Dynamics of Cross-Equatorial Flows

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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.