Dynamically downscaled future wave projections from SWAN model results for the main Hawaiian Islands

Projected wave climate trends from WAVEWATCH3 model output were used as input for nearshore wave models (for example, SWAN) for the main Hawaiian Islands to derive data and statistical measures (mean and top 5 percent values) of wave height, wave period, and wave direction for the recent past (1996-2005) and future projections (2026-2045 and 2085-2100). Three-hourly global climate model (GCM) wind speed and wind direction output from four different GCMs provided by the Coupled Model Inter-Comparison Project, phase 5 (CMIP5), were used as boundary conditions to the physics-based WAVEWATCH3 numerical wave model for the area encompassing the main Hawaiian islands. Two climate change scenarios for each of the four GCMs were run: the representative concentration pathway (RCP)-4.5 and RCP-8.5, representing a medium mitigation and a high emissions scenario, respectively. Simulation timeframes were limited to the years 2026-2045 and 2085-2100, as prescribed by the CMIP5 modeling framework. The WAVEWATCH3 modeled deep-water wave heights, wave periods, and wave directions, with current bathymetry were used as boundary conditions to drive simulations of mean and top 5 percent wave conditions at higher resolution over the insular shelves of the main Hawaiian islands using the 3rd-generation SWAN wave model. For each scenario, 12 simulations were made representing the month-averaged or top 5 percent conditions. The SWAN model is based on discrete spectral action balance equations, computing the evolution of random, short-crested waves. Physical processes such as bottom friction and depth induced breaking, and, non-linear quadruplet and triad wave-wave interactions are included. Wave propagation, growth, and decay are solved periodically throughout the model grid. The SWAN model has been shown to accurately model the propagation and breaking of waves over Pacific coral reefs.