A Thermodynamic Model of the Global Ocean Overturning Circulation

The majority of conceptual and theoretical models of the ocean overturning circulation endeavor to explain this circulation by identifying key dynamic processes in the ocean’s interior and at its surface. Here, a novel theoretical model of the ocean circulation is presented which instead endeavors to explain the ocean circulation by identifying its thermodynamic role in the global climate system.

From a thermodynamic perspective, the ocean must transport heat and freshwater from where these properties are gained at the surface at low latitudes, towards where they are lost (according to a complex spatial pattern) at higher latitudes. Arguably, the most fundamental thermodynamic function of the interior ocean dynamics is to sustain, through a combination of advective and diffusive processes, the transport of ocean heat and freshwater through space. This study develops a framework to understand relate ocean processes to the heat and freshwater transport they sustain.

We introduce the Buoyancy Transport Function, which quantifies the combined transport of heat and freshwater and can derived directly from the surface and diffusive buoyancy flux field using a Helmholtz-Hodge Decomposition. Its streamlines map pathways buoyancy perturbations take between spatially remote isopycnal outcrops, quantifying the spatial distribution of thermodynamic transport through the ocean system. The derivation of the Buoyancy Transport Function requires no explicit knowledge of the overturning circulation. However, because buoyancy directly couples heat and freshwater content to ocean volume, this function can be equivalently defined as a function of the ocean velocity field. As such, the advective, and diffusive components of the volume circulation streamfunction (traditionally streamfunction Psi) can be quantifiably linked to the global buoyancy circulation. This provides a framework with which the following questions can be addressed: how must the circulation and stratification be organized, given some distribution of surface buoyancy flux? Or conversely: what is the distribution of surface buoyancy flux that must arise, given particular overturning circulation?

The true utility of this perspective emerges when considering the argument that the distribution of heat and freshwater surface fluxes might be broadly constrained by the distribution of insolation and rotation rate of the earth, which greatly influence the meridional distribution of surface temperature, and atmospheric moisture convergence and divergence. In this case, such external constraints will enforce persistent features in its meridional distribution of surface buoyancy flux, regardless of ocean dynamics. A novel model of the global ocean circulation system is thus put forward, which is described using the coupled relationship between the Buoyancy Transport Function and the ocean volume circulation, Psi. This model constrains the internal ocean state, specifically the distribution of advective (wind-pumped) and diffusive (mixing driven) components of the total circulation, by the air-sea flux field on a global scale. It provides a novel method to quantitatively link meridional features in surface buoyancy flux, potentially persistent for a given climate state, to the global structure of the Overturning Circulation.