From Global to Regional-scale CMIP6-derived wind wave extremes: a HighResMIP and CORDEX downscaling experiment in South-East Australia

Abstract This study investigates the influence of high-resolution CMIP6 10-meter surface wind fields on wave climate dynamics in the South-East Australian region. We nest a regional unstructured grid spectral wave climate model within a global state-of-the-art spectral wave climate model to conduct our modelling experiments. The primary objective is to compare four distinct dynamical downscaling approaches of a similar GCM product: CMIP, AMIP, HighResMIP, and a CORDEX downscaled ocean surface wind speed product corrected for SST and sea ice bias and variance. Of particular interest is the comparative performance between HighResMIP's 25km spatial resolution wind speed forcing and CORDEX's 10km resolution wind speed downscaling approach in replicating wind-wave climate extremes, as these products are currently the most appealing to downscaling wind wave climate extremes at the regional level. Our findings emphasize the critical importance of climate model wind-forcing downscaling for ensemble statistics of future regional extreme wave climate projections, which go beyond the sole impact of spatial resolution. Through detailed analysis, we describe the characteristics of each climate model's downscaled wind speed input that impacts wind wave climate extremes in a region characterized by diverse wind wave climate conditions, ranging from local wind sea to swell conditions. These insights are valuable for estimating both past and future projected coastal flooding and erosion patterns and hold relevance for coastal risk assessment studies.