Sea Surface Height And Current Responses To Synoptic Winter Wind In The Bohai,Yellow And East China Seas

Coastal trapped waves(CTWs)dominate the sea surface height(SSH)and current responses to synoptic wind in shelf seas.CTWs play an important role in understanding the' physical process in shelf seas.They are also of great application value for improving the prediction of SSH and current in coastal area.CTWs features are very prominent in the Bohai,Yellow and East China Seas(BYES).However,the basin-scale propagating features and mechanisms of CTWs in the entire BYES are still unclear.Both in-situ observation and a numerical model(MITgcm)are used to investigate the SSH and current responses to synoptic winter wind in the BYES.The winter mean of simulated temperature,salinity,SSH and current show good agreement with previous studies.Moreover,the time series of simulated current can well reflect the synoptic variation of the observed current.The SSH and current responses to synoptic winter wind are very energetic at 2-5 days period throughout the entire BYES.The current is closely associated with SSH through geostrophic balance.The basin-scale responses can be well interpreted with two leading CTWs which are captured by complex empirical orthogonal function.The first CTW explains 76%of the total SSH vanance and approximately 28%of the total current variance.It dominates the SSH variation in the entire BYES and current variation mainly in the Bohai and Yellow Seas(BYS).The CTW is a free Kelvin wave and propagates cyclonically in the entire BYES around one SSH node to the southeast of Cheju Island as a relaxation response to strong northerly wind bursts in winter.The second CTW explains 17%of the total SSH variance and approximately 46%of the total current variance.It dominates the current variation mainly in the western East China Sea(ECS).This CTW is a wind-forced lowest shelf wave.It propagates cyclonically in the BYS and norttieastem ECS around two SSH nodes and southward along the western ECS with two SSH nodes on the northerm and southerm ECS shelf,respectively.Farther away from the nodes,the wavelengths are getting longer and the e-folding scales of wave amplitudes are getting larger.The CTWs show prominent regionally distinct propagating features and mechanisms in the ECS and BYS,respectively.In the western ECS,the wavelengths are very long and the e-folding scales of wave amplitudes are very small.The geostrophic currents associated with the CTWs are primarily perpendicular to the co-phase line of SSH,and the current magnitudes are proportional to the wave amplitude and inversely proportional to the e-folding scale of wave amplitude.In the semienclosed BYS,because of the wave superposition,the wavelengths are relatively short and the e-folding scales of wave amplitudes are very large.The geostrophic currents associated with the CTWs are primarily parallel to the co-phase line of SSH,and the current magnitudes are proportional to both the wave amplitude and wavenumber.These prominent spatial variations of wavelengths and e-folding scales of wave amplitudes are responsible for the regionally distinct features of CTWs.The momentum balances show that the major axis current is in a predominant geostrophic balance with the minor axis pressure gradient.In terms of the generation and propagation of CTWs,distinct dynamics are recognized in the ECS and BYS.In the western ECS coastal area,the major axis current is accelerated by the pressure gradient,wind stress and bottom stress,and the frictional effect is very important.In the BYS,the major axis current is mainly accelerated by the pressure gradient,and the frictional effect is not important.