Onshore and offshore seawater exchange off northeast Taiwan driven by the combined effects of Kuroshio intrusion and tidal currents

Abstract The exchange of materials and energy between boundary currents and the surrounding marginal seas largely influences the environment of those seas. In addition to the reported currents over the continental shelf northeast of Taiwan, a process of onshore and offshore water exchange (OOWE) across the northeast-southwest-oriented shelf break is newly introduced in this study. This exchange is constituted by the synergy of Kuroshio intrusion (northwestward) and ebb tide (southeastward). The total velocity obtained by combining Copernicus Marine Service (CMS) satellite geostrophic currents and modeled Ekman currents through the GLOBCURRENT project, along with numerical modeling based on the Regional Ocean Modeling System (ROMS), are used to describe the spatiotemporal characteristics of OOWE and explain its generation dynamics. OOWE occurs in a highly dynamic active area, sandwiched between two current jets: the Kuroshio Current (KC) and the Taiwan Strait Current (TSC). The seaward increase of northward currents off northeast Taiwan (due to a stronger KC and a weaker TSC) provides a favorable environment of positive vorticity that steers the Kuroshio onto the shelf, constituting the north branch of OOWE. Additionally, the ebb tide drives water away from the shore, producing a southeastward current along the north coast of Taiwan, forming the southern branch of OOWE. Uneven wind forcing in the spatial domain slightly modulates the OOWE pattern, though it plays only a secondary role in its generation. Given the potential to induce vigorous exchange and mixing between continental shelf waters and the Kuroshio, the OOWE identified in this study warrants further attention.