Marginal currents induced by the interaction between Kuroshio, tide, and coastal topography off the northeastern coast of Taiwan

The goal of the study around the Hoping Sea is to explore the causes of the southward mean current formation along the Hoping Coast, Hualien, using a three-dimensional general circulation model (MITgcm). Hoping Township is situated on the northeast coast of Taiwan, the offshore region by which the Kuroshio Current passes. Two current meter sites were deployed around this region, named CS (121.76507˚E, 24.29757˚N) to the south and CN (121.77197˚E, 24.32964˚N) to the north. A long-term observation analysis finds that the ocean current at CS usually flows northeastward, aligning with the direction or, at least, with the northeast component of the Kuroshio; on the contrary, a southward countercurrent along the Hoping coast appears at CN. In order to explore the cause of the characteristic flow field, the MITgcm model was used to simulate six different scenarios. The first two simulations of the MITgcm model were conducted solely with Kuroshio as a driving force and a realistic topographic, in which the model was executed under the Kuroshio mainstream nearshore and offshore cases, respectively. The results showed that the countercurrent exists in Yilan Bay but does not reach the Hoping Sea when the Kuroshio is the only driving force of the model. Subsequently, simulations were conducted based on the previous work, with a tidal generating force imposed on the model. Combining the Kuroshio and tides in the model, the southward countercurrent was not prominent when the mainstream of the Kuroshio was set to close the Hoping coast. However, when we moved the Kuroshio offshore, the simulated results showed that the model flow field was in good agreement with the in-situ data in all simulated cases. Further, to explore the influence of tides and topographic effects on the flow field, two ideal water-tank basins were designed to simplify the topography. One was a straight coastline, while the other was a coastline with a protruding headland in the model. The results showed that the currents during the ebb tides flow southward and adhere to the alongshore coast. The mean flow north of the headland flowed southward increasingly, but the northeastward mean flow south of the headland weakened slightly.