A Multi-Basin Residual-Mean Model for the Global Overturning Circulation

The ocean's overturning circulation inherently has a three-dimensional structure. Yet modern quantitative estimates of the overturning typically represent the sub-surface circulation as a two-dimensional, two-cell streamfunction that varies with latitude and depth, which suppresses information about zonal mass and tracer transport. Here we explore the dynamics of a ``figure-eight'' circulation that cycles through multiple basins.

We present two approaches that extend zonally-averaged overturning models to provide an idealized representation of the three-dimensional circulation. (i) We derive a two-basin residual-mean model of the overturning that permits mass transport between basins along isopycnals in a Southern Ocean-like channel. (ii) We introduce a multi-basin, isopycnal box model that solves for the stratification and water mass transport in two or more basins. In both cases, water mass modification, a component of the model solutions, occurs at the surface Southern Ocean and diffusively in northern basins. The residual-mean model accurately predicts the differing stratification between Atlantic and Pacific basins, indicating that a deeper stratification in the Pacific supports a larger flux of Antarctic Bottom Water into this basin. The dependence of the stratification and biogeochemical tracer distributions, in each basin, to external parameters such as the strength of deep and bottom water formation as well as the width of the two basins, is examined with the box model. A key result of both models is that most North Atlantic Deep Water is transformed into Antarctic Bottom water and upwells diffusively in the Pacific basin, which undermines the importance of an adiabatic overturning cell in the modern ocean. We discuss the utility of these models for understanding three-dimensional water mass and tracer distributions and for identifying transitions in the overturning circulation across climatic time scales.