Observations and Modeling of the Transition Layer

上层海洋是气候系统的重要组成部分。它调节着海-气之间的热量、动量、气体和营养物质的通量，在海洋生物地球化学中发挥着重要作用。目前的气候模式在地表混合层(ML)的深度方面通常有很大的误差，可能是因为它们的上层海洋参数化缺乏基本的物理。最近的努力改进了表面附近的物理表示，促使人们对上层海洋的另一个关键但鲜为人知的部分进行了研究？湍流ML和分层内部之间的过渡层(TL)。在这里，地表产生的湍流作用于下面的密度和示踪剂梯度，从而控制ML的深度以及进入ML的热量、盐、营养物质和其他示踪剂的通量。这项研究将结合新的理论见解、新的测量方法和高分辨率的数值模拟来了解TL动力学及其在新一代基于物理的上层海洋参数化中的作用。分析将描述TL结构，重点放在内波和不稳定性的物理上，它们的强迫和耗散机制，以及它们在设定上层海洋混合和夹带速率方面的作用。加强对这些过程的了解，对于改进区域和全球海洋和气候模型的性能至关重要。混合层深度和跨TL的营养物质通量在控制生物生产力的速率和特征，从而控制生物碳泵的强度方面起着关键作用。因此，改进的TL参数将直接应用于物理和生物地球化学海洋模型。大涡模拟被广泛用于发展边界层混合的参数化；这些分析将评估其对TL的精度。一名博士后将接受为期两年的支持，并接受该小组在过去25年开发的小规模拉格朗日海洋学方法的培训。该测量计划将结合APL/UW拉格朗日浮标已被证明的能力与最新开发的基于浮标的高精度声学多普勒海流剖面仪，以准确瞄准TL并对其进行采样。两个专门的彩车将部署在海洋气象站P(北纬50度，西经145度)，在2018年秋季执行为期100天的任务，这是风力驱动的ML快速加深的季节，因此TL活动最大。浮标将对TL的垂直速度、切变和层结进行有针对性的高分辨率测量。OWS-P现有的NOAA和OOI系泊设施将提供高质量的地表通量和长期的水文环境。现有的波纹浮标将被重新部署，以提供可靠的面波测量，这将确保这一宝贵的气候记录再延续两年。大涡模拟(LES)将是该项目的一个组成部分，指导采样策略的设计和数据的解释。本研究的中心假设是热释光的波动和不稳定性及其与湍流的相互作用控制了热释光中的混合和卷吸。TL被设想为一个共振的、通常是由ML从上方强迫的边缘不稳定的层。它对这种强迫的响应详细地取决于平均切变和层结廓线。解决这一假设的问题是：Q1：TL的平均切变和层结剖面是什么，这些与它的不稳定、波动、湍流和混合有什么关系？什么是理论波模式和不稳定模式？它们可以在过程模型或观察中识别吗？它们是如何分解为湍流和混合的？问题2：ML湍流如何设置TL的这些属性？TL波和不稳定性是由ML湍流直接强迫的，还是它们是TL内部的？Q3：模式模拟这些物理吗？现有的大涡模拟方案和总体参数化对观测到的结构和演化的描述有多好？怎样才能改善呢？

The upper ocean is a crucial component of the climate system. It mediates the air-sea heat, momentum, gas and nutrient fluxes and plays a large role in ocean biogeochemistry. Current climate models typically have large errors in the depth of the surface mixed layer (ML), likely because their upper ocean parameterizations lack essential physics. Recent efforts have improved representations of the physics near the surface, motivating this look at the other critical yet poorly understood part of the upper ocean? the transition layer (TL) between the turbulent ML and stratified interior. Here, the surface-generated turbulence acts upon the density and tracer gradients below, thereby controlling the depth of the ML and the fluxes of heat, salt, nutrients and other tracers into the ML. This study will combine new theoretical insights with new measurement methods and high-resolution numerical simulations to understand TL dynamics and its role in the new generation of physics-based upper ocean parameterizations. Analyses will describe the TL structure with a focus on the physics of internal waves and instabilities, their forcing and dissipation mechanisms, and their role in setting the upper-ocean mixing and entrainment rates. An improved understanding of these processes is crucial to improving the performance of regional and global models of the ocean and climate. Mixed layer depth and the fluxes of nutrients across the TL play a key role in controlling the rate and character of biological productivity and thus the strength of the biological carbon pump. Improved parameterizations of the TL would thus have direct application to both physical and biogeochemical ocean models. LES is widely used to develop parameterizations of boundary layer mixing; these analyses will assess its accuracy for the TL. A post-doc will be supported for two years and trained in the methods of small-scale Lagrangian oceanography developed by this group over the last 25 years.The measurement program will combine the proven abilities of APL/UW Lagrangian floats with the recent development of highly accurate float-based Acoustic Doppler Current Profilers to accurately target and sample the TL. Two specialized floats will be deployed at Ocean Weather Station P (50N 145W) for100-day missions in fall 2018, the season of rapid wind-driven ML deepening and thus maximum TL activity. The floats will conduct targeted high-resolution measurements of vertical velocity, shear and stratification across the TL. Existing NOAA and OOI moorings at OWS-P will provide high quality surface fluxes and long-term hydrographic context. An existing Waverider buoy will be redeployed to provide reliable surface wave measurements, which will ensure continuation of this valuable climate record for another 2 years. Large Eddy Simulation (LES) will be an integral part of the project, guiding the design of the sampling strategies and interpretation of the data. The central hypothesis of this research is that the waves and instabilities of the TL and their interaction with turbulence in the ML control the mixing and entrainment in the TL. The TL is envisioned as a resonant and often marginally unstable layer forced by the ML from above. Its response to this forcing depends in detail on the mean shear and stratification profiles. Questions that address this hypothesis are:Q1: What are the mean shear and stratification profiles of the TL, and how are these related to its instabilities, waves, turbulence and mixing? What are the theoretical wave and instability modes? Can they be identified in process models or observations. How do they break down to turbulence and mixing?Q2: How are these properties of the TL set by the ML turbulence? Are the TL waves and instabilities directly forced by the ML turbulence or are they internal to the TL?Q3: Do models simulate these physics? How well do existing LES schemes and bulk parameterizations describe the observed structure and evolution? How could they be improved?