Trend and attribution analysis of extreme wave intensity in the China Sea

As one of the most important dynamic processes in the ocean, ocean waves and the spatiotemporal variations in their extreme intensity are receiving increasing attention. To determine the variation characteristics of extreme wave intensity in the China Sea and analyze the main reasons for the variations, ERA-5 (released by the European Center for Medium range Weather Forecasts, ECMWF) wave and atmosphere reanalysis data are used. Based on a large number of buoy wave observation data, the accuracy of the ERA-5 significant wave height reanalysis data is evaluated. Then, the Sen slope estimation and Mann-Kendall trend detection methods are applied to analyze the temporal and spatial trends of extreme wave and average wave intensity in the China Sea over the past 40 a (1979−2018). The interannual trend and the trends during typhoon wave prevailing period (June to October) and cold air wave prevailing period (November to March of the following year) are analyzed by Sen and Mann-Kendall methods. In addition, by analyzing the trends of extreme sea surface wind speed in the China Sea, the causes of extreme wave intensity changes are investigated. The number and intensity of tropical cyclones that affect the south of the East China Sea, where the extreme wave intensity has altered significantly, are also analyzed for the attribution analysis. Herein, the 99.5% percentile of the significant wave height is used to characterize the extreme wave intensity. The analysis results show that: (1) The accuracy and stability of ERA-5 wave reanalysis data in the China Sea are very high, with the root mean square error of 0.264 m, relative error of 12.65% (when significant wave height is not less than 1 m), scatter index of 22.96%, and correlation coefficient of 0.956. (2) Regarding the trend of the average wave intensity, a significant interannual enhancement trend of significant wave height in the East China Sea and the South China Sea are observed, with the maximum amplitude of the trend being + 0.5 cm a−1. (3) The variation trend of extreme wave intensity denotes that in the south of the East China Sea, there is a significant interannual enhancement of the 99.5% percentile significant wave height, with the maximum amplitude of the trend being + 5 cm a−1, which is consistent with the trend characteristic and amplitude during the typhoon wave prevailing period. This maximum amplitude of the trend of extreme wave intensity is one order of magnitude larger than that of the average wave intensity. This shows that the extreme wave intensity has a greater enhancement trend than the average wave intensity. In addition, in the southern part of the South China Sea, the extreme wave intensity in the typhoon wave prevailing period and cold air wave prevailing period as well as interannually, shows increasing trends, with the maximum amplitude of the trend being + 3 cm a−1. (4) In terms of the attribution analysis for the above changes, the interannual enhancement trend of extreme sea surface wind speed is the direct reason for the interannual enhancement of extreme waves in the south of the East China Sea and the southern part of the South China Sea. In addition, the total number of strong typhoons and super strong typhoons per year that affect the south of the East China Sea has increased significantly, leading to the aggravation of the extreme wave intensity in this region.