Collaborative Research: Numerical Modeling of the Internal-Wave Cascade and Submesoscale Lateral Dispersion in the Ocean

横向搅拌是基本过程之一，平流和纵向混合决定了海洋中水质量、营养物质和溶解气体的分布和命运。海洋环流模式（ogcm）可以解析~30 km以上尺度的搅拌，也可以解析~1 km尺度的区域模式，但必须参数化亚中尺度（1 ~ 10 km）和精细尺度（10 ~1 km）过程的等环流（侧向）搅拌。示踪剂释放实验一致地发现，1-10公里尺度上的等压扩散系数比内波剪切色散的预测值大一个数量级。该项目将提供一个更完整和准确的评估内波和涡旋模态在亚中尺度（10米至10公里）横向搅拌中的作用，以及能量级联到耗散垂直尺度，以解决这一悖论。在观测上很难区分内波和涡旋模态。然而，明智地使用数值模型可以通过分离不同的物理学来梳理它们的影响。本项目方法的一个主要优点是，所有上述过程/动力学都可以在单一模型的背景下进行探索，使我们能够通过不同的机制在广泛的参数制度和强迫条件下无缝地探索内波级联和横向色散。所提出的模拟将阐明潜在的物理现象，并使我们了解这些过程如何在海洋中起作用，从而为ogcm的亚中尺度等典型扩散提供一阶参数化。本研究中制定的参数化将通过同行评议的出版物和会议报告提供。两名研究生将接受高分辨率数值模拟和数据分析技术的培训。一个公共项目网站也将维护，将我们模拟的初始条件和参数存档，并应其他有兴趣合作的研究人员的要求共享。提出的方法将使用Boussinesq伪谱模型来研究横向色散和内波能量级联。具体目标是确定(1)波/波和波/涡相互作用在小垂直尺度内波级联和湍流产生中的作用，(2)波/波和波/涡在等压搅拌中内波场的去相位，(3)内波剪切色散中的湍流间歇性，以及(4)涡旋模式剪切色散和搅拌。模型结果还将用于检验(5)细尺度滚转的物理学和(6)不稳定剪切事件的统计（理查森数小于四分之一）。模拟有和没有由内波破裂产生的细尺度位涡将被用来隔离涡型反级联的作用。被动示踪剂受横向混合和非扩散拉格朗日粒子将被用来区分剪切分散和搅拌。模拟将在不同的浮力频率、科里奥利频率、内波频谱能量水平和频谱形状下运行，以探索级联、细尺度滚转、理查德森数统计、横轴扩散和等轴扩散的基本参数依赖关系。将得到ogcm和区域模式的一阶亚中尺度水平扩散系数。不同的强迫（地面风，潮汐）维持在统计稳定状态的内波场将被测试。分析夏季在马尾藻海收集的浮子和染料数据将用于指导模型初始化并评估结果的真实性。在可能的情况下，将识别模型分散特性的差异，并将其用于识别观测数据集中的搅拌机制。

Lateral stirring is among the fundamental processes, along with advection and diapycnal (vertical) mixing that determines the distribution and fate of water-mass properties, nutrients and dissolved gases in the ocean. Ocean general circulation models (OGCMs) resolve stirring on scales larger than ~30 km, and regional models on scales ~1 km, but isopycnal (lateral) stirring by submesoscale (1 to 10 km) and finescale (10 m to 1 km) processes must be parameterized. Tracer-release experiments consistently find isopycnal diffusivities at scales of 1-10 km to be an order of magnitude larger than predictions for internal-wave shear dispersion. This project will provide a more complete and accurate assessment of the roles of internal waves and vortical mode in lateral stirring at the submesoscale of order 10 m to 10 km), and the cascade of energy to dissipative vertical scales, to resolve this paradox. Internal waves and vortical mode are difficult to distinguish observationally. However, judicious use of a numerical model can tease apart their influences by isolating different physics. A major advantage of the approach of this project is that all of the above processes/dynamics can be explored in the context of a single model, allowing us to seamlessly explore the internal-wave cascade and lateral dispersion by the different mechanisms across a wide range of parameter regimes and forcing conditions. The proposed simulations will elucidate the underlying physics and inform our understanding of how these processes work in the ocean so as to provide an order-one parameterization of submesoscale isopycnal diffusivities for OGCMs. Parameterizations developed in this study will be made available through peer-reviewed publications and conference presentations. Two graduate students will be trained in high resolution numerical modeling and data analysis techniques. A public project website will also be maintained, with initial conditions and parameters for our simulations archived and shared on request with other investigators interested in collaboration.The proposed approach will use a Boussinesq pseudo-spectral model to investigate lateral dispersion and the internal-wave energy cascade. Specific goals are to determine the roles of (1) wave/wave and wave/vortex interactions in the internal-wave cascade to small vertical scales and turbulence production, (2) wave/wave and wave/vortex de-phasing of the internal wave field in isopycnal stirring, (3) turbulent intermittency in internal-wave shear dispersion, and (4) vortical-mode shear dispersion and stirring. Model results will also be used to examine (5) the physics of the finescale roll-off and (6) the statistics of unstable shear events (Richardson number less than a quarter). Simulations with and without finescale potential vorticity production by internal-wave breaking will be used to isolate the roles of the vortical-mode inverse cascade. Passive tracers subject to diapycnal mixing and non-diffusive Lagrangian particles will be used to distinguish between shear dispersion and stirring. The simulations will be run with varying buoyancy frequency, Coriolis frequency, internal-wave spectral energy level and frequency spectral shapes to explore fundamental parameter dependences of the cascade, finescale roll-off, Richardson Number statistics, diapycnal diffusion and isopycnal diffusion. First-order submesoscale horizontal diffusivities for OGCMs and regional models will result. Different forcings (surface wind, tidal) for maintaining the internal-wave field in a statistically steady state will be tested. Profiling float and dye data collected in the Sargasso Sea during summer will be used to guide model initialization and assess the realism of the results. Where possible, differences in modelled dispersion characteristics will be identified and used to identify stirring mechanisms in the observational data sets.