Observations and Modeling of the Transition Layer

上层海洋是气候系统的重要组成部分。它调节海气热、动量、气体和养分通量，在海洋生物地球化学中起着重要作用。目前的气候模式在地表混合层（ML）的深度上通常有很大的误差，可能是因为它们的上层海洋参数化缺乏基本的物理特性。最近的努力已经改善了地表附近的物理表现，促使人们关注上层海洋的另一个关键但知之甚少的部分。湍流层与层状内部之间的过渡层（TL）。在这里，表面产生的湍流作用于下面的密度和示踪剂梯度，从而控制ML的深度以及热、盐、营养物质和其他示踪剂进入ML的通量。本研究将结合新的理论见解、新的测量方法和高分辨率数值模拟来理解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，重点是内波的物理性质和不稳定性，它们的强迫和耗散机制，以及它们在确定上层海洋混合和挟带率方面的作用。提高对这些过程的了解对于改善区域和全球海洋和气候模式的性能至关重要。混合层深度和养分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，改进的总温度参数化将直接应用于海洋物理和生物地球化学模式。LES被广泛用于边界层混合的参数化；这些分析将评估其对TL的准确性。一名博士后将获得为期两年的资助，并接受小规模拉格朗日海洋学方法的培训，该方法是该小组在过去25年中开发的。测量计划将结合APL/UW拉格朗日浮子的成熟能力和最近开发的高精度浮子声学多普勒电流分析器，以准确定位和采样TL。2018年秋季，两个专门的浮子将部署在海洋气象站P (50N 145W)，为期100天，这是风力驱动ML快速加深的季节，因此是最大的TL活动。浮标将对整个TL进行有针对性的高分辨率垂直速度、切变和分层测量。OWS-P现有的NOAA和OOI系泊将提供高质量的地表通量和长期水文背景。现有的乘波号浮标将被重新部署，以提供可靠的表面波测量，这将确保这项宝贵的气候记录在未来2年内继续存在。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，TL的波动和不稳定性以及它们与ML中的湍流的相互作用控制着TL中的混合和夹带。TL被设想为一个共振的、通常是边缘不稳定的层，由上面的ML强制。它对这种强迫的反应在细节上取决于平均切变和分层剖面。解决这一假设的问题是：Q1: TL的平均切变和分层剖面是什么，这些与它的不稳定性、波浪、湍流和混合有什么关系？理论波动模态和不稳定模态是什么？可以在过程模型或观察中识别它们吗？它们是如何分解成湍流和混合的？Q2: TL的这些特性是如何由ML湍流决定的？TL波和不稳定性是由ML湍流直接造成的，还是TL内部的？Q3：模型模拟这些物理吗？现有的LES方案和整体参数化如何很好地描述观测到的结构和演化？如何改进它们？

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?