Thermodynamic Controls on the Global Ocean Overturning Circulation

Abstract

This dissertation explores the fundamental relationship between the spatial distribution of air-sea flux and the structure of the ocean's Global Overturning Circulation (GOC). The GOC describes the circulation of ocean waters from the surface ocean at high latitudes to the deep and abyssal ocean, through the interior ocean, and ultimately back to ocean surface. In steady-state, this GOC persists even while the ocean density structure, which differs greatly with location, remains steady in time. To sustain the density structure, flow through the ocean must encounter sources and sinks of density at the ocean surface. The input of heat and freshwater fluxes at the surface supply the surface sources and sinks of density. Here, this thermodynamic requirement to maintain a steady state is exploited using an established framework - the Water Mass Transformation Framework - in a novel application: to arrive at a quantitative exploration of the relationship between the three-dimensional GOC and the distribution of surface heat and freshwater fluxes on global scale. This global analysis is approached in several steps. First, the downwelling branch of the GOC is explored. Specifically, the WMT framework is applied to examine how the dense water formation in the Southern Ocean relates to regional surface fluxes in a fully-coupled climate model. This study demonstrates that the surface processes mediating heat loss from the ocean have a fundamental influence on how dense water is circulated through the Southern Ocean. In the following study, the upwelling branch of the GOC (required to compensate for the formation of dense waters at polar surfaces) is considered. Specifically, to complete an overturning circulation, dense waters must form at the surface, circulate downward, and then return to the surface in a manner that maintains a steady global density structure. This study explores, from a theoretical perspective, how water can circulate between regions of the ocean surface in a thermodynamically consistent manner. A theoretical set of governing relationships between interior flow, surface density flux, and interior density transport are derived to reveal an important coupling between the spatial pattern of surface density flux and interior dynamics. In the final study of this dissertation, these theoretical relationships are tested in a fully-coupled global climate model. This global scale analysis reveals that the GOC transports buoyancy from the Indo-Pacific Oceans, through the Southern Ocean, and into the Atlantic Ocean; this transport is sustained by regional differences in surface heat and freshwater fluxes. To close, this dissertation offers an explanation for why the system may be organized into its current structure. Specifically, a hypothesis is advanced that argues for a preconditioning in the climate system towards an ocean circulation which conveys buoyancy out of the Indo-Pacific, through the Southern Ocean, and towards the North Atlantic. This preconditioning, it is suggested, arises from the location of these major ocean basins relative to the distribution of incoming solar radiation, two constrained features of the currently climate system.