基于WW3的南海北部有效波高时空变化及其极值重现期估算方法分析

Analysis of the spatio-temporal variations of significant wave height in the northern South China Sea and the return period estimation methods of its extreme based on WW3

本文基于第3代海浪模式WAVEWATCHⅢ(WW3)模拟的1996–2015年海浪后报数据,分析了南海北部有效波高及其极值的时空变化特征,并采用Pearson-Ⅲ和Gumbel两种极值分布方法对该区极值波高重现期进行了估算。结果表明,南海北部有效波高的季节变化和空间分布与季风风场基本一致,呈现秋冬高春夏低,并自吕宋海峡西侧向西南降低的特征,与ERA5再分析数据结果高度相似。有效波高极值(简称极值波高)的时空分布特征受时间分辨率强烈影响,采用极值数据的分辨率越高(如逐小时),所展现的台风型波浪特征越显著。扣除季节变化信号后的有效波高和年极值波高均体现出较强的线性增高趋势,近20年升高的比例分别为7.7%和31.6%,值得警惕和关注。该区多年一遇极值波高存在若干个大值区,且与台风的路径、强度有直接联系,表明台风是引发该区域极端大浪的最主要机制。对比Pearson-Ⅲ和Gumbel极值分布估算结果发现:若极值波高较低,频率随极值波高升高缓慢降低,此时两种极值分布的估算都比较准确,差异极小,可忽略不计;但当研究时间范围内,某年极值波高远超其他年份时,Pearson-Ⅲ极值分布估算结果明显高于Gumbel极值分布估算结果,且更接近实际极值波高,即Pearson-Ⅲ极值分布在此情况下表现更好。本研究表明对于特定海区,在出现超强台风引发极值波高远超出其他年份时,不同计算方法对极值波高的估算差异较大,会显著影响重现期的评估。此外,南海北部年极值波高的强烈增高趋势,也可能给计算未来极值波高重现期和海上工程防护带来不可忽视的影响。

The spatio-temporal variations of significant wave height and its extremum in the northern South China Sea from 1996 to 2015 are analyzed based on the wave hindcast simulation by the third-generation wave model WaveWatch Ⅲ(WW3).Two extreme value distribution methods,Pearson-Ⅲ and Gumbel methods,have been used to estimate the return period of the extreme significant wave height in the northern South China Sea.The results show that the seasonal variation and spatial distribution of significant wave height in the northern South China Sea are consistent with the monsoon field,similar to the results of reanalysis data.It is high in autumn and winter and low in spring and summer,and decreases from the west part of the Luzon Strait to the southwest.But the extremum of significant wave height is strongly affected by the temporal resolution.The higher resolution(such as hourly),the more typhoon wave characteristics are displayed.After deducting the seasonal cycle signal,both the significant wave height and its annual extremum show an intense linear trend,increasing by 7.7% and 31.6% in the last 20years respectively.There are several large value regions of the return period wave height in this area.They are related to the typhoon’s track and intensity directly,indicating that typhoons are the primary mechanism causing extreme waves in this region.Comparing the Pearson-Ⅲ and Gumbel distribution,it is found that if the extreme significant wave height was relatively low and the frequency decreased slowly with the growth of the extremum,the two methods are both accurate,with little difference which could be ignored.However,when the extreme significant wave height was much higher in one year than that in other years,the estimated result of the Pearson-Ⅲ would be much higher than that of the Gumbel method,and also closer to the actual value.In other words,the Pearson-Ⅲ extreme value distribution behaves better in this situation.This study shows that when the extreme significant wave height caused by a super typhoon is much higher than that in other years,the estimation from different methods differs greatly,which will significantly affect the assessment of the return period.Besides,the strong increasing trend of the extreme significant wave height in the northern South China Sea will also bring a non-negligible impact on the calculation of return period wave height and marine engineering protection in the future.