Collaborative Proposal: Diapycnal Mixing on the East Pacific Rise

观测研究表明，大洋中脊是流体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的海脊的测量，这些海脊的特点是地形非常粗糙，轴向深谷和断裂带峡谷。潮汐与这种地形的相互作用产生斜压流，这种斜压流局部增强了内波能量，增加了导致湍流混合的不稳定的可能性。这种类型的内波驱动混合已经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系数参数的基础。然而，最近的工作表明，在大洋中脊发生异常程度的湍流混合也可能归因于低频流动，特别是在缓慢扩散的脊上，那里无数的门槛和狭窄支撑着密度驱动的溢流。研究的主要目的是进行新的观测，以记录作用于快速传播的东太平洋海隆(EPR)的混合过程，与缓慢传播的海脊相比，东太平洋海隆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，导致以前的研究表明，只有在EPR附近才有微弱的混合。然而，作为美国国家科学基金会支持的梯子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平方厘米S-1的扩散系数，比典型的温跃层水平高两个数量级。对强流、切变和速度精细结构的观测表明，该地点的强烈湍流超出了简单的底部边界层，并有助于脊状地形上方分层水的显著混合。采样计划将包括微观结构和降低的声学多普勒剖面图，产生湍流耗散率的直接估计和精细结构参数。微结构测量将第一次表征任何快速扩散的山脊位置的耗散水平。我们有独特的机会加入梯子计划的最终巡航，并在现有和资助的船期框架内做出新的测量。智力优势：尽管进行了几十年的研究，但深海混合的特征仍然很差。特别是，目前基于粗糙度的混合参数所基于的所有现有测量都是在缓慢扩散的脊上进行的，在那里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展开的山脊是完全未知的。因此，拟议的微结构调查将把现有的混合数据集从深海扩展到具有不同地形特征的地点。测量的耗散率将被用来改进用于数值环流和气候模式的混合参数。广泛的影响：除了物理过程特征和由此对模式中过程参数的影响之外，这项研究对活动热液地点附近幼虫扩散的阶梯研究做出了基本贡献。拟议的工作还将支持佛罗里达州立大学的一名研究生和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.