High-frequency nonlinear internal waves in the Kuroshio observed by acoustic observations

High-frequency current fluctuations in the Kuroshio current were observed using a long-range Acoustic Doppler Current Profiler (ADCP) moored in the Tokara Strait. The fluctuations were separated as deviations from the 60-min running averaged currents. The duration of individual fluctuation was 3–5 min and the active fluctuation events occurred diurnally during the period of strong Kuroshio current, suggesting both influences from the Kuroshio and tidal currents. The statistical analyses revealed that the current fluctuations were confined in the mid-depths (158–262 m) and the westward component was dominant in the eastward-flowing Kuroshio currents. Close examination of a fluctuation event revealed that strong westward and downward currents occurred in a rank-ordered wave train consisted of 3–4 waves with a period ∼ 3 min. As this wave period was shorter than the ambient buoyancy period (8–13 min), the observed wave train was considered to be a packet of solitary waves (nonlinear internal waves; NLIWs). The observed current structure with a maximum amplitude at the mid-depths were similar to that of the second mode (mode-2) internal waves based on the observed stratification. In addition, the wave propagation direction estimated from the signal time lags between the ADCP transducers agreed well with the maximum current direction at mid-depths in the event. This strongly suggests that the observed current fluctuations were convex type mode-2 NLIWs. Although the NLIW generation was unclear, a role of small seamounts east of the mooring site was suggested.