Seasonal Variation Of Three-dimensional Structure Of Thermal And Salinity Fronts In The Northwestern South China Sea

Ocean front is an area where the intense exchange process between different water masses occurs.There are usually strong vertical motions in the frontal zone,and the exchanges of momentum and heat are extremely active.Ocean fronts are crucial process in ocean dynamics.Using high-resolution satellite data and in-situ observations in spring,summer 2018 and summer 2019 in the northwest South China Sea?NWSCS?,the features and processes of the thermal and salinity fronts are analyzed,including the seasonal variability,3D structure and formation mechanisms.The vertical velocity is diagnosed from observations with the generalized Omega equation,and the 3D velocity field of the front are firstly obtained in the area.The long-term satellite and numerical sea surface temperature?SST?data are used to analyze the seasonal variability of the surface thermal front in the NWSCS.The results show that the thermal fronts have the strongest intensity and the widest coverage in winter,following by spring and summer,and disappear in autumn.The thermal fronts are located far from the coast in winter and get closer in summer.The characteristic width of surface fronts is 37 km.We calculated the correlation coefficient and information flow between the surface wind and frontal indexes,revealing that the surface wind stress curl and along-coast wind are the most important factors affecting the surface thermal front.The in-situ observations analysis shows that there is a front existing in coastal area shallower than 20 m.It gets weaker and closer to the coast with the depth increasing,and is induced by convergence of warm-fresh coastal water and salty-cold shelf water.In the deeper region,the fronts in subsurface layer are stronger than that in the surface,and strengthen and shift to the coast with depth.These fronts may be caused by the upwelling,and their distribution depth is related to the outcrop of upwelling waters.The vertical velocities are diagnosed from observations with the generalized Omega equation.The results show that alternating positive and negative alongshore bands appear in cross-shore direction.The magnitudes of the vertial velocities are in order of?46?(10^-5)m s^-1.The large value appears in the frontal zone,which is downward,while on the two sides of the frontal zone exists upward velocity with a comparable magnitude.By calculating the Ertel potential vorticity?PV?and Richardson number?Ri?,we find that the low Ri exists near the bottom instead of the upper ocean frontal zone,indicating shear instability in the bottom associated the bottom friction.The PV in the frontal zone is negative,implying that symmetric instability will arise.The negative baroclinic component of PV avails the occurrence of symmetric instability in the front zonal,and thus favors the release of frontal energy.