Sverdrup Balance and Three Dimensional Variability of the Meridional Overturning Circulation

• Main Author: Thomas, Matthew
• Format: Thesis
• Language: English
• Published: 2012
• Subjects: Labrador Sea
• Online Access: https://ueaeprints.uea.ac.uk/id/eprint/48025/; https://ueaeprints.uea.ac.uk/id/eprint/48025/1/matthewthomas_thesis.pdf

Abstract Two numerical models are used to gain an understanding of the spatial structure of Atlantic Meridional Overturning Circulation changes and the dynamical framework within which those changes occur. Sverdrup balance is studied using the 16 year ECCO-GODAE state estimation. It is shown to hold well in the interior subtropics when integrating to a mid-depth level and when considered at spatial scales larger than approximately 5◦. Outside of the subtropics, in western boundary currents and at short spatial scales, significant departures occur mostly due to a failure in the assumption that there is a level of no motion that can be integrated to and partly due to the assumption of linear vorticity. Sverdrup balance is reached when enough time is allowed for the ocean to adjust to forcing by the propagation of baroclinic Rossby waves. A climate change simulation of the HiGEM high resolution coupled climate model is used to investigate to what extent a 30% reduction in the deep southward transport is balanced by a reduction in the northward flowing surface western boundary transport, or an increase in the southward upper interior transport. It is found that a reduction in the southwards deep transport is balanced solely by a weakening of the northward surface western boundary current. This is consistent with Sverdrup balance holding to a good approximation in the basin interior. Overturning calculations in depth space and density space are found to differ within the subpolar gyre of a 120 year Control simulation of HiGEM. Depth space overturning is found to depend strongly on the transports of the Labrador current, which are strengthened by a spin-up of the horizontal subpolar gyre. Density space overturning is found to be strongly dependent on the densities of the Labrador Current, which increase following Labrador Sea water mass transformation and strong flow through the Denmark Straits.