Collaborative Proposal: Diapycnal Mixing on the East Pacific Rise

观测研究表明，洋中脊是流体动力过程增强湍流混合的场所。以前的研究包括对大西洋和印度洋缓慢扩张的海脊进行的测量，这些海脊的特点是地形非常粗糙，有很深的轴向山谷和赤道带峡谷。潮汐与这种地形的相互作用产生斜压流，局部增强内波能量，增加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，并已成为海洋模型中使用的扩散率参数化的基础。然而，最近的工作表明，在大洋中脊发生的异常湍流混合水平也可能归因于低频流动，特别是在缓慢扩展的洋脊上，那里有无数的底坎和收缩支撑密度驱动的溢流。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海隆（EPR）的混合过程，与缓慢扩张的海脊相比，EPR具有更平滑的地形起伏。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EPR附近只有弱混合。然而，示踪剂释放实验，作为NSF支持的阶梯项目的一部分进行，展示了湍流水平对应的扩散率为10厘米2秒-1，两个数量级以上的典型温跃层水平。强流，剪切，和速度精细结构的观测表明，在这个网站的强湍流延伸到一个简单的底部边界层之外，并有助于显着混合的分层水以上的山脊地形。采样程序将包括微观结构和降低声学多普勒轮廓，产生湍流耗散率和精细结构参数的直接估计。微观结构的测量将是第一个表征耗散水平在任何快速扩展的脊网站。我们有独特的机会加入LADDER计划的最后一次巡航，并在现有和资助的船时框架内提供新的测量结果。智力优势：尽管经过几十年的研究，深海混合仍然很难表征。特别是，目前基于粗糙度的混合参数化所基于的所有可用测量都是在缓慢扩展的山脊上进行的，其中不同的过程涉及所观察到的高水平混合。因此，完全不知道这些参数化在多大程度上适用于快速扩张的洋脊。因此，拟议的微结构调查将大大扩大现有的混合数据集，从深海到具有不同地形特征的地点。测得的耗散率将被用来提高混合参数化用于数值环流和气候models.Broader影响：除了物理过程的表征和过程参数化模型的影响，这项研究有助于在一个根本的方式LADDER研究活跃的热液站点附近的幼虫扩散。拟议的工作还将支持FSU的研究生和LDEO的博士后研究员。

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.