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。大西洋子午线翻转环流(AMOC)是全球气候系统的一个关键组成部分。古证据表明，这种环流经历了快速的重组，特别是在最后一次冰川时期。要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，就需要深入了解维持AMOC的动力过程。在这里，我们将着重于建立其下沉分支的过程。这个项目将对北大西洋次极地热力驱动环流的动力学进行全面的研究。总体目标是了解AMOC-S大尺度下降分支的基本三维动力学，包括其对水平和垂直混合、浮力通量、风、海底地形和盆地几何以及南半球强迫的依赖性。为了解决这一目标，我们将开发海洋环流模式(OGCM)的解的层次结构，并在有用的情况下，获得层模式的相应解。在我们的层次结构的基础上，是一种热强迫的、无粘性的OGCM解决方案，它不会发生昼夜颠覆。因此，我们研究的第一个任务是详细了解破坏这种零态的过程(水平与垂直混合、扩散与粘度、表面加热强度)。未来的任务将依次考虑我们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亚极下沉的强度和位置的过程及其与大尺度AMOC的关系。通过对AMOC下沉分支物理的关注，这项研究将提供重要的动力学见解，有助于评估AMOC在密度扰动方面的稳定性。它还将有助于解释显示北大西洋地区AMOC快速过渡证据的古替代数据，并有助于评估未来气候变化模型模拟的非涡旋解析。

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.