Characteristics of Offshore Currents at Taiwan Ocean Energy Test Site

There is an offshore ocean energy test site in the northeastern waters of Taiwan. To understand the local ocean currents, a bottom-mount acoustic Doppler current profile (ADCP) was set up at a water depth of about 30 meters near the coast to measure the nearshore ocean current profiles. This ADCP also contains temperature and pressure sensors. The first measurement period was made from June 6, 2023 to July 18, 2023. After the data was downloaded, the ADCP was deployed back to the same location and is still being measured. Measurements were ensembled using 50 pings every 10 minutes, outputting one layer per meter of depth. The measurement data were analyzed by the harmonic method. The result shows that although the sea level in this area is dominated by the M2 tide, the tidal current is led by the M4 tide, followed by the M2 tide. M4 tides are caused by non-linear effects in shallow waters. The tidal ellipse shows that the semi-major axis and the inclination are 10.04 cm s^-1 and 89.98°, respectively for the M4 tide and 5.35 cm s^-1 and 86.18° for the M2 tide on the surface layer. Therefore, the main direction of the tidal current is north-south. However, sea surface current statistics show that most of the flow direction is north-northwest, which is different from the main direction of the tidal current. After checking the wind data from nearby weather stations, the wind direction during the measurement period was mainly easterly, with an average wind speed of about 3 m s^-1. The effect of wind on surface current velocity at this latitude is approximately 1% of the wind speed, resulting in a westward surface velocity of approximately 3 cm s^-1. Combining M4 and M2 tidal currents with wind-driven current, the results are in good agreement with the observations.