Impact Of Four-dimensional Data Assimilation On The Modelling Of The East China Sea Mesoscale Dynamics And Its Application In Research Of Cross-shelf Penetrating Front

In this work,the four-dimensional variational(4D-Var)data assimilation(DA)of the Regional Ocean Modelling System(ROMS)is applied in the East China Sea(ECS),and the model performance is evaluated systematically.The assimilated data in this study include sea surface temperature(SST),sea surface height(SSH),in situ temperature and salinity profiles,as well as surface drifters and a surface ocean current analysis(OSCAR).Overall,4D-Var performs well in reducing model-data misfit for all observation types.As tidal forcing plays important roles in the shelf circulations of the ECS,tidal forcing was included in 4D-Var and its impact was evaluated by comparing two experiments with and without tides.The biases of SST in DA analyses are small on the continental shelf in both experiments.However,compared with experiments with tides,the absence of tidal forcing will make temperature higher near the surface layer and lower below mixed layer in the background simulation(3-day forecast using DA analyses as IC)in the warm season.The difference in temperature profile is associated with two factors: stratification and tidal mixing.The relative importance of the two factors varies with depth.With the aid of the adjoint model and observation impact method,the impacts on the Kuroshio volume transport(KVT)and the Kuroshio onshore intrusion(KOI)contributed by different types of observations are evaluated,as well as the contributions of IC,BC and FC.SSH,SST and in situ temperatures have large total impacts while in situ temperatures have the largest impact per datum.The geographical distributions of observation impacts are similar for different observation types.Large observation impacts extend along the Kuroshio path from the northeast of Taiwan to the southwest of Japan.Several factors control the geographical distribution,which include the model forecast skills,the dynamic processes that are responsible for the transferring of assimilated information,and the specified error covariance.Tracking how the assimilated information propagate in space helps to advance the understanding of the dynamics of KVT and KOI.Based on the DA results,an important phenomenon of cross-shelf transport in the ECS is analyzed.That is the cross-shelf penetrating fronts(CPFs),which refer to frontal intrusions from mean fronts of oceanic properties like temperature,salinity and chlorophyll-a.CPFs induce significant cross-shelf exchange of water properties and nutrients,and thus are important to coastal environments.In this study,the characteristics and mechanisms of realistic CPFs in the East China Sea(ECS)in summer were investigated based on a data assimilative model.The model reproduced CPFs matched well with satellite observations.Although the cross-shelf currents were predominantly offshore off the Zhe-Min Coast,only three strong CPFs occurred in the summer of 2014.The three-dimensional structure of CPF in the model was similar with that observed in previous research.Two different mechanisms were responsible for the formation of observed CPFs.Two CPFs formed as a result of the convergence of the Taiwan Warm Current(TWC)and the Zhe-Min Coastal Current(ZMCC),while the other one was caused by the undulation of thermocline.In the two mechanisms,the offshore currents associated with CPFs were in geostrophic balance,and they were driven by along-shelf pressure gradient.Sensitivity experiments suggest that southerly wind relaxation and tidal forcing are indispensable conditions for CPF formation.Tidal forcing makes the axis of the ZMCC shift offshore by ~50 km,so that the ZMCC could impinge right against the axis of the TWC.The relaxation of the southerly winds allows the ZMCC to extend southward.Southerly wind relaxation in summer is mostly associated with tropical cyclones(TCs).Without winds and synoptic variation of the TWC,CPFs form periodically due to the strengthening of the ZMCC during neap tide period.For the case of interest,winds and tidal forcing were dominant,and river discharge played a secondary role in the formation of the CPF.