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Complex small-scale processes and energetic turbulence are observed at a sill located on the I-Lan Ridge that spans across the strong Kuroshio off Taiwan. The current speed above the sill is strong (1.5 m s−1) and unsteady (±0.5 m s−1) due to the Kuroshio being modulated by the semidiurnal tide. Above the sill crest, isothermal domes, with vertical scales of ∼20 and ∼50 m during the low and high tides, respectively, are generated by turbulent mixing as a result of shear instability in the bottom boundary layer. Tidally modulated hydraulic character modifies the small-scale processes occurring on the leeward side of the sill. Criticality analysis, performed by solving the Taylor–Goldstein equation, suggests that the observed lee waves and intermediate layer sandwiched by two free shear layers are related to the mode-1 and mode-2 critical control between the sill crest and immediate lee, respectively. Around high tide, lee waves are advected further downstream, and only mode-1 critical control can occur, leading to a warm water depression. The shear instabilities ensuing from the hydraulic transition processes continuously mediate flow kinetic energy to turbulence such that the status of marginal instability where the Richardson number converges at approximately 0.25 is reached. The resultant eddy diffusivity Kρ is concentrated from O(10−4) to O(10−3) m2 s−1 and has a maximum value of 10 m2 s−1. The sill on the western flank of the Kuroshio is a hotspot for energetic mixing of Kuroshio waters and South China Sea waters.

The two-way Lagrangian particle-tracking model (PTM) is proposed for specifying sources of objects drifting with random-walk processes on the sea surface. First, to determine object source candidates, modeled particles are released from the point (hereafter, “receptor”) where an observer finds the objects using a backward-in-time PTM with modeled ocean currents of which directions are reversed in sign. Second, the modeled particles are released from these source candidates in a forward-in-time PTM using ocean currents originally computed in hydrographic models. Third, the source candidates are considered to be reliable at a 5% significance level if the observed receptor is located inside the ellipse whose center is the mean position of the modeled particles at the time when the observer found the objects and whose axis length is twice the standard deviation computed using all modeled particle positions. The two-way PTM experiments are carried out in a realistic hydrographic model over the East China Sea shelf for the period from June through August 2004. Statistically significant sources are well specified close to the true source because 58%–90% of source candidates are rejected in the experiments.