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The Kuroshio intrusion plays a vitally important role in carrying nutrients to marginal seas. However, the key mechanism leading to the Kuroshio intrusion remains unclear. In this study we postulate a mechanism: when the Kuroshio runs onto steep topography northeast of Taiwan, the strong inertia gives rise to upwelling over topography, leading to a left-hand spiral in the stratified ocean. This is called the topographic beta spiral, which is a major player regulating the Kuroshio intrusion; this spiral can be inferred from hydrographic surveys. In the world oceans, the topographic beta spirals can be induced by upwelling generated by strong currents running onto steep topography. This is a vital mechanism regulating onshore intruding flow and the cross-shelf transport of energy and nutrients from the Kuroshio Current to the East China Sea. This topographic beta spiral reveals a long-term missing link between the oceanic general circulation theory and shelf dynamic theory.

Plain Language Summary The water and material exchange between global ocean and marginal sea is attracting immense research interest because it significantly influences the coastal hydrodynamic environment and ecosystem. We first report the topographic beta spiral inferred from observations, which is generated by strong upwelling induced when strong western boundary current runs onto steep topography in coastal sea. We postulate that this mechanism plays a vital role in regulating intrusion/bifurcation of western boundary currents. This mechanism may be at work in many places in the world oceans; thus, it can be used as a clue in predicting the intrusion path and the bifurcation pattern of strong boundary currents. Similar to Ekman spiral, topographic beta spiral is one kind of very important and common spirals in the world oceans. In some cases, topographic beta spiral suggests that the intruding water mass originates from the deep water of western boundary current rather than surface water. Since temperature, oxygen and nutrients in the deep water are significantly different to the shelf water, this intruding water can influence coastal ecosystem and hypoxia.