Characteristics of turbulence in the Changjiang River Estuary and its adjacent waters in summer
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摘要: 利用2019年7月在长江口科学考察实验研究夏季航段(NORC2019-03-02)中获得的MSS90L湍流剖面仪的直接观测数据，本文计算并分析了该断面的湍动能耗散率ε和垂向湍扩散系数KZ的分布情况。湍动能耗散率的大小为1.72×10−10～2.95×10−5 W/kg；垂向湍扩散系数的大小为3.24×10−7～4.55×10−2 m2/s。湍动能耗散率和垂向湍扩散系数的分布相似，均为上层最强，底层次之，中层最弱。上层由于风应力的作用，使得湍动能耗散率和垂向湍扩散系数较大；温跃层处层化较强，抑制了湍动能的耗散和垂向上的湍混合。盐度锋面的次级环流会促使低盐水团脱离，锋面引起的垂向环流会加强海洋的湍混合。低盐水团与外界的能量交换较少，湍动能耗散率较弱。长江口海区存在明显的上升流和下降流，它们是由锋面的次级环流产生的；上升流和下降流的存在促进湍动能的耗散与湍混合。Abstract: Using the direct observation data of the MSS90L turbulence profiler obtained in the summer cruise of the Changjiang River Estuary scientific investigation and experimental study (NORC2019-03-02) in July 2019, in this paper, the distribution of the turbulent kinetic energy dissipation rate ε and the vertical turbulent diffusion coefficient Kz are calculated and analyzed. The turbulent kinetic energy dissipation rate is 1.72×10−10 W/kg to 2.95×10−5 W/kg, and the vertical turbulent diffusion coefficient is 3.24×10−7 m2/s to 4.55×10−2 m2/s. The distribution of the turbulent kinetic energy dissipation rate and the vertical turbulent diffusion coefficient are similar, both of which are the strongest in the upper layer, the next in the bottom layer and the weakest in the middle layer. In the upper layer, due to wind stress, the turbulent kinetic energy dissipation rate and vertical turbulent diffusion coefficient are larger; the stratification is stronger at the thermocline, which suppresses the dissipation of turbulent kinetic energy and vertical turbulent mixing. The secondary circulation of the salinity front will promote the separation of the low salt water mass, and the vertical circulation caused by the front will enhance the turbulent mixing of the ocean. The energy exchange between the low salt water mass and the outside world is less, and the turbulent kinetic energy dissipation rate is weaker. There are obvious upwelling and downwelling in the Changjiang River Estuary, which are produced by the secondary circulation of the front; the existence of upwelling and downwelling promotes the dissipation of turbulent kinetic energy and turbulent mixing.
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图 2 剪切功率谱与相应的Nasmyth谱的比较
蓝线代表剪切功率谱；黑线代表Nasmyth谱
Fig. 2 Comparison of the shear power spectrum with the corresponding Nasmyth spectrum
The blue line represents the shear power spectrum; the black line represents the Nasmyth spectrum
图 3 夏季A4断面的温度分布
黑细线代表以0.2℃/m为标准计算得到的温跃层范围；黑粗线代表温跃层强度最强的位置
Fig. 3 The temperature distribution of Section A4 in summer
The thin black line represents the range of the thermocline calculated with 0.2℃/m as the standard; the thick black line represents the strongest location of the thermocline
图 4 夏季A4断面的盐度分布
红线为盐度为31的等值线；黑细虚线代表以0.1 m−1为标准计算得到的盐跃层范围；黑粗虚线代表盐跃层强度最强的位置
Fig. 4 The salinity distribution of Section A4 in summer
The red line is the contour line with a salinity of 31; the black thin dashed line represents the range of the halocline calculated based on the standard of 0.1 m−1; the thick black dashed line represents the location of the strongest halocline
图 5 夏季A4断面的浮性频率（lgN2）分布
黑细线代表以0.2℃/m为标准计算得到的温跃层范围；黑粗线代表温跃层强度最强的位置；N2单位：s−2
Fig. 5 The distribution of the buoyancy frequency (lgN2) of Section A4 in summer
The thin black line represents the range of the thermocline calculated with 0.2℃/m as the standard; the thick black line represents the strongest location of the thermocline; the unit of N2 is s−2
图 6 夏季A4断面的湍动能耗散率（lgε）示意图
红线代表盐度为31的等值线；黑虚线为盐跃层的范围；黑实线为温跃层的范围；ε单位：W/kg
Fig. 6 Schematic diagram of the turbulent energy dissipation rate (lgε) of Section A4 in summer
The red line represents the contour line with a salinity of 31; the black dotted line is the range of the halocline; the black solid line is the range of the thermocline; the unit of ε is W/kg
图 7 夏季A4断面的垂向湍扩散系数（lgKZ）的分布
红线代表盐度为31的等值线；黑虚线为盐跃层的范围；黑实线为温跃层的范围；KZ单位：m2/s
Fig. 7 The distribution of vertical turbulent diffusion coefficient (lgKZ) of Section A4 in summer
The red line represents the contour line with a salinity of 31; black dotted line is the halocline range; the black solid line is the thermocline range; the unit of KZ is m2/s
图 8 水平盐度梯度
红色虚线包含的范围代表盐度锋面（0.03 km−1）；红线代表盐度为31的等盐线；黑实线代表温跃层的位置；黑虚线代表盐跃层的位置
Fig. 8 Horizontal salinity gradient
The range covered by the white line represents the salinity front (0.03 km−1); the red line represents the isosalt with a salinity of 31; the black solid line represents the location of the thermocline; the black dotted line represents the location of the halocline
图 9 上升流（a）和下降流（b）的形成示意图
Fig. 9 Schematic diagram of the formation of upwelling (a) and downwelling (b)
图 10 水平温度梯度
红线包含的范围代表温度锋面（0.04℃/km）；红线代表盐度为31的等盐线；黑实线代表温跃层的位置；黑虚线代表盐跃层的位置
Fig. 10 Horizontal temperature gradient
The range covered by the white line represents the temperature front (0.04°C/km); red line represents the isosalt with a salinity of 31; the black solid line represents the location of the thermocline; and the black dotted line represents the location of the halocline
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