Title: Mixing and formation of layers by internal wave forcing Show Chinese title

Title: 内部波强迫下的混合与层状结构形成

Abstract: The energy pathways from propagating internal waves to the scales of irreversible mixing in the ocean are not fully described. In the ocean interior, the triadic resonant instability is an intrinsic destabilization process that may enhance the energy cascade away from topographies. The present study focuses on the integrated impact of mixing processes induced by a propagative normal mode-1 over long term experiments in an idealised setup. The internal wave dynamics and the evolution of the density profile are followed using the light attenuation technique. Diagnostics of the turbulent diffusivity $K_{T}$ and background potential energy $BPE$ are provided. Mixing effects result in a partially mixed layer colocated with the region of maximum shear induced by the forcing normal mode. The maximum measured turbulent diffusivity is 250 times larger than the molecular value, showing that diapycnal mixing is largely enhanced by small scale turbulent processes. Intermittency and reversible energy transfers are discussed to bridge the gap between the present diagnostic and the larger values measured in Dossmann et al, Experiments in Fluids, 57(8), 132 (2016). The mixing efficiency $\eta$ is assessed by relating the $BPE$ growth to the linearized $KE$ input. One finds a value of $\Gamma=12-19\%$ larger than the mixing efficiency in the case of breaking interfacial wave. After several hours of forcing, the development of staircases in the density profile is observed. This mechanism has been previously observed in experiments with weak homogeneous turbulence and explained by argument. The present experiments suggest that internal wave forcing could also induce the formation of density interfaces in the ocean. Show Chinese abstract

Abstract: 能量路径从传播的内波到海洋中不可逆混合的尺度尚未完全描述。 在海洋内部，三重共振不稳定性是一种内在的失稳过程，可能会增强远离地形的能量级联。 本研究关注在理想化设置中长期实验中由传播的正常模态-1引起的混合过程的综合影响。 使用光衰减技术跟踪内波动力学和密度剖面的演变。 提供了湍流扩散率$K_{T}$和背景位能$BPE$的诊断结果。 混合效应导致与由强迫正常模态引起的最大剪切区域共存的部分混合层。 测得的最大湍流扩散率是分子值的250倍，表明由小尺度湍流过程大大增强了垂向混合。 讨论了间歇性和可逆能量传递，以弥合目前的诊断结果与Dossmann等人在《流体中的实验》（Fluids, 57(8), 132 (2016)）中测得的较大值之间的差距。 通过将$BPE$的增长与线性化的$KE$输入相关联来评估混合效率$\eta$。 发现一个比破折界面波情况下的混合效率更大的$\Gamma=12-19\%$值。 在数小时的强迫作用后，观察到密度剖面中阶梯结构的发展。 这种机制之前在弱均匀湍流实验中被观察到，并通过论点进行了解释。 目前的实验表明，内波强迫也可能在海洋中诱导密度界面的形成。