Transverse‐vertical structure and temporal variability of the Kuroshio current across the Tokara Strait during 2003–2012 measured by a ferryboat acoustic Doppler current profiler with a 2‐km horizontal resolution and a two‐day interval are presented. The Kuroshio passing through the Tokara Strait exhibits a multicore velocity structure. Its seasonal volume transport variation is biannual for baroclinic components relative to 700 m, peaking in July and December–January. However, the barotropic transport component exhibits an annual cycle with a maximum in December. Empirical orthogonal function analysis of the cross‐sectional velocity is performed. The first two empirical orthogonal function modes reveal the north‐south shift of the Kuroshio current axis and the change in Kuroshio volume transport, respectively. Temporal variabilities of the leading two modes correspond to those of the Kuroshio Position Index and the sea level difference across the strait, respectively. The third empirical orthogonal function mode, with a relatively smaller horizontal scale, was examined in terms of turbulent mixing. The banded structure captured by this mode is likely induced by flow‐topography interaction because islands in the Kuroshio route could cause horizontal and vertical flow separation. Additional analysis based on high‐resolution reanalysis data suggested that (1) inertial instability, which is expected in the areas with negative Ertel's potential vorticity, arises to enhance vertical mixing around the islands in the Tokara Strait, and (2) when the Kuroshio directly impinges the islands, flow divergence in the lee of the islands drives upwelling and leads to uplift of isotherms.

From 2003 to 2011, current surveys, using an acoustic Doppler current profiler (ADCP) mounted on the Ferry Naminoue, were conducted across the Tokara Strait (TkS). Resulting velocity sections (1234) were used to estimate major tidal current constituents in the TkS. The semidiurnal M₂ tidal current (maximum amplitude 27 cm s⁻¹) was dominant among all the tidal constituents, and the diurnal K₁ tidal current (maximum amplitude 21 cm s⁻¹) was the largest among all the diurnal tidal constituents. Over the section, the ratios, relative to M₂, of averaged amplitudes of M₂, S₂, N₂, K₂, K₁, O₁, P₁, and Q₁ tidal currents were 1.00:0.44:0.21:0.12:0.56:0.33:0.14:0.10. Tidal currents estimated from the ship‐mounted ADCP data were in good agreement with those from the mooring ADCP data. Their root‐mean‐square difference for the M₂ tidal current amplitude was 2.0 cm s⁻¹. After removing the tidal currents, the annual‐mean of the net volume transport (NVT) through the TkS ± its standard derivation was 23.03 ± 3.31 Sv (Sv = 10⁶ m³ s⁻¹). The maximum (minimum) monthly mean NVT occurred in July (November) with 24.60 (21.47) Sv. NVT values from the ship‐mounted ADCP were in good agreement with previous geostrophic volume transports calculated from conductivity temperature depth data, but the former showed much finer temporal structure than those from the geostrophic calculation.

To investigate the dynamics of tidal and residual currents in a shallow bay with narrow bay mouth and large tidal flat area, this study conducted three sets of coastal acoustic tomography (CAT) observations at 13 stations in deep area of Jiaozhou Bay (JZB), China, from December 6–18, 2015. Thereafter, travel time difference data observed via CAT were assimilated into a barotropic tidal model to realize an extrapolated study in tidal flats. The root mean square difference was 6.5 cm/s between assimilation and acoustic Doppler current profile data. Harmonic analyses indicated that the semidiurnal tide (M2) was predominant; however, relatively large nonlinear tidal currents (M4 and M6) were also found. Moreover, the spatial mean amplitudes of M2, M4, and M6 were 0.35 (excluding the tidal flat), 0.09, and 0.04 m/s, respectively. Dynamic analyses showed that continuity and advection terms were the main contributors to M4 in the bay mouth, whereas M4 in the tidal flats was generated by continuity and friction terms. Furthermore, bottom friction was the primary contributor to M6. In addition, residual currents with multi‐vortex structures appeared near the bay mouth, the maximum speed reached 0.60 m/s, and advection, continuity, and bottom friction contributed more to the generation of residual current eddies. Horizontal pressure gradient of residual elevation and advection of tidal and residual currents were the dominant factors maintaining the residual current near the bay mouth. The generation mechanisms of M4 and M6 were first discussed based on the assimilation of the current field data in JZB.

An experiment for synchronous mapping of current velocity (V_CAT) and temperature (T_CAT) distributions was successfully carried out using four coastal acoustic tomography (CAT) systems, from March 17 to April 1, 2013, in the Qiongzhou Strait, China. During the CAT observation period, shipboard acoustic Doppler current profiler (ADCP) surveys were also performed to observe the current velocity (V_ADCP) and temperature (T_ADCP) for the purpose of comparison with the CAT data. Position offsets, including a long-time averaged offset and a short-period fluctuation offset, were corrected for temperature estimation. The horizontal distribution of V_CAT and T_CAT was reconstructed by an inverse technique using the CAT data. V_CAT was nearly parallel to the strait while T_CAT was higher in the southern area than in the northern area. During the whole observation period, T_CAT gradually increased by approximately 2°C, indicating seasonal warming. V_CAT was in good agreement with V_ADCP, while the root-mean-square differences (RMSDs) between V_CAT and V_ADCP for eastward and northward velocities were 0.10 m/s and 0.08 m/s, respectively. T_CAT was in approximate agreement with T_ADCP, with an RMSD of 0.30°C.