Keywords:swell, surface wave, rias coast, offshore surface wind, coastal shape
Real-time monitoring of wind and surface waves in Otsuchi Bay, a ria located in the northeastern portion of Japan, had been continued over four years since October 2012, using a mooring buoy with an ultrasonic anemometer and a single-mode GPS wave sensor. The monitoring revealed that waves in the bay were dominated by swells propagated from the northeastern offshore region in all seasons. Time-lag correlation coefficient between the significant wave height in the bay and the offshore wind velocity component blowing in the direction to the bay was maximized in the region approximately 300 km northeast of the bay, from which waves developed by the local wind are accessible within the corresponding time-lag to the bay at a speed of the group velocity of the swell observed in the bay. As an interesting feature common to swell-dominant waves observed in the rias bays adjacent to Otsuchi Bay, the region of the significant correlation between the wave height and the offshore wind velocity extends to the direction that faces the bay mouth. It implies that the source region of the swell depends heavily on the coastal shape of the bay. Consequently, the relation between the offshore wind field and the decay-rate of the swells propagating to Otsuchi Bay indicates that the source region of the swell influential in the bay is located in the offshore region several hundreds of kilometers northeast of the bay, which is on the typical migration route of high and low pressures in the Northern temperate zone.