Boundary constraints on the large-scale ocean circulation

• Main Author: Jonathan, T
• Other Authors: Johnson, H, Marshall, D, Bell, M, Munday, D
• Format: Thesis
• Language: English
• Published: 2022
• Subjects: Meridional overturning circulation; Boundary Decomposition; Physical oceanography; AMOC variability; Antarctic circumpolar current; Antarctic; Drake Passage; The Antarctic; Antarc*
• Online Access: https://ora.ox.ac.uk/objects/uuid:7b703739-186d-4608-b886-240fe1a4397e

The Meridional Overturning Circulation (MOC) is a system of surface and deep currents encompassing all ocean basins, crucial to the Earth's climate, transporting heat, carbon and nutrients around the globe. 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). In this thesis, the role of boundary properties (including density and velocity) 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, providing a means to quantify the relative importance of boundary contributions on different timescales. 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, in terms of magnitude and spatial structure. The time-mean maximum overturning streamfunction is relatively constant with latitude, despite its underlying boundary contributions varying considerably, especially in the northern hemisphere. Regression modelling, supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics, provides a natural ranking of the various boundary components in explaining total transport variability. At short timescales the bottom, western boundary and Ekman components play a dominant role. At decadal timescales, the importance of both boundary density components is revealed. 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 ...