Boundary constraints on the large-scale ocean circulation

TL;DR

This paper investigates how boundary properties influence the large-scale ocean currents, specifically the Atlantic MOC and Antarctic Circumpolar Current, using models to decompose and analyze their variability and boundary contributions.

Contribution

It introduces a boundary-based decomposition method for the MOC and ACC, revealing their structure and variability, and highlights the importance of boundary information in ocean circulation characterization.

Findings

Boundary properties significantly influence MOC and ACC dynamics.

Decomposition captures key features of the overturning circulation.

Model simulations show spatial resolution impacts ACC transport estimates.

Abstract

The Meridional Overturning Circulation (MOC) is a system of surface and deep currents encompassing all ocean basins, crucial to the Earth's climate. Detecting potential climatic changes in the MOC first requires a careful characterisation of its inherent variability. Key components of the MOC are the Atlantic MOC (AMOC) and the Antarctic Circumpolar Current (ACC). The role of boundary properties in determining the AMOC and ACC is investigated, as a function of cross-sectional coordinate and depth, using a hierarchy of general circulation models. The AMOC is decomposed as the sum of near-surface Ekman, depth-independent bottom velocity and eastern and western boundary density components. The decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. The estimated total basin-wide AMOC overturning streamfunction, reconstructed using only boundary information, is in good agreement with direct calculations of the overturning using meridional velocities. The time-mean maximum overturning streamfunction is relatively constant with latitude, despite its underlying boundary contributions varying considerably, especially in the northern hemisphere. Applying a similar decomposition diagnostic to the ACC provides insight into the differences between model simulations, revealing spatial resolution dependence of ACC transport through the Drake Passage. All models exhibit a weak ACC compared to observations. Density maps along sloping ocean boundaries are produced, incorporating the western and eastern Atlantic boundaries and the Antarctic coastline. Isopycnals are flat over long portions of eastern ocean boundaries, and slope linearly on western boundaries. Results of this thesis serve to indicate the importance of boundary information in characterising the AMOC and the ACC, and the relative simplicity of along-sloping-boundary density structure.