Collaborative Proposal: Diapycnal Mixing on the East Pacific Rise

观测研究表明，洋中脊是水动力过程增强湍流混合的场所。之前的研究包括对大西洋和印度洋缓慢扩张脊的测量，这些脊的特征是非常粗糙的地形，有深轴向山谷和断裂带峡谷。潮汐与这种地形的相互作用产生斜压流，这在局部增强了内波能量，增加了导致湍流混合的不稳定性的可能性。这种类型的内波驱动混合已成为深海湍流发生的范例，并已成为海洋模式中使用的扩散率参数化的基础。然而，最近的研究表明，在大洋中脊发生的异常水平的湍流混合也可能归因于低频流动，特别是在缓慢扩张的脊上，无数的基板和收缩支撑着密度驱动的溢流。研究的主要目标是进行新的观测，以记录作用于快速扩张的东太平洋隆起（EPR）的混合过程，与缓慢扩张的山脊相比，东太平洋隆起与更平滑的地形起伏有关。相对不那么粗糙的地形阻止了在其他地方看到的内波能量的显著增强，导致先前的研究表明EPR附近只有弱混合。然而，作为nsf支持的LADDER项目的一部分进行的示踪剂释放实验表明，湍流水平对应于10 cm2 s-1数量级的扩散，比典型的温跃层水平高两个数量级。对强流动、剪切和速度精细结构的观测表明，该地点的强湍流超出了简单的底边界层，并有助于山脊地形上方分层水的显著混合。采样程序将包括微观结构和降低声学多普勒剖面，从而直接估计湍流耗散率和精细结构参数。微结构测量将是第一个表征任何快速扩散脊的消散水平。我们有独特的机会加入梯子计划的最后一次巡航，并在现有和资助的船舶时间框架内贡献新的测量方法。知识优势：尽管经过了几十年的研究，深海中的混合仍然缺乏特征。特别地，所有可用的测量数据都是基于当前基于粗糙度的混合参数化，这些测量数据都是在缓慢扩散的脊上进行的，在这些脊上观察到的高混合水平涉及不同的过程。因此，这些参数化在多大程度上适用于快速扩展的脊是完全未知的。因此，拟议的微观结构调查将大大扩展现有的混合数据集，从深海到具有不同地形特征的地点。测量到的耗散率将用于改进数值环流和气候模式中使用的混合参数化。更广泛的影响：除了物理过程表征和由此产生的模型过程参数化影响外，本研究还为活性热液点附近幼虫分散的LADDER研究做出了根本性的贡献。这项工作还将支持FSU的一名研究生和LDEO的一名博士后研究员。

Observational studies indicate that mid-ocean ridges are sites where hydrodynamic processes act to enhance turbulent mixing. Previous studies have included measurements from slow-spreading ridges in the Atlantic and Indian Oceans, where the ridges are characterized by very rough topography with deep axial valleys and fracture-zone canyons. The interaction of tides with such topography gives rise to baroclinic currents, which locally enhance the internal-wave energy and increase the potential for instabilities that lead to turbulent mixing. Internal- wave driven mixing of this type has become the paradigm for the occurrence of turbulence in the deep ocean, and has been the basis for parameterizations of diffusivity for use in ocean models. However, recent work has demonstrated that the exceptional levels of turbulent mixing occurring at mid-ocean ridges may also be attributable to low-frequency flow, in particular on slow-spreading ridges where innumerable sills and constrictions support density driven overflows. The main goals of study is to make new observations to document the mixing processes acting on the fast-spreading East Pacific Rise (EPR) which, in comparison to slow-spreading ridges, is associated with much smoother topographic relief. The comparatively less rough topography prevents the considerable enhancement of internal wave energy seen elsewhere, leading previous studies to suggest only weak mixing near the EPR. However, a tracer-release experiment, conducted as part of the NSF-supported LADDER project, demonstrated turbulence levels corresponding to diffusivities of order 10 cm2 s-1, two orders of magnitude above typical thermocline levels. Observations of strong flow, shear, and velocity finestructure suggest that strong turbulence at this site extends beyond a simple bottom boundary layer, and contributes to significant mixing of stratified water above the ridge topography. The sampling program will consist of microstructure and lowered acoustic Doppler profiling, yielding both direct estimates of the turbulent dissipation rates and finestructure parameters. The microstructure measurements will be the first to characterize dissipation levels at any fast-spreading ridge site. We have the unique opportunity to join the final cruise of the LADDER program and contribute new measurements within the framework of existing and funded shiptime.Intellectual Merits: In spite of several decades of research, mixing in the deep ocean remains poorly characterized. In particular, all available measurements on which current roughness- based mixing parameterizations are based, were taken on slow-spreading ridges where different processes have been implicated in the observed high levels of mixing. Therefore, it is entirely unknown to what degree these parameterizations apply on fast-spreading ridges. The proposed microstructure survey will thus significantly extend the available mixing data set from the deep ocean to sites with different topographic characteristics. The measured dissipation rates will be used to improve mixing parameterizations to be used in numerical circulation and climate models.Broader Impacts: In addition to the physical process characterization and resulting implications for process parameterization in models, this study contributes in a fundamental way to the LADDER study of larval dispersion near active hydrothermal sites. The proposed work will also support a graduate student at FSU and a postdoctoral investigator at LDEO.