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

侧向搅拌是决定海洋中水团性质、营养物质和溶解气体的分布和命运的基本过程之一，它与平流和日(垂直)混合一起决定。海洋环流模式(OGCM)解决了大于30公里尺度的搅动，区域模式解决了~1公里尺度的搅动，但次中尺度(1~10公里)和细尺度(10 m~1公里)过程的等速(横向)搅动必须被参数化。示踪剂释放实验一致地发现，在1-10公里尺度上的等向扩散系数比预测的内波剪切弥散大一个数量级。该项目将更全面和准确地评估内波和涡旋模式在横向搅动中的作用(在10米至10公里量级的亚中尺度)，以及能量到耗散垂直尺度的级联，以解决这一悖论。内波和涡旋模式在观测上很难区分。然而，明智地使用数值模型可以通过隔离不同的物理来梳理它们的影响。这个项目方法的一个主要优点是，上述所有过程/动力学都可以在一个单一模式的背景下进行探索，使我们能够在广泛的参数制度和强迫条件下，通过不同的机制无缝地探索内波级联和横向频散。所提出的模拟将阐明潜在的物理机制，并使我们了解这些过程是如何在海洋中工作的，从而为OGCM提供亚中尺度等周期扩散系数的一阶参数。本研究中制定的参数将通过同行评议的出版物和会议发言提供。两名研究生将接受高分辨率数值建模和数据分析技术方面的培训。还将维护一个公共项目网站，将我们模拟的初始条件和参数存档并应要求与其他有兴趣合作的研究人员共享。拟议的方法将使用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)等轴搅拌中内波场的波/波和波/涡退相，(3)内波剪切频散中的湍流间歇性，以及(4)涡模剪切弥散和搅拌。模型结果也将被用来检查(5)精细尺度滚落的物理和(6)不稳定切变事件(理查森数小于四分之一)的统计。通过内波破裂产生FineScale位涡和不使用FineScale位涡的模拟将被用来分离涡模逆级联的作用。被动示踪剂受昼夜混合和非扩散拉格朗日粒子的影响，将用于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、内波频谱能级和频谱形状下运行，以探索叶栅、精细尺度滚降、Richardson数统计、双日扩散和等周期扩散的基本参数依赖关系。将产生OGCMS和区域模式的一阶亚中尺度水平扩散系数。将测试用于将内波场维持在统计稳定状态的不同作用力(地面风、潮汐)。夏季在马尾藻海收集的剖面、浮标和染料数据将用于指导模型初始化和评估结果的真实性。在可能的情况下，将确定模拟弥散特征的差异，并用于确定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.