Weather-type statistical downscaling for ocean wave climate in the Chinese marginal seas

A comprehensive understanding of wave characteristics and their variability, based on reliable long-term wave data, is essential in the design, construction, operation and management in offshore and coastal applications. This study aims to downscale multivariate wave data in the Chinese marginal seas at different time scales by employing two weather-type statistical downscaling models. The calibration of both models involves the usage of historical data from ERA5 reanalysis, with sea level pressure and the squared sea level pressure gradient as the predictors and wave parameters as the predictands. The predictor definition considers the swell components in the local wave data. Both models categorize the atmospheric predictors into different weather types using the K-means algorithm (KMA), with and without regression-guided clustering (referred to as WTD-RG and WTD, respectively). Each weather type is associated with sea state parameters. The downscaled wave data from both models are validated against ERA5 data. The results show that both models can generally reproduce the wave statistics for the significant wave height, mean wave direction, mean wave energy flux, and 95th percentile of the significant wave height. However, the WTD-RG model performs better than the WTD model. Additionally, the ERA5 data reveal increasing trends in the mean significant wave height for most areas of the East China Sea and South China Sea and decreasing trends in the Bohai Sea, Yellow Sea, and Beibu Gulf from 1959 to 2021. These patterns are well captured by the WTD-RG model but not by the WTD model.