Study on tidal characteristics of coral reef lagoon of Xisha Islands in the South China Sea based on mooring observation
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摘要: 本文利用南海西沙群岛潟湖区29 d的全水深浮标观测资料研究了潟湖区内正压潮和内潮的基本特征，采用深度平均方法分析海流的适用性，并讨论潟湖区内潮的主要来源。深度平均流的动能谱显示全日潮流占主导，其动能占整体海流动能的41%。对比分析深度平均流和Tpxo7.2模式预测的全日、半日潮流的调和常数，两者均表明全日正压潮流受地形调制，主轴方向为西北−东南向，而半日正压潮流主轴方向为东−西向。两种方法得到的全日正压潮流大−小潮存在半个相位（6～7 d）的差异，进一步分析发现全日正压潮和全日内潮潮龄不同，存在部分相互抵消，且全日内潮大潮发生时间在深度上存在差异，推测由于缺少海表和海底的测量数据，导致深度平均方法得到的全日正压潮仍然包含全日内潮信号。调和分析结果表明，全日内潮的动能中相干部分占比高达91%，说明潟湖区的全日内潮是正压潮与局地岛礁地形相互作用产生，而从远场传播而来的可能性很小。Abstract: Based on the 29-day full-depth mooring data of coral reef lagoon of Xisha Islands in South China Sea, we investigated the characteristics of the internal tides (ITs), the applicability of depth-averaging method for current analysis, and the source of ITs. The depth-averaged currents indicated that the diurnal tides were dominant, their HKE (horizontal kinetic energy) accounted for 41% of the total currents. The harmonic analyses of depth-averaged current and Tpxo7.2 model current indicated that the barotropic diurnal currents were modulated by topography, its main axis orientation was northwest-southeast. The spring-neap periods of barotropic diurnal currents with two methods have the half-phase shift (6−7 days). It is proposed that the depth-averaged barotropic diurnal currents contain the baroclinic diurnal components (diurnal internal tides) due to the lacking of measurements near the surface and bottom ocean. Further analysis showed that the barotropic and baroclinic diurnal currents had different amplitudes with the variable phase shift. The coherent diurnal ITs contributed 91% HKE to the total diurnal ITs, which implied that ITs was most generated in Xisha Islands.
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Key words:
- coral reef lagoon /
- internal tide /
- coherent internal tide /
- barotropic tide
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图 1 南海地形图（a）和浮标示意图（b）
红色方框表示西沙群岛，红色十字表示浮标观测点
Fig. 1 Bathymetry of the South China Sea (a) and mooring diagram (b)
Red square represents Xisha Islands, red cross represents location of the mooring
图 3 正压潮4个主要分潮的潮流椭圆
蓝线：深度平均；红线：模式预报
Fig. 3 The ellipses of four dominant barotropic tidal currents
Blue line: depth-averaged; red line: predicted by model
图 4 正压潮的时间序列
蓝线：观测的全日潮；红线：模式预测的全日潮。a. 东西方向的速度；b. 南北方向的速度；c. CTD观测的海底压强变化和模式预测的海平面高度变化的时间序列
Fig. 4 Time series of barotropic current
Blue lines: observed diurnal tides; red lines: diurnal tides predicted by model. a. East-west velocity; b. north-south velocity; c. time series of sea bottom pressure measured by CTD and sea level height predicted by model
图 5 O1分潮相位差的概率密度函数分布
Fig. 5 Probability density function distribution of O1 tidal phase difference
图 6 K1分潮相位差的概率密度函数分布
Fig. 6 Probability density function distribution of K1 tidal phase difference
图 7 全日相干内潮水平动能的时间–深度剖面
Fig. 7 Horizontal kinetic energy of diurnal coherent internal tide
图 8 深度积分后全日相干/非相干内潮和Tpxo7.2的全日正压潮的水平动能的时间序列
Fig. 8 Time series of depth-integrated of horizontal kinetic energy of diurnal coherent/incoherent internal tide and diurnal barotropic tide of Tpxo7.2
表 1 深度平均流的主要分潮的椭圆要素
Tab. 1 Elliptical elements of four major constituents of depth-averaged currents
分潮 长轴/（cm·s−1） 短轴/（cm·s−1） 倾角/（°） 迟角/（°） O1 9.95 –3.62 152.24 350.27 K1 10.91 –5.94 155.35 214.78 M2 3.42 0.64 178.86 319.17 S2 2.42 1.12 13.99 245.89 
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表 2 模式预测的主要分潮的椭圆要素
Tab. 2 Elliptical elements of four major constituents of model
分潮 长轴/（cm·s−1） 短轴/（cm·s−1） 倾角/（°） 迟角/（°） O1 11.81 –4.01 164.95 199.07 K1 12.92 –4.25 148.06 262.06 M2 3.03 0.47 2.10 148.73 S2 0.37 0.01 12.62 248.55
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表 3 O1分潮概率密度函数峰值对应的相位差
Tab. 3 Phase difference corresponding probability density function peak of O1 constituent
深度/m 相位差/（°） 8 207 36 191 64 167
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表 4 K1分潮概率密度函数峰值对应的相位差
Tab. 4 Phase difference corresponding probability density function peak of K1 constituent
深度/m 相位差/（°） 8 116 36 154 64 175
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