Study On The Influence Of Typhoon Process On Marine Dynamic Environment In Coastal Waters Of East China Sea

Under the background of global warming,typhoons affecting the coast of mainland China are strengthened and their landing locations are moving northward.The East China Sea is a more frequent area affected by typhoons.Typhoon is one of the weather events with the most severe impact on the Marine environment in short time scale,and it has a significant impact on the Marine dynamic environment.Based on field observation data and multi-source satellite remote sensing reanalysis data,this paper used the methods of wavelet analysis and wavelet coherence analysis,the influence of five typhoon processes with different paths and intensities in 2019 on the coastal currents in the East China Sea and their environmental effects were comprehensively analyzed.And discussed the influence process of super typhoon"Lekima"on the coastal upwelling structure in the East China Sea.The evolution of coastal current and upwelling in the East China Sea under the influence of typhoons has been systematically understood.Firstly,the main influencing factors of the coastal currents in the East China Sea under normal sea conditions and the evolution process under the influence of typhoons are studied.The results show that the local wind was the main dynamic factor affecting the evolution of the current.The surface current was mainly affected by the wind,and the bottom current was also controlled by the wind during typhoons.Typhoons of different paths had different impacts on the coastal current.Typhoons Lekima and Mitag directly through the East China Sea had greater impacts on the coastal current,while the three typhoons farther away had less impacts.The typhoon injected energy into the ocean,the tide signal disappeared,and typhoons Lekima and Mitag stimulated the current to produce a new periodic signal for 3～6 days and 1～3days,respectively.During the typhoon,the change of wind field caused the change of flow field,which resulted in the northeast transport of Zhejiang and Fujian coastal current to southwest transport in summer.Before typhoon landfall,under the action of the continuous northeasterly wind,the southwest current was generated,and the sea water kept piling up to the shore,causing the coastal water level to rise.After the typhoon landed,the wind direction changed,the coastal circulation shifted to the northeast direction,offshore sea water transport occurred,the coastal water level decreased,the sea surface temperature near the typhoon path decreased,the temperature range of the right side of the path were greater than the left side.Then,the evolution process of the upwelling system in the coastal waters of the East China Sea from normal sea conditions to typhoon and its influence mechanism are studied.The results showed that there was a low temperature zone in the coastal area,and there were low temperature centers outside the Changjiang estuary and Jiaojiang estuary,respectively.It proved that there is upwelling in the coastal of the East China Sea in summer,and the upwelling outside the Jiaojiang estuary is generated in June,strong in July,and gradually weakened in August.Under the action of persistent and stable southwest wind,the upwelling in the coastal waters strengthened and extended to the coastal area.During the approach of the Typhoon"Lekima",the locally stable southwesterly winds disappeared and the strength of the offshore upwelling weakened.With the typhoon moving,the vertical circulation gradually disappeared,and the low-temperature subsurface water in the outer sea was inhibited and could not climb along the terrain to the coastal waters,resulting in the disappearance of upwelling in the nearshore shallow waters.Finally,based on the analysis of the change of coastal current and upwelling current under the influence of typhoon,the influence of typhoon on the Marine environment in the inner shelf area of the East China Sea was studied.The results showed that typhoons trigger a strong vertical mixing of the ocean,with large amounts of underlying material carried to the upper layers through re-suspension.Under normal sea conditions,the daily mean flux in the coastal direction of the current at W1 station is about-2.0×10~5～2.0×10~5m~3.When the super Typhoon"Lekima"directly affected the study area,the water transport to the northeast was 4times as much as that to the normal period in a single day,and to the southwest was2-3 times as much as that to the normal period in a single point.During the typhoon period,the heavy rainfall caused the river runoff to increase rapidly,and increased the water transport from the river to the sea,it had a certain influence on the biological growth and the stability of the ecosystem in this area.In conclusion,typhoon process had immeasurable impact on Marine deposition,hydrodynamics and Marine biological ecological environment of inner shelf of East China Sea.