Diurnal coastal trapped waves propagating along the east coast of the Shimokita Peninsula, Japan

In this study, we investigated diurnal coastal trapped waves in the eastern coastal area of the Shimokita Peninsula near the Tsugaru Strait. The coastal trapped waves in this area have not yet been observed. We observed current velocities at three sites on the coast to clarify the propagation and seasonal features. We also used an ocean general circulation model to determine the detailed structure and the causes underlying the seasonal characteristics of the waves. The coastal trapped waves propagated southward along the coast from the strait, where significant tidal currents exist. Coastal trapped waves depend on cross-shelf length and stratification strength. The coastal trapped waves propagate as internal Kelvin waves from summer to early winter in the northern part of the peninsula (where the shelf is narrow) and as shelf waves in the southern part (where the shelf is wide). From late winter to spring, the coastal trapped waves practically disappear in the northern part of the peninsula owing to the vertical uniform density off the east coast and the small cross-shelf width in the northern part. In autumn, the tidal current flows north of the sill near Cape Shiriya at the eastern mouth of the straits owing to the northward shift of the Tsugaru Warm Current in the strait; thus, the coastal trapped waves along the Shimokita coast weaken.