Studies Residual Flow Dynamics In Shallow Waters

Residual flow in shallow waters plays a key role in determining the net exchange of salt,water,and other biologically and chemically important materials in the waters such as nutrients and pollutants.Due to the complicated coupled interactions among possible driving factors,such as wind,river input,tidal current,density distribution and circulation in the open sea.it is difficult to quantify the contribution of each factor to the total residual flow,especially for the regions where nonlinear interactions are strong.Therefore,the dynamics of residual flow in many coastal seas remain unclear.Review of the previous researches indicates the following difficulties.The residual nonlinearity is neglected improperly.The linearization of bottom friction needs theoretical support.In addition.the horizontal diffusive coefficient is difficult to be determined.Therefore.a new scheme for linearizing bottom friction is proposed based on the assumption that the residual flow is much weaker than tidal flow.Two kinds of residual flow dynamics are considered for residual flow dynamics in this study,one with neglection of the residual nonlinearity and the other with neglection of the horizontal diffusive process.The traditional former has two problems:(1)real open boundary condition is unavailable;(2)the influence of the neslection of residual nonlinearity may be accumulative.:Fhey may induce unexpected deviation.The latter,based on natural Helmloltz-Hodge decomposition,is proposed to in this study indicate the local influence of driving stress with curl.It is a kind of local potential residual flow,and it completely avoids the previous two problems.Potential residual flow solution makes it possible to evaluate the contribution of each driving force to residual circulation.Ideal numerical experiments are designed and conducted to investigate the advantages and disadvantanges of traditional linear residual flow dynamics.In reference case,the horizontal diffusive coefficient is set very high to make the diffusive process dominant in residual flow dynamics.High diffusive coefficient is also good for neglection of residual nonlinearity.Based on result of reference case,the new scheme of linearizing bottom friction is estimated and tested.It shows that the new scheme could present the averaged bottom friction better than the traditional method.The influence of open boundary condition,residual nonlinearity and horizontal diffusive coefficient are also discussed.The results show that the averaged flow in reference case could be reproduced with real open boundary condition;and neglection of residual nonlinearity has little influence on the averaged flow in reference case;and the horizontal diffusive coefficient has significant influence on reproduced averged flow in reference case.Ideal cases are also designed to investigate the influence of important parameters(bathymetry,bottom friction coefficient,Coriolis parameter and horizontal diffusive coefficient)on residual flows.The numerical results show that residual flows are very sensitive to the previous parameters.This means that an accepted ocean current numerical model plays important role in analysis of residual flow dynamics as observation of most ocean state variables are not available.The operational ocean model of the Taiwan Strait(TFOR)based on Regional Ocean Modeling System(ROMS)is introduced and its Performance is assessed.Model-data comparisons based on tracers,temperature,and salinity demonstrated that the model can reproduce the 3-D structure of the hydrographic properties and identify important oceanic processes,such as upwelling and river plumes.Comparisons of the SST patterns between the MODIS data and the model results indicated that the intrusion of cold Chinese coastal water into the eastern Taiwan Strait during the winter changes the SST dramatically and that this change is difficult to replicate,especially in the northwestern part of Penghu island,where it connects with the Kuroshio Branch Current and China Coast Current.Nevertheless,the TFOR can reproduce the variations in the SST to an adequate extent and it is useful for further investigation.The analyses of the tidal characteristics showed that the distributions of the M2 tide constants were consistent with previous studies,although the ellipse currents deduced by the model differed slightly from those based on the buoy data.Statistical analyses of the model simulations showed that the mean annual and seasonal transports were comparable to those reported in previous studies.Moreover,suecessful applications to maritime search and rescue operations indicated the reliability and usefulness of the TFOR products.Based on the TFOR and its products,the ocean model with high resolution for XMB is setup.The XMB averaged flow is simulated in 20 June to 20 July 2011 and 20 December 2010 to 20 January 2011 with meteorological climatological variables and the TFOR product for open boundary.Both XMB averaged flows in winter and summer are presented.They show that relative vorticity of the averaged flows may be too strong to be neglected.Based on natural Heimholtz-Hodge decomposition,the drivng stress with curl includes tide-induced stress,wind-inducde stress and density-induced stress.Their potential residual flows are presented to indicate the contribution of each driving force to residual flow.The averaged flow,apart from total residual flow,is treated as the contribution of open boundary condition.The density-induced flow and the contribution of the open boundary condition is treated as the influence of JRE and the circulation in the Taiwan Strait.Some important conclusions are made in this study.Wind-induced residual flow in winter is stronger than that in summer,but both of them are weak.XMB averaged flow is dominated by tidal stress all the year around.Density.induced residual flow in summer is stronger than that in winter,and both of them move toward the JRE.Finally,this study shows that,averaged elevation has important influence on the distribution of the averaged flow,and the averaged elevation could only be obtained via observation.