Currents of the 2011 Tohoku tsunami south of Oahu, Hawaii, from high-frequency doppler radio scatterometer

Abstract

A 16-MHz high-frequency Doppler radio scatterometer (HFDRS) deployed on the south shore of Oahu, Hawaii, detected oscillatory radial currents following the 2011 Tohoku tsunami. Observational data on tsunami currents over a two-dimensional area provided an opportunity to validate the currents and their spatial patterns in a non-hydrostatic wave model of the tsunami from its generation to the shores of the Hawaiian Islands. Over Penguin Bank, a 50-m bank extending west from Molokai, currents were split into two distinct areas, with stronger, longer period currents that persisted longer on the south part of the bank (0.27 ms-1 currents at 43 min lasting 6 hours versus 0.14 ms-1 currents at 27 min lasting 2 hours). The EOF spatial maps suggested that standing half-wave and full-waves in sea level formed over the bank. Modeled currents over Penguin Bank were similar to observations, but with the north-south asymmetry less pronounced. In the near-shore, along-shore observations showed a long-period current oscillating at 43 min that stretched the entire coastline, while modeled currents showed strong evidence for edge waves. EOF analysis of the near-shore revealed that the HFDRS and model showed different processes. Despite HFDRS data issues with azimuthal side lobe contamination and decreased angular resolution at high steering angles, the spatial comparison between observations and model currents were encouraging for a first attempt at two-dimensional model validation of tsunami currents.