Collaborative Research: A Study of Submesoscale Mixed-Layer Dynamics at a Mid-Latitude Oceanic Front: Isolating the Sub- and Super-Inertial Response to Atmospheric Forcing

海洋混合层是大气和海洋之间动量、热量和气体交换的管道和屏障。最近的模拟揭示了一系列相互竞争的亚中尺度混合层过程，它们可以促进或抑制海气交换。这些过程是一个观察性挑战，因为需要无混叠的时间和横向分辨率，以及建立持久机制的足够持续时间。这项研究计划通过在北太平洋副热带锋区进行为期28天的实验，极大地改善对三维混合层动力学中1-10公里尺度横向过程的理解，北太平洋副热带锋区是一个大气强迫高于平均水平、典型的大洋中尺度平流和应变以及典型的亚中尺度活动的地区，最大限度地增加了发现重要信号的可能性。这一观测计划的结果将改善混合层参数化的物理基础，导致对海-气通量、气体输送和生物生产力的更好的模式预测。该项目将培养一名研究生，并为本科生提供研究经验。一个邮轮博客和实验网站将向更广泛的科学界和一般公众提供该项目的成果，其中包括多船调查、概况浮标阵列部署(16-20个浮标测量温度、盐度、水平速度和微观结构)和一个漂移的海气通量平台，该平台将采样在存在混合层锋面的情况下上层海洋对冬季风暴强迫的反应，而混合层锋面是中央大洋涡旋的特点。这些阵列将每30-40分钟同步一次剖面，以消除时空混叠，即在地表和100-150米深度(过渡层以下)之间循环。在部署期间和部署之间，将使用(I)拖体和舰载声学多普勒海流剖面仪反复调查该区域，以提供更大尺度的背景和(Ii)多深度流动温度和盐度系统，以解决上层2-5m内较小的水平尺度。地面混合层中的次中尺度变化导致的动力过程不容易满足常用的准地转或一维表层近似。虽然这些过程是在孤立的或理想化的条件下研究的，但迄今为止的观测证据不足以澄清这些亚惯性过程是否能够在高频大气强迫和强烈的内波切变存在的情况下运行。计划的5-7阵列部署，每次3-5天，将具有足够的持续时间，将低频动力学的演变与主导短时间尺度的近惯性切变分开，从而能够通过混合层涡旋、强迫和垂直过程来评估混合层再层化和去层化率。

The ocean mixed-layer is both conduit and barrier for exchange of momentum, heat and gases between the atmosphere and ocean. Recent modeling has revealed a host of competing submesoscale mixed-layer processes which can either promote or inhibit air-sea exchange. These processes are an observational challenge because of the need for unaliased temporal and lateral resolution, as well as sufficient duration to establish persistent mechanisms. This study plans to dramatically improve understanding of 1-10 km scale lateral processes in three-dimensional mixed-layer dynamics with a 28-day experiment in the North Pacific Subtropical Front, a region of above-average atmospheric forcing, typical mid-ocean mesoscale advection and straining, and typical submesoscale activity, maximizing the likelihood of finding significant signals. The results of this observational program will improve the physical basis of mixed-layer parameterizations, leading to better model predictions for air-sea fluxes, gas transfer and biological productivity. The project will train one graduate student and provide research experiences to undergraduate students. A cruise blog and experiment website will make the results of the project available to the wider scientific community and general publicMultiple ship surveys, profiling float array deployments (16-20 floats measuring temperature, salinity, horizontal velocity and microstructure) and a drifting air-sea flux platform will sample the upper-ocean's response to winter storm forcing in the presence of the mixed-layer fronts that characterize the central ocean gyres. The arrays will profile in sync every 30-40 minutes to eliminate space-time aliasing, cycling between the surface and 100-150 m depth (below the transition layer). During and between deployments, the region will be surveyed repeatedly with (i) a tow-yo body and a shipboard Acoustic Doppler Current Profiler to provide larger-scale context and (ii) a multi-depth flow-through temperature and salinity system to resolve smaller horizontal scales in the upper 2-5 m. Submesoscale variability in the surface mixed-layer leads to dynamical processes which do not readily satisfy the quasigeostrophic or 1-D surface layer approximations in common use. Although these processes have been examined in isolation or idealized conditions, observational evidence to date is insufficient to clarify whether these subinertial processes are able to operate in the presence of high-frequency atmospheric forcing and strong internal-wave shears. The planned 5-7 array deployments of 3-5 days each will be of sufficient duration to separate the evolution of low-frequency dynamics from near-inertial shear that dominates short timescales, allowing evaluation of mixed-layer restratification and destratification rates by mixed-layer eddies, wind-forcing and vertical processes.