夏季南海北部粤东陆架锋面的动力特征分析*

本文利用现场观测资料和卫星遥感数据, 并结合ROMS(regional ocean modeling system)数值模拟对南海北部粤东陆架的锋面特征及其影响因素进行探讨。观测结果显示, 夏季南海北部陆架存在活跃的上升流温度锋面, 其水平尺度约为50km, 强度达到0.06℃∙km^-1, 大于同时期卫星遥感观测结果, 垂向影响深度超过20m, 且具有一阶理查森数(Richardson number, Ri)的典型动力学特征。进一步的ROMS 模式诊断分析结果显示, 锋面处水平梯度增强, 且动力学上表现出一阶Ri数, 为锋面不稳定的发生提供了有利条件。高分辨率模拟结果显示, 在夏季西南风的驱动下, 沿锋面地转流方向的风应力引起的跨陆架Ekman输运将锋面处冷水向暖水运移, 导致水平浮力梯度和锋面强度增强并形成负Ertel位涡(Ertel potential vorticity, EPV)。因此, 夏季风场强迫引起的Ekman浮力通量(Ekman buoyancy flux, EBF)可能是南海北部锋面不稳定现象的主要贡献者, 对局地动力环境有重要影响。

Analysis of the dynamic characteristics of the east Guangdong shelf front in the northern South China Sea in summer

This study investigates the characteristics of the east Guangdong shelf front and its dynamical regime using in-situ measurements, satellite data, and high resolution simulations by the regional ocean modeling system (ROMS). Observation results show active upwelling thermal fronts with horizontal scale of 50km on the northern shelf of the South China Sea in summer. The horizontal temperature gradient at the front is up to 0.06 ℃∙km^-1 and is stronger than satellite observed results in the same period. The front can reach 20m depth, and has the characteristics of the order-one Richardson number. Further diagnostic analysis using ROMS model output also show that the horizontal buoyancy gradient is enhanced at the front, with order-one Richardson number, which is favorable for frontal instabilities. High-resolution simulation results indicate that driven by the southwest wind in summer, the Ekman transport across the continental shelf caused by down-front wind, will accumulate cold water of upwellings to the warm water, enhancing the horizontal buoyancy gradient and front sharpness, and change the frontal baroclinicity, which leads to negative Ertel potential vorticity (EPV). As such, the Ekman buoyancy flux caused by summer monsoon may significantly contribute to the formation and instability of the continental front in the northern South China Sea, and it has an important impact on the local dynamic environment.