Dynamics of the descending branch of the Atlantic Meridional Overturning Circulation

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。大西洋经向翻转环流（AMOC）是全球气候系统的重要组成部分。古证据表明，这种环流经历了快速的重组，特别是在最后一个冰期。了解这一气候引擎的驱动机制及其对过去和未来扰动的敏感性，需要深入了解维持AMOC的动力过程。在这里，我们关注建立其下沉分支的过程。该项目将对北大西洋次极区热驱动环流的动力学进行全面研究。总体目标是了解AMOC的基本三维动态。包括其对水平和垂直混合、浮力通量、风、底部地形和盆地几何以及南半球强迫的依赖。为了实现这一目标，我们将开发海洋环流模型（OGCM）的层次解，并在有用的情况下获得层模型的相应解。在我们的层次结构的基础上是一个热强迫的、不粘的、OGCM解决方案，它不会产生明显的倾覆。因此，我们研究的第一项任务是详细了解打破这种零状态的过程（水平与垂直混合，扩散与粘度，表面加热强度）。进一步的任务将依次考虑我们的目标中列出的其他过程的影响。预期的结果是更好地理解决定深次极下潜强度和位置的过程及其与大尺度AMOC的关系。通过我们对AMOC下沉分支物理学的关注，本研究将提供重要的动力学见解，这将有助于评估AMOC相对于密度扰动的稳定性。它还将有助于解释显示北大西洋地区AMOC快速转变证据的古代理数据，并有助于评估非涡旋解决未来气候变化模式模拟。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate system. Paleo-evidence suggests that this circulation has experienced rapid reorganizations, particularly during the last glacial period. Understanding the driving mechanisms of this climate engine and its sensitivity to past and future perturbations requires an in-depth understanding of the dynamical processes that maintain the AMOC. Here, we focus on the processes that establish its sinking branch.This project will conduct a comprehensive study of the dynamics of thermally-driven circulation in the subpolar North Atlantic. The overall goal is to understand the fundamental, 3-dimensional dynamics of the AMOC?s large-scale descending branch, including its dependence on horizontal and vertical mixing, buoyancy fluxes, winds, bottom topography and basin geometry, and southern-hemisphere forcing. To address this goal, we will develop a hierarchy of solutions to an ocean general circulation model (OGCM) and, when useful, obtain corresponding solutions to layer models. At the base of our hierarchy is a thermally forced, inviscid, OGCM solution that develops no diapycnal overturning. A first task in our study, then, is to understand in detail the processes (horizontal vs. vertical mixing, diffusion vs. viscosity, strength of surface heating) that breakdown this null state. Further tasks will sequentially consider the impacts of the other processes listed in our goal.The expected outcome is an improved understanding of the processes that determine the strength and location of deep subpolar downwelling and its relationship to the large-scale AMOC.Through our focus on the physics of the AMOC sinking branch, this research will provide important dynamical insights that will help to assess the stability of the AMOC with respect to density perturbations. It will also help in the interpretation of paleo-proxy data that show evidence of rapid AMOC transitions in the North Atlantic region and in the evaluation of non-eddy resolving future climate change model simulations.