夏季南海北部粤东陆架锋面的动力特征分析*
曾毅港(1997—), 男, 湖南省湘潭市人, 硕士研究生, 主要从事锋面观测研究。email: |
Copy editor: 姚衍桃
收稿日期: 2021-12-04
修回日期: 2022-02-16
网络出版日期: 2022-02-21
基金资助
国家自然科学基金项目(92058201)
国家自然科学基金项目(41776040)
国家自然科学基金项目(41949907)
国家自然科学基金项目(42149907)
中国科学院基础前沿科学研究计划原始创新项目(ZDBS-LY-DQC011)
广州市科学研究计划(201904010420)
Analysis of the dynamic characteristics of the east Guangdong shelf front in the northern South China Sea in summer
Copy editor: YAO Yantao
Received date: 2021-12-04
Revised date: 2022-02-16
Online published: 2022-02-21
Supported by
National Natural Science Foundation of China(92058201)
National Natural Science Foundation of China(41776040)
National Natural Science Foundation of China(41949907)
National Natural Science Foundation of China(42149907)
Original Innovation Project of Basic Frontier Scientific Research Program of Chinese Academy of Sciences(ZDBS-LY-DQC011)
Guangzhou Science and Technology Project(201904010420)
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。
曾毅港 , 经志友 , 黄小龙 , 郑瑞玺 . 夏季南海北部粤东陆架锋面的动力特征分析*[J]. 热带海洋学报, 2022 , 41(4) : 136 -145 . DOI: 10.11978/2021172
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.
Key words: northern South China Sea; shelf front; in-situ observation; front instability; ROMS
图1 南海北部海域陆架地形和航次观测站点分布该图基于国家测绘地理信息局标准地图服务网站下载的审图号为 GS(2016)2893的标准地图制作。图中红点为CTD观测站点, 虚线、灰色实线、黑色实线分别为0m、20m、100m等深线 Fig. 1 Bathymetry and CTD stations in the northern South China Sea. Red dots are the CTD stations in summer. Topography is shown by the dotted, grey and black isobaths at 0m, 20m and 100m, respectively |
图2 卫星遥感观测到的温度、温度锋(等值线, 单位: 10-2℃∙km-1)和风场(箭头, 单位: m∙s-1)a. 气候态的温度、温度锋和风场; b. 2017年观测期间的平均温度、锋面强度和风场。该图基于国家测绘地理信息局标准地图服务网站下载的审图号为 GS(2016)2893的标准地图制作, 底图无修改 Fig. 2 Climatic temperature, thermal front (contour in 10-2℃∙km-1) and wind field (a), temperature, thermal front and wind field during in-situ observation period in 2017 (b). Contours in (a) and (b) denote the thermal front |
图3 多年现场观测的温度、锋面强度(等值线, 单位: 10-2℃∙km-1)和风场(箭头, 单位: m·s-1)水平分布a. 2017年10m层; b. 2017年20m层; c. 2018年10m层; d. 2018年20m层; e. 2019年10m层; f. 2019年20m层。图a中的D1、D2为观测断面; 该图基于国家测绘地理信息局标准地图服务网站下载的审图号为 GS(2016)2893的标准地图制作, 底图无修改 Fig. 3 Years of in-situ observation temperature, front intensity (black contour, unit: 10-2℃∙km-1), wind field (vector, unit: m·s-1) horizontal distribution |
图4 2017年6月现场观测典型温度和锋面强度断面a. D1断面温度; b. D1断面锋面强度; c. D2断面温度; d. D2断面锋面强度。图中白色等值线为理查森数, 黑色等值线为海水密度(单位: kg∙m-3) Fig. 4 Vertical temperature (a, c) and thermal fronts (b, d) distribution of the sections D1 and D2 in June 2017. The white lines in (b) denote Richardson number, and the black contour is density |
图5 模式第21年7月4日南海北部数值模拟的海表温度(a)和水平浮力梯度分布(b)该图基于国家测绘地理信息局标准地图服务网站下载的审图号为 GS(2016)2893的标准地图制作, 底图无修改。图a中灰色直线为研究断面; 图a中箭头为风速; 图b中箭头为流速 Fig. 5 Maps of SST (shading) (a) and horizontal buoyancy gradient (shading) from the ROMS1 simulation. The grey line in (a) is the study section; the black vectors in (a) and (b) represent wind speed and velocity, respectively |
图6 典型锋面海域的温度与盐度(a)、浮力频率(lg|N2|)与密度(b, c)断面分布图a和b为ROMS模拟结果, 图c为现场观测结果; 图a中等值线表示盐度(单位: ‰), 图b和c中等值线为密度(单位: kg∙m-3) Fig. 6 Vertical profile of temperature, salinity (a) and buoyancy frequency (lg|N2|), density (b) from the ROMS; buoyancy frequency from in-situ observations (c). Contours in (a), (b) and (c) are salinity, density and density, respectively |
图7 典型锋面海域的水平浮力梯度(lg|$ \bigtriangledown $hb|, a)、Ertel位涡斜压分量(EPVh, b)、Ertel位涡(EPV, c)和理查森数(lg|Ri|, d)断面分布图中黑色等值线为密度(单位: kg∙m-3), 其中粗黑线为22kg∙m-3密度线; 图c中紫色线为EPV=0等值线; 图d中黄色线为0.25理查森数等值线 Fig. 7 Vertical slices of the log of the magnitude of horizontal buoyancy gradient (a), EPV (c) and its baroclinic component (b), the log of Richardson number (d). The yellow contours in (d) denote the critical value of 0.25 for Richardson number. The thick black lines represent density equal 22 kg∙m-3. The magenta contours in (c) show the critical value of zero for EPV |
图8 典型锋面海域的Ekman浮力通量(EBF)水平分布(a)及研究断面表层EBF与Ertel位涡(EPV)曲线图(b)图a基于国家测绘地理信息局标准地图服务网站下载的审图号为 GS(2016)2893的标准地图制作, 底图无修改。图a中绿色箭头为地转流, 黑色箭头为风速; 图b为 EBF和EPV沿断面的变化图, 断面位置如图a中蓝色实线所示 Fig. 8 Snapshot of the EBF(a), curves of the sea surface EBF and EPV of study section (b). The location of the study section (b) is marked by the blue line in (a). The black (green) arrows denote wind speed (geostrophic currents) |
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