Dynamics of the subpolar North Atlantic ocean circulation—the role of surface forcing and topography

Thesis

The ocean is a fundamental component of the global climate system, acting as a colossal sink of heat and carbon dioxide. Its effectiveness as a sink is enhanced by the transport of these tracers into the deep ocean by the Atlantic Meridional Overturning Circulation (AMOC). Hence, due to its location near the northern terminus of the AMOC, the subpolar North Atlantic plays a crucial role within the global climate system. Unfortunately, this is a fiendishly difficult region to model. As such, a better understanding of the physical mechanisms setting the large-scale flow at these latitudes is required.

In this thesis, the roles played by wind stress, surface buoyancy forcing, and topography in determining the mean barotropic and overturning circulations in the subpolar North Atlantic Ocean are explored. An idealised model of the region is introduced, and the response of the mean circulation to different combinations of wind, buoyancy forcing and bathymetry is presented. The overturning circulations in both depth, and density, space are split into shear, Ekman, and external mode components.

A novel diagnostic is then derived allowing for the isolation of the force balances controlling the large-scale flow. This diagnostic is based on the previously proposed rotational momentum framework, whereby the rotational component of an applied force is extracted in the form of a set of force func- tions. An error in the original derivation of this framework is highlighted which limits its utility to domains without topography. A new derivation generalising the approach to apply to domains with arbitrary topography is presented, and the derived force functions are related to the barotropic and overturning circulations.

This diagnostic is then applied to the aforementioned experiments, allowing the dominant force balances in each to be identified. A key result emerging from this analysis is that the presence of sloping sidewalls drastically alters the dominant force balance in the mean barotropic circulation.

Authors: Boneham, J

• DOI: 10.5287/ora-zvpqyxg98
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: streamfunction; North Atlantic; ocean modelling; barotropic; vorticity; large-scale ocean circulation
• Subjects: Physical Oceanography