Structural Characterization Of The Upper Mixed Layer In The South China Sea

The Upper Mixed Layer(UML)is an oceanic boundary layer with strong vertical mixing and dissipation,and almost vertically uniform temperature,salinity and density,formed under the joint action of external forcing at the sea surface and internal dynamical processes in the ocean.The oceanic mixed layer is not only a gathering place for oceanic primary productivity,but also a bridge for direct momentum and heat transport between the ocean and the atmosphere,contributing significantly to large-scale sea-air interactions.The study of mixed layer dynamics and thermodynamics has been a hot issue in physical oceanography.Mixed Layer Depth(MLD),which is the depth where the mixed layer is located on the ocean,represents the maximum depth that can be reached by the vertical mixing of ocean turbulence,and directly reflects the degree of response of the upper ocean to various external forcing.In this paper,we use the measured CTD(Conductivity Temperature Depth,CTD)and turbulent dissipation data in the South China Sea from 2015-2018 to conduct a study related to the calculation of the upper mixed layer thickness and its spatial and temporal variability.In this paper,by introducing the ratio of the standard deviation of temperature from the sea surface to the mixed layer depth as the Quality Index(QI),and using QI as the only judgment criterion for the accuracy of MLD,the calculation results of the MLD calculation method have a unified judgment criterion.This thesis adopts the threshold method and the relative change method to optimize the pooling method,and uses QI as the judgment criterion,so that the optimized pooling method is applicable to most sea areas,with simple calculation,stable performance,and high accuracy of results.The MLD and its spatial and temporal variation characteristics of the South China Sea derived from the 2015-2018 cruise observations are calculated by the ensemble method,and the mixed layer structure characteristics of the central South China Sea basin are analyzed to explore the variation mechanism.The MLD in the southern part of the South China Sea is generally larger than that in the northern shelf area,and the average depth of the mixed layer in the southern part of the South China Sea is about 35 m,while that in the northern part of the South China Sea is about 14 m.The MLD in the central basin of the South China Sea is significantly larger than that in other areas,and the maximum value of the MLD is located in the eastern side of 10°N,with a maximum value of about 70 m.The MLD in the sea near the mouth of the Pearl River has a strong mixing effect due to the frequent exchange of water bodies,and its MLD is 3-10 m deeper than that in adjacent areas.The analysis of the MLD in the northern shelf area of the South China Sea during the summerautumn transition shows that the surface layer of seawater warms quickly in summer and the subsurface layer warms slowly,and the convective mixing is weak in summer,so the upper layer of seawater does not mix strongly and the mixing layer is shallow.In autumn,the sea surface temperature decreases continuously,convective mixing strengthens,and the mixing layer deepens continuously.The relationship between the MLD and the thickness of the mixing layer(XLD)during the autumn transition period was investigated by combining the measured hydrographic and meteorological observations and reanalysis data of the 2018 Central South China Sea Basin cruise.The average MLD was about 21 m,and the maximum value(58 m)was found in the eastern side of the 10°N sea area.From the Monin-Obukhov stability parameters,we know that the mixing in the upper layer of the South China Sea is controlled by the wind field and buoyancy fluxes jointly during this period,and becomes dominated by buoyancy fluxes due to the enhanced shortwave radiation during the daytime,and becomes dominated by wind fluxes due to the larger momentum flux input from the surface wind field on the west side of the 12°N sea area.From the quantitative calculations of the mixing components,it is found that the mixing is mainly caused by the net seaair heat flux during the daytime,and the net sea-air heat flux is driven by the combination of wind,current and sea-air heat flux at night.It is assumed that the difference between MLD and XLD is smaller due to the increase of solar shortwave radiation flux which strengthens the seawater lamination,and the weakening of wind kinetic flux input which reduces the turbulent motion of seawater,resulting in the decrease of MLD and XLD.The thesis has 22 figures,3 tables,and 67 references.