When monitoring exchanges between adjacent marginal seas, measurements of the volume transport through connecting straits are essential. In such cases, acoustic Doppler current profilers (ADCPs) mounted at the bottom of ferries that regularly cross such straits can provide useful platforms for directly monitoring strait currents. However, since their observations would naturally be confined to each ferry’s set route, it would be impossible for them to observe all relevant current patterns. In addition, a ship-mounted ADCP is very expensive to install, maintain, and (when necessary) remove. In this chapter, we will describe alternative indirect remote sensing techniques that can be used to observe surface velocities in the Korea Strait between Japan and Korea. One such technique involves a high-frequency (HF) ocean radar system that can provide synoptic views of the surface velocity field with high resolutions in both space and time, even though additional processes are required to separate the geostrophic ocean currents from the ageostrophic tidal and wind-driven currents. Alternatively, slopes of the sea surface dynamic height (SSDH) can provide a geostrophic component of the surface velocity. Unfortunately, the satellite altimeters that are most commonly used to measure the SSDH field in open oceans are unsuitable for use in narrow straits. Instead, the use of global navigation satellite systems (GNSS) is showing promise as a way to obtain the coastal SSDH, even though proper spatial smoothing processes are still required.
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