Dynamics of Cross-Equatorial Flows

海洋在全球气候系统中最重要的功能之一是经向（南北）输送热量和淡水。这些运输需要将热量（淡水）增益区域与净热量（淡水）增益区域连接起来，这些区域通常位于很远的地方。大西洋表面的热通量就是一个明显的、气候上重要的例子。在亚极地的北大西洋和北欧海存在净损失，这被南半球向北的净热通量所平衡。海洋还将淡水和二氧化碳、氧气等重要气体输送到赤道彼岸。虽然控制这种大规模输送的动力学是人们普遍感兴趣的，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限制。本项目的重点是更好地了解经向翻转环流的中深海和上层海水如何穿过赤道。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。本项目将培养1名理论物理海洋学、数值方法和气候科学方面的博士后。这个项目的结果将被纳入研究生水平的课程和地球物理流体动力学暑期学校的讲座。这项提议的工作解决了全球尺度环流的一个基本方面，这个方面已经得到了广泛的认识，但仍然知之甚少：上层海洋和中层海洋是如何穿过赤道的。基本位涡的考虑表明，赤道对从一个半球到另一个半球的气流具有独特的动力转变。行星涡度在赤道上的符号变化意味着，如果这些水要被平流到赤道带外，某些非保守过程必须变得活跃起来。Ertel位涡方程给出的基本动力约束为该工程提供了理论框架。该方法将利用理想的、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，它们如何与经向热量和淡水的输送联系起来，以及它们是否可以在低分辨率气候模式中被参数化以准确表示。首先，将开发在浮力和/或风的推动下，横跨赤道的上层海洋和中深度交换的理想的高分辨率模式。然后，将开发诊断并应用于解释模型运行中的动力学，包括Ertel潜在涡量预算、拉格朗日粒子跟踪和稳定性分析。将确定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场和涡旋场的潜在联系。分析的要素与中纬度动力学有关，如位涡的涡流混合、转换欧拉平均值诊断和边界层标度。最后，控制跨赤道气流的机制将与更广泛关注的次级量有关，如经向热量和淡水通量、涡旋驱动的平均流量和海气交换。

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.