Inthelightoftheproblemofoilpollutionbroughtaboutbyships,inthispaperwepresenttheconceptofbackwardtracingoilspills.Inthecourseofbackwardcalculationofthetwo-dimensionalconvection&diffusionequation,ontheonehand,theadvectiontermitselfhasthestrongunilateralproperty,whichmeansinformationintheupperreachesistransmitteddownstreamviatheadvectionterm;ontheotherhand,becauseoftheoppositedirectionofcalculation,itisessentialforinformationtobeconveyedupstreambymeansoftheadvectionterm.Inaddition,unlikethatintheforwardcalculation,thediffusionterminthebackwardcalculationispronetoaccumulateerrors,andthusrendersthewholeschemeunstable.Therefore,weadoptthecentraldifferencetodealwithboththeconvectionaltermandthediffusionterm.Byexaminingtwopracticalexamples(1)undertheunlimitedboundarycondition,and(2)underthelimitedboundarycondition,itisproventhatthismethodcouldachievefundamentallysatisfactoryresultsnotonlyintheopenoceanbutalsointheclosedorsemi-closedbay.
