This study investigates the submesoscale fronts and their dynamic effects on the mean flow due to frontal instabilities in the wind-driven summer offshore jet of the western South China Sea (WSCS), using satellite observations, a 500 m-resolution numerical simulation, and diagnostic analysis. Both satellite measurements and simulation results show that the submesoscale fronts occupying a typical lateral scale of O(∼10) km are characterized with one order of Rossby (Ro) and Richardson (Ri) numbers in the WSCS. This result implies that both geostrophic and ageostrophic motions feature in these submesoscale fronts. The diagnostic results indicate that a net cross-frontal Ekman transport driven by down-front wind forcing effectively advects cold water over warm water. By this way, the weakened local stratification and strong lateral buoyancy gradients are conducive to a negative Ertel potential vorticity (PV) and triggering frontal symmetric instability (SI) at the submesoscale density front. The cross-front ageostrophic secondary circulation caused by frontal instabilities is found to drive an enhanced vertical velocity reaching O(100) m/d. Additionally, the estimate of the down-front wind forcing the Ekman buoyancy flux (EBF) is found to be scaled with the geostrophic shear production (GSP) and buoyancy flux (BFLUX), which are the two primary energy sources for submesoscale turbulence. The large values of GSP and BFLUX at the fronts suggest an efficient downscale energy transfer from larger-scale geostrophic flows to the submesoscale turbulence owing to down-front wind forcing and frontal instabilities. In this content, submesoscale fronts and their instabilities substantially enhance the local vertical exchanges and geostrophic energy cascade towards smaller-scale. These active submesoscale processes associated density fronts and filaments likely provide new physical interpretations for the filamentary high chlorophyll concentration and frontal downscale energy transfer in the WSCS.

这项研究使用卫星观测，500 m分辨率数值模拟和诊断方法研究了亚中尺度尺度的前沿及其对南海西部风驱夏季海上射流中锋面不稳定性造成的平均流量的动态影响。分析。卫星测量和模拟结果均表明，占据典型横向尺度O（〜10）km的亚中尺度锋面具有Rossby（Ro）和Richardson（Ri）中的数字。这一结果表明，在这些亚中尺度的锋面都存在地转运动和地转运动。诊断结果表明，由前风强迫驱动的净跨额叶曼运输有效地将冷水平移到温水上。通过这种方式，减弱的局部分层和强大的侧向浮力梯度有利于负Ertel势涡度（PV），并在亚中尺度密度前沿触发锋面对称不稳定性（SI）。发现由额叶不稳定性引起的跨界变质次生环流驱使垂直速度达到O（100）米/天 此外，发现迫使埃克曼浮力通量（EBF）的前风的估计值与地转剪切产量（GSP）和浮力通量（BFLUX）成比例，这是亚中尺度湍流的两个主要能源。正面的GSP和BFLUX值较大，这表明由于正面风向和正面不稳定性，从较大规模的地转流向次中尺度湍流的有效的向下尺度能量转移。在这种内容下，亚中尺度的前沿及其不稳定性大大增强了局部垂直交换和地转能级联向较小的尺度发展。