minimal. We hypothesize that strong IW dissipation at critical layers along with nonlinear effects can generate the observed depth-averaged velocity component.