Satoru Okajima

Swells can cause coastal hazards. A unique example of such hazards is the Yorimawari‐Nami (YM‐wave) events that have been observed once every few years in Toyama Bay in the Japan Sea. Based on coastal in situ observations and hindcast simulations of ocean waves, the forcing mechanism for the record‐high 2008 YM‐wave event is investigated, focusing on the surface winds and the atmospheric disturbances that drove wind seas and swells in the Japan Sea. Detailed analysis of atmospheric mesoscale data along with cyclone tracking and wave spectral analysis revealed that the 2008 event was forced by an exceptionally persistent northerly wind off the Tohoku and Hokkaido regions associated with two successive extra‐tropical cyclones, passing across the Japan Sea and accompanying a well‐defined upper‐tropospheric cut‐off low. The first cyclone generated the initial northerly wind forcing toward the northwest of Hokkaido Island that was aligned with the direction of southward‐propagating wind seas toward Toyama Bay. This initial fetch caught up with the second cyclone, leading to an extension in the area of a large northerly wind component (>12 ms −1 ) that lasted longer than 24 hr and extended southward due to the southward extension of the cut‐off low with a propagation speed that matches the wave group velocity, reinforcing the initially generated swell.