Numerical study of diurnal tidal currents on the Pacific shelf off the southern coast of Hokkaido, Japan

We developed a triply nested, high-resolution (1/50 degrees) ocean model to reproduce tides and tidal currents under realistic oceanographic conditions, mainly on the Pacific shelf around Hokkaido, Japan. The model reproduced observed properties of tides and tidal currents reasonably well. Dominant diurnal tidal currents were simulated and accounted for mainly by propagation of coastal-trapped waves (CTWs) of the first mode. The first-mode CTWs were generated mainly around the Four Islands, which are located to the east of Hokkaido, where the massive energy of gravity waves was scattered to CTWs. After leaving the forcing area around the islands, diurnal CTWs propagated along the Hokkaido coast to Cape Erimo. Around the cape, CTWs were also excited via scattering of gravity waves and superposed with first-mode CTWs propagating from the Four Islands. The K-1 (O-1) currents associated with the CTWs weakened (strengthened) around and west of the cape because of out-of-phase (in-phase) superposition. The K-1 currents therefore ended up being smaller than the O-1 currents by one order of magnitude. Moreover, the first-mode CTWs generated around the Four Islands were attenuated along the propagation route through complicated processes. The unsteadiness of the K-1 and O-1 currents related to their 13.67-day beats therefore increased gradually along the propagation route. The unsteadiness on the shelf west of the cape was further enhanced by superposition with CTWs forced around the cape, which were occasionally unstable.

Automated summary

A high-resolution ocean model was developed to simulate tides and tidal currents around Hokkaido, Japan, accurately reproducing observed oceanographic conditions. The study found that dominant diurnal tidal currents were generated mainly by coastal-trapped waves along the Hokkaido coast, with the first-mode CTWs being generated near the Four Islands and propagating towards Cape Erimo. Additionally, the interaction of different tidal currents around Cape Erimo led to varying strengths and directions of currents, with the K-1 currents being weaker than the O-1 currents by one order of magnitude. Furthermore, the unsteadiness of the tidal currents gradually increased along the propagation route, especially on the shelf west of the cape due to superposition with unstable CTWs.