Characteristics And Variation Trend Of Wind And Waves In The Yangtze Estuary And Adjacent Seas Over The Past 40 Years

The trend change of wind waves in estuarine and coastal areas under the background of global climate change is a scientific question with important application background.Climate change is one of the emphases of Marine and atmospheric science research.Sea surface wind speed and various wave elements,including significant wave height,have important influences and reflections on the climate change,and are closely related to the energy exchange between the sea-air interface and the mixing layer on the ocean.Existing studies of wind and wave changes have focused on both global distribution changes and local changes in China′s offshore areas.The Yangtze estuary is located on the coast of the east China sea.As one of important dynamic conditions estuary coastal waters,the wind and waves change not only affects the flow field,sediment transport,water,turbidity,the natural process,such as bed stability will also affect the safety of navigation condition and coast protection engineering,so the research of the elements of the Yangtze river estuary waters waves distribution characteristics and change tendency has the vital significance and value.Based on the latest ERA5 and ERA-interim reanalysis data of the European Centre for Medium-Range Weather Forecasts（ECMWF）,this paper conducts a systematic and grid analysis of wind and wave elements in the Yangtze estuary and its adjacent waters in the past 40 years（1979-2018）.In this paper,the wind and wave element data of ERA-Interim data were statistically analyzed,including statistical index of climate state,frequency distribution of each grade,seasonal characteristics,etc.The spatial distribution characteristics and their relations of wind and wave fields were analyzed by EOF decomposition,the periodic time changes were analyzed by Morlet wavelet transform.The mann-kendall test was used to analyze the the trend changes of wind and wave elements in this sea area,and the possible change trend of wave height in this sea area in the future was analyzed in combination with the change of sea level under the background of global warming.In addition,this paper makes a comparative analysis of the latest ERA5 reanalysis data product and ERA-interim.The main research conclusions are as follows:1)Analysis of statistical indicators of wind and wave pentads in the past 40 years:The average wind speed in the study area was 5.76 m/s,the average effective wave height was 0.90 m,and the average wave period was 4.90 s.According to the comparison of quartiles,extreme values and mean values,when extreme weather events occur,the values of each element will far exceed the normal fluctuation range,or even reach several times above the mean value,with the maximum amplitude of the effective wave height.According to the distribution frequency statistics of wind and wave element sites,most of the sea surface wind speed was distributed in the 2⁵ wind force in the past 40 years,with a cumulative frequency of 91.61%,with a maximum of 29.97%in the wind grade 4,while most of the significant wave height was distributed in the range of 0².5 m,and the highest frequency in the light wave grade of 0.5¹.25 m was 58.86%.2)There are seasonal differences in the distribution of wind and wave elements in the sea area:In terms of the mean value,quartile and 90^th%extreme value,the wind speed and wave height in spring and summer are lower than that in autumn and winter,while in terms of the extreme value and maximum value of 99^th%,summer and autumn are higher than that in winter and spring.The statistical results are consistent with the characteristics that the East China sea where the Yangtze river estuary is located is affected by the prevailing east Asian monsoon.The distribution frequency statistics of each monsoon direction and wave direction were carried out according to the sixteen azimuth table,and the distribution frequencies of each monsoon direction and wave direction were calculated.The wind direction with the largest distribution frequency was SSE,S,NNE and N,and the wave direction with the largest distribution frequency was NNE,SSE,NE and N,respectively.The distribution of wind direction and wave direction has a certain similarity,which reflects that the local wind direction is one of the important factors affecting the wave propagation direction.3)Spatial distribution characteristics of wind and wave elements in the study area:The first mode of monthly average wind speed distribution is generally positive deviation,and the distribution is higher and higher in the offshore sea from the inland to the offshore sea,while the contour distribution is similar to the contour of the longitude or coastline,indicating that the sea-land difference may be the main factor affecting the distribution form of wind speed field.The first mode of wave height is positively deviated in the study sea area,and the offshore sea is getting higher and higher than the offshore sea.The offshore isoline east of 122°30′is distributed along the longitude line,and there are two low value areas west of 122°30′,indicating that the sea-land difference and the island topography distribution are the main factors affecting the spatial distribution of wave height.4)Wavelet analysis of monthly wind speed and wave height near sheshan station shows that the strongest periodic scale change in 40 years is 227 months（about 18.92years）,and there is still a 1.5 year periodic scale in local location.The results of trend analysis show that the average annual sea surface wind speed and effective wave height of the changjiang estuary and adjacent sea areas have been on the rise in the past 40years（1979-2018）,with the wind speed increasing by 2.1%and the wave height increasing by 5.4%.The results pass the test of significance level of 5%.The variation trend of monthly average wind speed and wave height calculated by seasonal classification is positive and negative.The variation trend of wind speed and wave height calculated based on ERA5 reanalysis data is consistent with the calculation result of era-interim.