Collaborative Research: Kelvin-Helmholtz Instabilities at a Kuroshio Seamount (KHIKS)

由风驱动的西部边界流，如西太平洋的黑潮，向北输送大量的热量、盐和动量，有助于海洋翻转环流和气候系统热平衡。西部边界流如何在时间和空间上变化，以及它们如何向周围环境损失能量，是长期存在的问题。与地形特征罕见的大西洋湾流不同，黑潮经常与山脊、岛屿和海底山相互作用。因此，通过湍流混合从大规模流动中提取能量，特别是在水平速度剪切较大的情况下。众所周知，这些支持各种剪切不稳定性，如开尔文-亥姆霍兹（KH）或霍尔姆波（Holmboe），其中流动中的小扰动放大，成为湍流产生的重要机制。虽然KH剪切不稳定性已经在理想的实验室实验和数值模拟中得到了广泛的研究，但在海洋中的直接观测很少，而且主要局限于河口和浅海。因此，在开阔的海洋中，对高雷诺数海洋流中KH-like巨浪的产生、演化和衰减、它们被背景流调制和衰减为湍流的观测不足。该项目是与台湾科学家的合作，利用他们可用的船舶时间，在黑潮路径上的海底山的背风处测量高分辨率的温度、盐度和速度精细结构。还将部署几个系泊设施，以获得海底山附近变化的长期空间评估。将评估观测到的类kh巨浪内的湍流耗散和混合，以确定它们在热、盐度、质量和营养物质的湍流通量、黑潮和局部水团的改变以及与环流尺度环流相关的黑潮能量耗散中的作用。该项目支持两名早期职业科学家向观测海洋学家转型。国际合作将提供船舶时间和进入具有全球科学重要性的黑潮水域的途径。威斯康星大学本科生的暑期项目活动包括对K-12教室的教育访问，以及在威斯康星大学和当地科学中心的推广活动。具有开尔文-亥姆霍兹（KH）样特征的剪切不稳定性将在台湾东部黑潮路径上的海山背风处测量。利用拖曳式CTD链测量，并辅以船上ADCP和回声测深仪，将精细三维（垂直、沿流和跨流）密度结构分解为水平10米和垂直1米。在如此高雷诺数的海洋流动中进行如此详细的测量是前所未有的，并将阐明其运动结构，动态演变和相关湍流，为现实的数值模型模拟和实验室实验提供指导。停泊的adcp阵列和温度传感器将提供额外的时间序列剖面(i)海山上游的剪切和分层，（ii）海山山顶的高时间和垂直分辨率的3-D速度和回声测深图像，以及（iii）海山背风处沿流和跨流的3-D速度场时间序列剖面。多种估计方法和直接微观结构测量的湍流耗散和混合将有助于确定这些流动/地形相互作用对西部边界流水团转化和耗散的作用。从科学的角度来看，理解平衡环流的耗散和洋内潜流混合的来源仍然是物理海洋学中最紧迫的挑战之一。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Wind-driven western boundary currents, such as the Kuroshio in the western Pacific, transport large amounts of heat, salt and momentum northward, contributing to the ocean overturning circulation and the climate system heat balance. How western boundary currents vary in time and space, and how they lose energy to their surroundings, are long-standing questions. Unlike the Gulf Stream in the Atlantic where topographic features are rare, the Kuroshio frequently interacts with ridges, islands and seamounts. Energy is thus extracted from the large-scale flow through turbulent mixing, particularly in settings with large shear in horizontal velocities. These are known to support a variety of shear instabilities, such as Kelvin-Helmholtz (KH) or Holmboe, whereby small perturbations in the flow amplify, to become important mechanisms for turbulence generation. Although KH shear instability has been widely studied in idealized laboratory experiments and numerical simulations, direct observations in the ocean are rare and mostly confined to estuaries and sills. Consequently, the generation, evolution and decay of KH-like billows in high-Reynolds-number oceanic flows, their modulation by background flow and decay into turbulence are under-observed in the open ocean. The project is a collaboration with Taiwanese scientists, using their available ship-time, to measure high-resolution temperature, salinity and velocity finestructure of primary KH-like billows and evolution in the lee of a seamount in the path of the Kuroshio. Several moorings will also be deployed to gain longer-term spatial assessment of variability in the seamount vicinity. Turbulent dissipation and mixing within the observed KH-like billows will be assessed, to determine their roles in turbulent fluxes of heat, salinity, mass, and nutrients, modification of Kuroshio and local water-masses, and energy dissipation in the Kuroshio, of relevance to the gyre-scale circulation. This project supports two early-career scientists in their transition to observational oceanographers. International collaborations will provide shiptime and access to Kuroshio waters of global scientific importance. Activities within the summer program for UW undergraduate students are included, along with educational visits to K-12 classrooms, and outreach at UW and local science centers. Shear instabilities with Kelvin-Helmholtz (KH)-like characteristics will be measured in the lee of a seamount in the path of the Kuroshio east of Taiwan. The finescale 3-D (vertical, along- and across-stream) density structure will be resolved to 10-m horizontally and 1-m vertically using towed CTD chain surveys, augmented with shipboard ADCP and echosounder. Such detailed measurements in such high-Reynolds-number oceanic flows are unprecedented, and will elucidate their kinematic structure, dynamic evolution and associated turbulence, to offer guidance for realistic numerical model simulations and laboratory experiments. A moored array of ADCPs and temperature sensors will provide additional time-series profiles of (i) shear and stratification upstream of the seamount, (ii) high temporal and vertical resolution of 3-D velocity and echosounder images on the seamount summit, and (iii) 3-D along- and across-stream time- series profiles of velocity fields in the lee of the seamount. Turbulent dissipation and mixing from multiple estimation methods and direct microstructure measurements will help determine the role of these flow/topography interactions on water-mass transformation and dissipation of a western boundary current. Scientifically, understanding the dissipation of the balanced circulation and the sources of diapycnal mixing in the ocean remain among the most pressing challenges in physical oceanography.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.