The Numerical Simulation Of Circulations, Mesoscale Eddies And Submesoscale Processes In The East China Sea

The East China Sea(ECS)is located in the mid-latitude part of the Northwest Pacific Ocean,adjacent to mainland China,with a latitude range of approximately 24°N-41°N and a longitude range of approximately 120°E-129°E.ECS has a unique dynamic system forced by monsoons,tides,western boundary currents,diluted water and boundary water exchanges,and modulated by various characteristic terrains,including semi-closed ocean basins,continental shelves and trenches.This work uses a three-layer online nested model,the Coastal and Regional Ocean COmmunity model(CROCO),to study ECS under large-scale,mesoscale and sub-mesoscale frameworks.The sea surface temperature(SST),sea surface salinity(SSS),sea surface height anomaly(SSHA),mean temperature vertical structure,surface current field,and tidal simulation from the 1/12° model(first-level nesting model)results are similar to those from the observational data.On a large scale,SST,SSS,and sea surface height(SSH)fields from the 1/36° and1/108° model(second-and third-level nesting model)are almost the same as those from the first-level nesting results.The boundary transitions between each level are smooth.The circulation system of the ECS is analyzed under the large-scale framework using the results of the first-level model.Water mass transports in boundary channels show annual,semiannual,30-month,50-day to 200-day variation periods.Local wind changes and mesoscale eddies propagating from the Northwest Pacific can induce water mass transport anomalies,while the central part of ECS basically follows the geostrophic equilibrium.Ageostrophic occurs in the coastal areas and around the islands where the advection and vertical mixing are important.Wind stress and tidal forcing have a greater impact on the mesoscale activity of the open ocean,which further affects the path and intensity of the Kuroshio and change the volume transport in relate starits.Under the mesoscale framework,the mesoscale eddies in ECS in 2018 are studied using altimeter merged data generated based on a two-dimensional variational(2-DVar)method and the second-level model results.The results of merged data show that the number of cyclonic and anticyclonic eddies is the same.The lifespans and mean radii of most eddies are less than5 weeks and 50 km-60 km,respectively.The eddies detected in the open ocean are abundant,with large radii and long lifespans.The model results show that the eddy polarities are asymmetric across the Kuroshio.Most of the radii are below 50 km,and most have lifespans that are less than 1 week.Instability may be present on both sides of the Kuroshio,and the eddy generation is mainly caused by the baroclinic instability induced by horizontal shear and buoyant flux.Under the submesoscale framework,the third-level model results are used to analyze the submesoscale processes in the Kuroshio front and its vicinity in the central ECS.Submesoscale processes can be characterized by large Rossby numbers,strong vertical velocity,and strong horizontal density gradients.Submesoscale processes in the mixing layer of the upper ocean are more active than those below the mixing layer.The higher the model resolution,the more active the submesoscale processes are.Selecting the sub-mesoscale process analysis in January2018,it is found that there are various instabilities in the flow field.The mixed layer instability is the main mechanism for the generation of sub-mesoscale processes in this case,and the frontogenesis process is the secondary mechanism.The energy storage and conversion in this case is quantified,where the background kinetic energy is the largest,and the sub-mesoscale kinetic energy and potential energy are 3 orders of magnitude smaller than the background kinetic energy.The main source of sub-mesoscale kinetic energy is the release of potential energy caused by baroclinic instability,accounting for 89.8%,and the shear effect of the flow field accounts for 9.2%;the sub-mesoscale kinetic energy loss is mainly caused by forcing and dissipation,accounting for 64.7%,and buoyancy accounting for 35.3%.The source of submesoscale potential energy is mainly caused by forcing and dissipation,accounting for 96.3%;the loss of sub-mesoscale potential energy is mainly caused by baroclinic instability,accounting for 87.6%.