| The expansion of the Changjiang diluted water (CDW) is a dominant hydrographic phenomenon in the East China Sea. Observations clearly show that there are some isolated low-salinity water lenses (LSWLes) in the expansion area of the CDW, which change the traditional structure of the Changjiang River plume, and also the characteristic of salinity in the East China Sea. However, studies on this phenomenon are scarce.In this paper, basing on the analysis of the advantages and disadvantages of some vertical coordinates applied in the numerical calculation on the expansion of the CDW, includingσ-level, z-level and general hybridσ-z coordinates, we design a new vertical hybirdσ-z coordinate, which useσcoordinate for current and hybridσ-z coordinate for salinity. The new hybridσ-z coordinate is in the form of upperσcoordinate and lower z coordinate, and the interfacial line betweenσand z coordinates located at level H0 (z = H0). A new model, namely POM-σ-z, is developed by using the new vertical hybridσ-z coordinate into POM. In POM-σ-z, the baroclinic pressure gradient force is calculated on the salinity layers, and Euler-Lagrange method (ELM) is used to join the calculation of current and salinity.In order to compare the capability of POM-σ-z and POM, they are applied into the numerical simulations of the expansion of CDW and LSWL. Expansions of the CDW in September 1996 and August 2000 are simulated firstly. Results clearly show that POM-σ-z can better reveal the characteristic of the expansion of CDW than POM. Simulated results of POM-σ-z can display the estuarine front and the plume front of CDW, and also the front in Hangzhou Bay. Moreover, simulation results of POM-σ-z compare well with the observation data including seven stations in Sep 1996 and six vertical sections in Aug 2000. Previous modeling studies on the expansion of the CDW mostly can not reveal the LSWL. In this paper, the actual LSWL in Aug 1975, Aug 1977, Aug 1980, Aug 1983, Aug 1986, Aug 1991, Aug 1996 and Aug 1997 have been simulated with POM-σ-z and POM. Simulation results show that the two models all can reveal some of LSWL. POM-σ-z can simulate six LSWL, and its simulated LSWLes in Aug 1977, Aug 1983, Aug 1986 and Aug 1997 are consistent with the observations. While POM can only simulate five LSWLes, and its simulated LSWLes in Aug 1977 and Aug 1997 are consistent with observations.A numerical model for an idea estuary similar to the Changjiang River Estuary is set up to examine the impact ofβeffect, baroclinic effect, bottom topography, river outflow and wind on the expansion of river plume and the formation of LSWL. The simulated results show that the scales of the model-LSWLes from the change of wind are consistent with the observations, which indicate that the change of wind may be an important mechanism for the LSWLes.Basing on the numerical simulations of POM-σ-z, we analyze the characteristics of model-simulated LSWLes in Aug 1977, Aug 1983, Aug 1986 and Aug 1997, and classify them into three types: wind-induced LSWL, tidal-induced LSWL, wind-tide induced LSWL.A series of numerical experiments using POM-σ-z are set up to analyze the dynalmical mechanisms of the LSWLes in Aug 1977, Aug 1983, Aug 1986 and Aug 1997. The results indicate that wind makes important influence on each LSWL. Wind is the main dynamical mechanism for the formation and evolution of the wind-induced LSWL and the wind-tide induced LSWL, and the tide-induced LSWL can formed usually in the appropriated wind condition that it is helpful for CDW expanding eastward and northeastward. The influence of tide on LSWL is complex. In general, the effect of tidal mixing restrain the formation of LSWL.While the vertical tidal mixing from neap tide to spring tide vails to the formation of LSWL. And the tidal Lagrange residual current is useful to LSWL’s formation in the northeast out of the Changjiang river mouth. The river discharge and the Taiwan Warm Current make little influence on the formation of LSWL. |
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