Spatio-temporal Variations Of Sea Surface Wind And Analysis Of The Impact Of Tropical Cyclones On Coral Reef Areas Of The South China Sea

Using remote sensing data to analyse the wind speed in coral reef areas,including sea surface wind(SSW)and tropical cyclone(TC),is an important content to investigate the environmental stress of coral reef areas,which can lay a foundation for the protection of coral reef resources.Based on the SSW,this study analysed the spatio-temporal variations of SSW in the coral reef areas of the South China Sea from 1988 to 2017.The distributions of TCs in coral reef areas of the South China Sea from 2010 to 2019 were analysed by using the best track data of TCs,and the impact degree grades of TCs on different coral reef areas were also evaluated.The main results are as follows:(1)The maximum annual SSW appeared in Dongsha Islands,and the minimum appeared in Nansha Islands.The multiyear monthly variations of SSW in the coral reef areas presented a ‘W'-shaped structural variation.The maximum annual SST appeared in Nansha Islands,and the minimum appeared in Dongsha Islands.In spring,the SSW in the four coral reef areas were relatively small and the wind directions were complex.In summer,the frequency of southwest wind over Nansha Islands was the highest,and the SSW was about 7 m/s.The SSW in Dongsha Islands was the lowest.In autumn,northeast winds almost prevailed in the coral reef areas,and the SSW was about6 m/s.Nansha Islands has both a high frequency of northeastly wind and southwestly wind.In winter,northeastly wind prevailed in the four coral reef areas,with a frequency at about 80%.Dongsha Islands had the highest wind speed,and Nansha Islands had the lowest.The maximum SSW appeared in Dongsha Islands,and the minimum appeared in Nansha Islands.The multiyear monthly variations of SST in the coral reef areas approximately presented an‘M'-shaped structural variation,which was contrary to the ‘W'-shaped structural variation of SSW.The SST change in the South China Sea not only lagged behind the El Ni(?)o 3.4 index by about 7 months,but also decreased in intensity.(2)The EOF analysis of the SSWA showed that the changes of SSWA in Dongsha Islands,Xisha Islands,Zhongsha Islands and Nansha Islands were consistent in the first mode.The spatial distribution of the second mode of SSWA in the South China Sea showed that there was an inverse relationship between the variation of Nansha Islands and the other three coral reef areas.The EOF analysis of the zonal wind showed that the increase and decrease of the zonal wind in Dongsha Islands,Xisha Islands,Zhongsha Islands and Nansha Islands were consistent in the first mode.The spatial distribution of the second mode of zonal wind in the South China Sea showed that there was an inverse relationship between the variation of Nansha Islands and the other three coral reef areas.The EOF analysis of the meridional wind showed that the changes of the meridional wind in Dongsha Islands,Xisha Islands,Zhongsha Islands and Nansha Islands were in the same phase in the first mode.The spatial distribution of the second mode of meridional wind in the South China Sea showed that there was an inverse relationship between the variation of Nansha Islands and the other three coral reef areas.The time coefficients of the first mode of SSWA,zonal wind and meridional wind all showed an obvious one-year cycle.The time coefficients of the second mode all showed obvious annual and semi-annual cycles.(3)In the past 10 years,the total number of TCs occurred in 2010 and 2011 was relatively small,and the frequency of TCs in other years was not significantly different.STY frequency was the least,and Super TY frequency was the most.The TC grade affecting the coral reef areas in the South China Sea was mostly Super TY,and STS rarely affected the coral reef areas.The peak occurrence period of TCs affecting the coral reef areas was from August to October.The wind speeds in the coral reef areas were mostly below 41.5 m/s.No TCs affected coral reef areas in 2011,2015 and 2019.In 2012,2013 and2017,coral reef areas were affected by TCs for a long time.TCs had a greater impact on the northern part of the South China Sea,and most TCs were STY and Super TY.From 2010 to 2019,Dongsha Islands was the most severely affected by TCs,with the highest impact degree up to grade V,followed by Xisha Islands,with the highest impact degree up to grade IV.Zhongsha Islands and Nansha Islands were less affected by TCs,with the impact degree almost at grade ?.