[1] Low-frequency acoustic imaging of internal oceanic structure (''seismic oceanography'') is providing unprecedented views of thermohaline finestructure with the potential to provide quantitative information on such processes as internal waves, eddy dynamics, and turbulent dissipation. Producing seismic images clear enough to confidently extract such information requires accurate sound-speed models. Because sound-speed modeling is time-intensive, and oceanic sound speed relatively simple compared to the solid earth, simplistic assumptions of ocean sound speeds have some appeal in seismic processing of ocean reflections. Here, we consider the effect on seismic images of four sound-speed models: (1) a uniform sound speed of 1500 m/s, (2) regional data from an archived database, (3) temperature profiles collected concurrently with the seismic data, and (4) standard user-selected profiles. Our results show the inadequacy of simple ''shortcut'' models (1 and 2) and indicate the necessity of rigorous, locally derived sound-speed models (3 and 4).
The Luzon Passage generates some of the largest amplitude internal waves in the global ocean as the result of coupling between strong tides, strong stratification, and topography. These internal waves propagate into the South China Sea (SCS) and develop into soliton‐like internal wave pulses that are observed by moored instruments and satellite backscatter data. Despite the observation of these waves, little is known of the mechanisms related to their evolution into nonlinear wave pulses. Using seismic data, we find evidence that the geometry of bathymetric conditions between the Heng‐Chun and Lan‐Yu ridges drive nonlinear internal wave pulse generation. We produce three seismic images and associated maps of turbulent diffusivity to investigate structure around the two ridges and into the SCS. We do not observe large amplitude soliton‐like internal waves between the ridges, but do observe one outside the ridges, a finding in accord with the interpretation that wave pulses form due to geometrical resonance. Additionally, we find no evidence for lee wave activity above the ridges in either the seismic images or associated turbulence maps, suggesting an unlikelihood of hydraulic jump driven generation around the ridges. Our results show increased levels of turbulent diffusivity (1) in deep water below 1000 m, (2) associated with internal tide pulses, and (3) near the steep slopes of the Heng‐Chun and Lan‐Yu ridges as explored in this paper.
Abstract. Breaking internal waves play a primary role in maintaining the meridional overturning circulation. Oceanic lee waves are known to be a significant contributor to diapycnal mixing associated with internal wave dissipation, but direct measurement is difficult with standard oceanographic sampling methods due to the limited spatial extent of standing lee waves. Here, we present an analysis of oceanic internal lee waves observed offshore eastern Costa Rica using seismic imaging and estimate the turbulent diffusivity via a new seismic slope spectrum method that extracts diffusivities directly from seismic images, using tracked reflections only to scale diffusivity values. The result provides estimates of turbulent diffusivities throughout the water column at scales of a few hundred meters laterally and 10 m vertically. Synthetic tests demonstrate the method's ability to resolve turbulent structures and reproduce accurate diffusivities. A turbulence map of our seismic section in the western Caribbean shows elevated turbulent diffusivities near rough seafloor topography as well as in the mid-water column where observed lee wave propagation terminates. Mid-water column hotspots of turbulent diffusivity show levels 5 times higher than surrounding waters and 50 times greater than typical open-ocean diffusivities. This site has steady currents that make it an exceptionally accessible laboratory for the study of lee-wave generation, propagation, and decay.
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