Dynamics of Cross-Equatorial Flows

海洋在全球气候系统中最重要的功能之一是热量和淡水的经向(南北)输送。这些传送器需要将热量(淡水)获取区域与净热量(淡水)厕所区域连接起来，后者通常位于很远的距离。这方面的一个明显且气候重要的例子是大西洋的表面热流。亚极地北大西洋和北欧海域有净损失，南半球向北的净热流平衡了这一损失。海洋还将淡水和二氧化碳、氧气等重要气体输送到赤道。虽然控制这种大规模输送的动力是普遍感兴趣的，但赤道对流动动力学以及在哪里、如何以及可能有多少可以跨半球输送构成了严重的制约。该项目的重点是更好地了解经向翻转环流的中深水和上层海水如何穿越赤道。这项研究有可能将中纬度和赤道的动力学和环流机制联系起来，并进一步加深我们对全球尺度气候系统的理解。该项目将培养理论物理海洋学、数值方法和气候科学方面的博士后研究员。该项目的成果将被纳入地球物理流体动力学暑期学校的研究生课程和讲座。这项拟议的工作解决了全球范围的大气环流的一个基本方面，这一方面得到了广泛的承认，但仍然知之甚少：上层海洋和中层气流如何穿越赤道。基本的位涡考虑表明，对于从一个半球到另一个半球的气流，赤道构成了一个独特的动力学转变。赤道上行星涡度符号的变化意味着，如果这些水要平流到赤道带之外，一些非保守的过程必须变得活跃起来。Ertel位涡方程提出的基本动力学约束为该项目提供了理论框架。该方法将利用理想化的、非常高分辨率的数值模式和标度理论来确定哪些过程是活跃的，它们如何与热量和淡水的经向输送联系起来，以及它们是否可以被参数化以在低分辨率气候模式中准确地表示。首先，将开发由浮力和/或风强迫的赤道上海洋和中深度交换的非常高分辨率的理想化模式。然后，将开发和应用诊断学来解释模式运行中的动力学，包括Ertel位涡预算、拉格朗日粒子跟踪和稳定性分析。与动量和浮力的横向和垂直混合有关的机制，以及它们与大尺度平均和涡旋场的潜在联系，将被确定。分析的要素与中纬度动力学有关，例如位涡的涡旋混合、转换的欧拉平均诊断和边界层尺度。最后，控制越赤道气流的机制将与更广泛关注的次生量有关，例如经向热量和淡水通量、涡动驱动的平均气流和海-气交换。

One of the most important functions of the oceans in the global climate system is the meridional (North-South) transport of heat and freshwater. These transports Tare required to connect regions of heat (freshwater) gain with regions of net heat (freshwater) loos, which are often located at great distances. One clear and climatically important example of this is the surface heat flux in the Atlantic Ocean. There is a net loss in the subpolar North Atlantic and Nordic Seas, which is balanced by a net northward heat flux from the southern hemisphere. The ocean also transports freshwater and important gases like carbon dioxide and oxygen across the equator. While the dynamics that control such large-scale transports are of general interest, the equator poses an acute constraint on the flow dynamics and where, how, and perhaps how much can be transported across hemispheres. The focus of this project is to better understand how the mid-depth and upper ocean waters of the meridional overturning circulation cross the equator. This study has the potential to connect mid-latitude and equatorial dynamics and circulation regimes and further our understanding of the global-scale climate system. This project will train a post-doctoral fellow in theoretical physical oceanography, numerical methods, and climate science. The results from this project will be incorporated into graduate level classes and lectures at the Geophysical Fluid Dynamics Summer School. The proposed work addresses a fundamental aspect of the global-scale general circulation that is widely recognized yet still poorly understood: how upper ocean and mid-depth flow cross the equator. Basic potential vorticity considerations indicate that the equator poses a unique dynamical transition for flows crossing from one hemisphere to the other. The change in sign of the planetary vorticity across the equator implies that some non-conservative process must become active if these waters are to be advected outside the equatorial band. The basic dynamic constraints posed by the Ertel potential vorticity equation provides the theoretical framework for the project. The approach will make use of idealized, very high resolution numerical models and scaling theory to determine which processes are active, how they are connected to the meridional transport of heat and freshwater, and whether or not they can be parameterized for accurate representation in low resolution climate models. First idealized, very high resolution models of upper ocean and mid-depth exchange across the equator forced by buoyancy and/or wind will be developed. Then, diagnostics will be developed and applied to interpret the dynamics in the model runs, including Ertel potential vorticity budgets, Lagrangian particle tracking, and stability analysis. Mechanisms related to lateral and vertical mixing of momentum and buoyancy, and their potential connections to the large-scale mean and eddy field, will be identified. Elements of the analysis have connections to mid-latitude dynamics, such as eddy-mixing of potential vorticity, transformed Eulerian mean diagnostics, and boundary layer scaling. Finally, the mechanisms controlling the cross-equatorial flow will be related to secondary quantities of broader interest, such as meridional heat and freshwater fluxes, eddy-driven mean flows, and air-sea exchanges.