Tidal currents and eddies around islands - ship-based measurements in Zhoushan Archipelago on the East China continental shelf

The measurements of tidal currents in the ocean can be done by instruments deployed in the ocean, or by a moving research vessel carrying a current meter. The latter has the advantage of resolution of spatial structures of the flow field both in the vertical and in the horizontal. In an archipelago region where the bathymetry and geomorphology in general are complicated and tidal currents swift between islands, the measurements with either method can be challenging. Aimed at the resolution of eddies and the complicated flow structures, two ship-based surveys were conducted in the Zhoushan Archipelago region in summer of 2017 and summer of 2018. A 300 KHz acoustic Doppler current profiler (ADCP) was used in measuring the current velocity profile time series along a planned route around a selected island. We observed eddies and velocity field around small islands in the area with strong tides, narrow channels, dramatic change in depth, and strong velocity shears. Our preliminary analysis has provided interesting results. This report briefly summarizes the ongoing work done thus far. Basically, a few eddies were apparent on the leeward side of the island. These eddies were changing with tidal phase. Harmonic analysis allowed us to resolve both the tidal elevation from the depth data and tidal current from the velocity profile time series. The data are expected to be used in subsequent model validation for comprehensive numerical experiments on hydrodynamics including transport, dispersion, storm surge, and other related dynamical processes.