High-frequency bottom-pressure and acoustic variations in a sea strait: internal wave turbulence
van Haren, H. (2012). High-frequency bottom-pressure and acoustic variations in a sea strait: internal wave turbulence. Ocean Dynamics 62(8): 1123-1137. dx.doi.org/10.1007/s10236-012-0550-8
In: Ocean Dynamics. Springer-Verlag: Berlin; Heidelberg; New York. ISSN 1616-7341; e-ISSN 1616-7228, more
During a period of 3 days, an accurate bottom-pressure sensor and a four-beam acoustic Doppler current profiler (ADCP) were mounted in a bottom frame at 23 m in a narrow sea strait with dominant near-rectilinear tidal currents exceeding 1 m s(-1) in magnitude. The pressure record distinguishes small and short surface waves, wind- and ferry-induced near-surface turbulence and waves, large turbulent overturns and high-frequency internal waves. Typical low-frequency turbulent motions have amplitudes of 50 N m(-2) and periods of about 50 s. Such amplitudes are also found in independent estimates of non-hydrostatic pressure using ADCP data, but phase relationships between these data sets are ambiguous probably due to the averaging over the spread of the slanted acoustic beams. ADCP's echo amplitudes that are observed in individual beams show much better phase correspondence with near-bottom pressure, whether they are generated near the surface (mainly air bubbles) or near the bottom (mainly suspended sediment). These 50-s motions are a mix of turbulence and internal waves, but they are not due to surface wave interactions, and they are not directly related to the main tidal flow. Internal waves are supported by stratification varying between extremely strong thin layer and very weak near-homogeneous stratification. They are driven by the main flow over 2-m amplitude sand-wave topography, with typical wavelengths of 150 m.
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