海洋底混合层研究进展

Research progress of ocean bottom mixed layer

  • 摘要: 海洋底混合层是海洋底层水体中受强湍流混合作用影响的水层,其作为海洋物理、化学和生物过程交汇的重要界面,在全球海洋能量、物质交换和生物地球化学循环中扮演着关键角色。本文系统综述了海洋底混合层的研究进展,重点探讨了其结构特征、影响机制、厚度的确定及参数化方法以及全球海洋底混合层厚度分布规律,并展望了其未来研究方向。详细分析了底混合层温度、盐度和密度在垂直方向的结构;探讨了底层剪切流引发湍流破坏水体密度层结、内波破碎产生强湍流混合、地形改变底层流流速方向并增大流速梯度、潮汐作用周期性调节底层流流速和流向、地热通量加热底层海水减弱层结稳定性等多种影响机制;介绍了阈值法、曲率法、最大角度法和相对方差法等确定海洋底混合层厚度的方法,并讨论了底混合层厚度与海水深度和湍流扩散系数呈正相关、与浮力频率呈负相关等参数化关系;总结了全球大洋底混合层厚度分布不均匀性特征,洋中脊附近较厚,大西洋、印度洋和太平洋的底混合层厚度中值分别为40 m、42 m和64 m,全球中值为47 m;最后对海洋底混合层未来的研究手段和研究方向做出了展望,提出需发展深海坐底潜标、自主水下机器人等先进观测技术,加强底层流剪切、内波破碎和地形效应相互作用的理论研究,发展底混合层精细数值模拟等方法。

     

    Abstract: The ocean bottom mixed layer (BML) is a water layer near the seabed influenced by intense turbulent mixing, serving as a critical interface where physical, chemical, and biological processes converge. It plays a key role in global ocean energy and material exchange, as well as biogeochemical cycles. This paper systematically reviews the research progress on the BML, focusing on its structural characteristics, influencing mechanisms, methods for determining and parameterizing its thickness, and the global distribution patterns of BML thickness. Furthermore, future research directions are outlined. The vertical structures of temperature, salinity, and density within the BML are analyzed in detail. The influencing mechanisms are explored, including turbulent mixing induced by bottom shear flows disrupting density stratification, strong turbulence generated by internal wave breaking, topographic effects altering flow velocity and direction while enhancing shear, tidal forces periodically modulating near-bottom currents, and geothermal heating reducing stratification stability. Various methods for determining BML thickness are introduced, such as the threshold method, curvature method, maximum angle method, and relative variance method. Parameterization relationships are discussed, indicating that BML thickness is positively correlated with water depth and turbulent diffusivity but negatively correlated with buoyancy frequency. The global distribution of BML thickness exhibits significant heterogeneity, with thicker layers near mid-ocean ridges. The median BML thickness in the Atlantic, Indian, and Pacific Oceans is 40 m, 42 m, and 64 m, respectively, with a global median of 47 m. Finally, future research directions are proposed, emphasizing the need for advanced observational technologies such as deep-sea bottom-mooring systems and autonomous underwater vehicles, enhanced theoretical studies on the interactions between bottom shear flows, internal wave breaking, and topographic effects, and the development of high-resolution numerical simulation methods for the BML.

     

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