The 2011 Tohoku tsunami south of Oahu: High-frequency Doppler radio observations and model simulations of currents

Abstract

A 16 MHz high-frequency Doppler radio (HFDR) deployed on the south shore of Oahu (Hawaii) detected oscillatory radial currents following the arrival of the 2011 Tohoku tsunami. The observations over a two-dimensional area provided an opportunity for intercomparison with the spatial patterns of currents and the resonant modes predicted by a nonhydrostatic model. Over the 50 m deep Penguin Bank, extending west from Molokai, the observed currents are intensified in two areas: 43 min period currents of 0.27 m s-1 lasting 6 h are observed on the south part of the bank, while 27 min period currents of 0.14 m s-1 lasting 2 h are observed on the north. The spatial EOFs suggest that standing full-waves and 3/2 waves formed over the bank. Modeled currents over Penguin Bank are similar to the observations but their north-south asymmetry is less pronounced than observed. Nearshore, observed alongshore currents showed long-period oscillations of 43 min that stretched along the entire coastline, while modeled currents show strong evidence for edge waves. EOF analysis of the nearshore signal suggests that the HFDR and model reveal different processes. The discrepancy might be attributed to the fact that both the Penguin Bank and nearshore observations are limited by HFDR sensitivity to azimuthal sidelobe contamination and decreased angular resolution at high steering angles.