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

“全球纬向翻转环流”是一种洋流，它将温暖的、通常是咸的水从热带带到较冷的极地地区。这种环流的作用是在地球仪周围重新分配热量，并对地球气候产生深远的影响，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翻转产生了强烈的影响。然而，在南大洋，人们对这种海流的动力学知之甚少。在这个项目中，研究员（Christopher Chapman）将与巴黎LOCEAN-IPSL的赞助科学家Jean-Baptiste Sallée一起，利用简单的海洋模型、海洋学仪器的观测和复杂的全球气候模型，努力了解大型海底地形和规模小得多的湍流（称为“漩涡”）在全球翻转中所起的作用。我们的研究结果将有助于减少未来气候变化预测的不确定性，并将有助于政策制定者了解其决策的潜在影响。南大洋对全球翻转环流的影响很大。正是在南大洋，寒冷的深层沃茨涌上海面，表面浮力强迫和底涡混合过程导致水团转变，在那里，高能斜压涡旋起作用，对抗风强迫驱动的总体向北翻转环流。然而，由于缺乏对该地区的观测，以及运行大规模涡解模拟的高昂费用，人们对南大洋翻转的了解甚少。特别是，迄今为止，大多数研究利用南大洋洋流主要是纬向的事实，将MOC视为二维系统，忽略了经度的变化。然而，最近的观测表明，南大洋不是二维的。湍流、示踪剂混合和水团俯冲强烈地局限于大型测深特征的下游区域，如凯尔古勒高原。最近的工作表明，这些水深特征附近的湍流过程可能导致海洋物理结构的大规模重新排列。因此，这些局部尺度的动力学可能会对全球翻转的大尺度结构产生影响。该项目旨在混合描述性和基于过程的方法来研究“局部”动态对“全球”翻转的影响。该项目的第一阶段将利用来自卫星、阿尔戈浮标、船舶和装有仪器的象海豹的数据，以确定由于水流与大型测深特征相互作用而产生的动力过程如何影响大规模海洋结构，特别是位涡结构和热量和质量的纬向通量。该项目的第二阶段将使用一个高分辨率（涡流分辨率）数值海洋模型，以理想化的配置运行，旨在代表南大洋。该模型将在有和没有大规模水深测量以及有和没有湍流涡流的情况下运行。最后，将对CMIP 5数据库中的气候模式的输出进行检查，以确定南大洋水流/水深相互作用的准确表示对全球翻转的响应有什么影响。

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.