OCE-PRF Track 2 (International): Towards a 3-Dimensional Understanding of the Meridional Overturning Circulation

“全球经向翻转环流”是一种洋流，将温暖的、通常是含盐的水从热带带到较冷的极地地区。这种环流在全球范围内重新分配热量，并对地球气候产生深远影响，因此有时被称为“全球海洋传送带”。环绕南极大陆的南大洋对全球的颠覆产生了强烈的影响。然而，在南大洋，人们对这股洋流的动态知之甚少。在这个项目中，该研究员(Christopher Chapman)将与巴黎国际空间物理研究所的赞助科学家Jean-Baptiste Sallée一起，利用简单的海洋模型、海洋仪器的观测和复杂的全球气候模型，致力于理解大的海底地形和小得多的尺度湍流在全球倾覆中所起的作用。我们的结果将有助于减少未来气候变化预测中的不确定性，并将帮助政策制定者了解他们的决定的潜在影响。南大洋强烈影响着全球翻转环流。正是在南大洋，寒冷的深水上升到海面，表面浮力强迫和昼夜混合过程导致水团变化，在那里，高能斜压涡旋与风强迫驱动的普遍向北颠覆的环流相反。然而，该地区缺乏观测，而且运行大规模涡旋解析模拟的费用高昂，这意味着人们对南大洋的倾覆知之甚少。特别是，迄今为止的大多数研究都利用了这样一个事实，即南大洋洋流主要是以纬向为导向的，将MOC视为一个二维系统，忽略了经度的变化。然而，最近的观测表明，南大洋并不是二维的。湍流、示踪剂混合和水团俯冲强烈局限于大型测深特征的下游地区，如克尔古林高原。最近的工作表明，这些水深特征附近的湍流过程可能会导致海洋物理结构在大范围内重新排列。因此，这些局部尺度的动力可能会对全球颠覆的大尺度结构产生影响。本项目旨在将描述性方法和基于过程的方法相结合，以研究“局部”动态对“全球”颠覆的影响。该项目的第一阶段将使用来自卫星、Argo花车、船只和仪表式象海豹的数据，以确定由于水流与大水深特征相互作用而产生的动力过程如何影响大尺度海洋结构--特别是潜在涡度结构以及热量和质量的经向通量。该项目的第二阶段将使用一个高分辨率(涡旋分辨率)的海洋数值模式，以一种理想化的配置运行，旨在代表南大洋。该模型将在有或没有大尺度水深测量的情况下运行，在有或没有湍流涡流的情况下运行。最后，将审查CMIP5数据库中所载气候模型的输出，以确定南大洋流动/测深相互作用的准确表示对全球倾覆的反应有什么影响。

The "global meridional overturning circulation" is an ocean current that brings warm, often salty water from the tropics to the colder polar regions. This circulation acts to redistribute heat around the globe and has a profound influence of the Earth's climate, and for this reason is sometimes called the "global ocean conveyor belt." The Southern Ocean, which encircles the Antarctic continent, exerts a strong influence on the global overturning. However, in the Southern Ocean, the dynamics of this current are very poorly understood. In this project, the fellow (Christopher Chapman) together with sponsoring scientist Jean-Baptiste Sallée, at LOCEAN-IPSL, Paris, will work towards understanding the role that large undersea topography and much smaller scale turbulent flows, called "eddies" play in global overturning using a mixture of simple ocean models; observations from oceanographic instruments; and sophisticated global climate models. Our results will help reduce uncertainties in projections of future climate change and will aid policy makers to understand the potential impacts of their decisions. The Southern Ocean strongly influences the global overturning circulation. It is in the Southern Ocean that cold, deep waters upwell to the surface, that surface buoyancy forcing and diapycnal mixing processes cause water mass transformations and where energetic baroclinic eddies act to oppose the general northward overturning circulation driven by wind forcing. However, a lack of observations in the region and the prohibitive expense of running large-scale eddy-resolving simulations has meant that the Southern Ocean overturning is poorly understood. In particular, most studies to date exploit the fact that the Southern Ocean currents are primarily zonally oriented to treat the MOC as a two dimensional system, ignoring variations with longitude. However, recent observations have shown that the Southern Ocean is not 2-dimensional. Turbulence, tracer-mixing and water-mass subduction are strongly localized to regions downstream of large bathymetric features, such as the Kergulean Plateau. Recent work has indicated that turbulent processes near these bathymetric features can cause a rearrangement of the ocean's physical structure on large-scales. As such, these local-scale dynamics may have an influence of the large-scale structure of the global overturning. This project aims to blend descriptive and process based approaches to study the influence of "local" dynamics on the "global" overturning. The first stage of the project will use data from satellites, Argo floats, ships and instrumented elephant seals to determine how dynamical processes originating due to flow interaction with large bathymetric features influence the large-scale oceanic structure - in particular the potential vorticity structure and the meridional flux of heat and mass. The second stage of the project will use a high-resolution (eddy-resolving) numerical ocean model, run in an idealized configuration, meant to be representative of the Southern Ocean. The model will be run both with, and without, large-scale bathymetry and with, and without, turbulent eddies. Finally, the output of climate models contained in the CMIP5 database will be examined to determine what influence, if any, the accurate representation of flow/bathymetry interaction in the Southern Ocean has on the response of the global overturning.