Numerical simulations of a storm‐generated island‐trapped wave event at the Hawaiian Islands

The structure and generation of subinertial trapped waves at the Hawaiian Islands are investigated using a primitive equation model with realistic bathymetry, stratification, and wind forcing. The strongest wave event measured on the island of Hawaii during the 1980s is selected as a case study. An extratropical cyclone traveling to the east provided the forcing. Three model runs are considered: a single island and a chain of islands forced by synoptic‐scale winds and a chain of islands with enhanced winds to simulate features of the mesoscale forcing. Each model run documents the establishment of high and low surface elevation anomalies (0.05 m amplitudes) on opposite sides of the island of Hawaii due to convergences and divergences in the surface wind‐driven current. Predicted along‐shelf current amplitudes exceed 0.4 m/s, with the strongest currents near the surface and weak amplitudes (<0.1 m/s) below 100 m depth. Following the storm, a gravest mode island‐trapped wave with a 59 hour period progresses around the island for at least five wave cycles. Although the storm itself lasts only 2 days, the rotation of the wind field in time reinforces the gravest mode wave at the island of Hawaii. A similar forced pattern is established at the other major islands; however, the free wave response is much weaker than at Hawaii because of a mismatch in the forcing and resonant wave mode frequencies. In comparison to tide gauge measurements and a current meter record from the northwest coast of Hawaii, the model shows similar time dependence, although amplitudes are underpredicted by approximately a factor of 2. Increasing the wind speeds near Hawaii yields closer agreement with the observations. Exchange of wave energy between the islands is weak (<10%) but noticeable, particularly in determining the decay rate of the Hawaii island‐trapped wave.