Variations of the Kuroshio Current System, which is the western boundary current of the North Pacific, influence both large-scale climate changes and regional waters. Satellite altimeters have been widely used to monitor various aspects of the Kuroshio, such as its position and volume transport. Although both meanders of the Kuroshio axis and isolated mesoscale eddies result in similar mesoscale temporal anomaly, they can be distinguished when the mean sea surface dynamic height is retrieved, which cannot be obtained by altimetry data alone. This enables us to describe the shedding and merging of offshore mesoscale eddies and the development of the stable large meander south of Japan. Spatial and temporal resolutions of satellite altimetry data are, however, not sufficient to describe fast-moving small-scale variations of the Kuroshio itself. The high-resolution surface velocity field around the Kuroshio can be directly obtained by high-frequency (HF) ocean radars, which can provide thorough descriptions of the Kuroshio in terms of variations of the maximum speed of the Kuroshio and the displacement of its axis position. In addition, short-term variations of the Kuroshio, such as responses to extraordinary winds of a typhoon, can be studied quantitatively.
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