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CSIRO-UTAS PhD Program in Quantitative Marine Science

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Sensitivity of the ocean's overturning circulation to changes in climate

Supervision Team

Maxim Nikurashin

Steve Rintoul

Andy Hogg



The meridional overturning circulation (MOC) is a planetary-scale oceanic flow which is of direct importance to the climate system: it transports heat meridionally and regulates the exchange of CO2 with the atmosphere. The MOC is forced by wind and heat/freshwater fluxes at the surface and turbulent mixing in the ocean interior. The MOC is closely tied to the distribution of water masses in the ocean and consists of two overturning cells: the upper cell, corresponding to sinking of dense water mass in the North Atlantic, and the lower cell, corresponding to sinking of the densest water mass around Antarctica. Paleoclimate reconstructions suggest that the MOC was quite different in past climates: the boundary between the two overturning cells was substantially shallower and the strength of the MOC was likely weaker during the Last Glacial Maximum than in the present climate. Various studies suggest that variations in the MOC may have been responsible for the low CO2 in the atmosphere in glacial climates.

Schematic of the global overturning circulation of the ocean

Figure: Schematic of the global overturning circulation of the ocean (Kuhlbrodt et al, 2007)


Project outline, objectives and methods

While being crucially important for the climate system, the dynamics of the MOC remain poorly understood. A number of conceptual theories have been developed and tested with idealized numerical simulations. However, the relevance of simple conceptual theories to the MOC simulated with higher complexity models or observed in nature still remains unclear. The overall goal of this project is to test and further improve our understanding of the dynamics that governs the MOC in the ocean. The project will focus on the sensitivity of the MOC to changes in forcing conditions across models with various complexities, ranging from simple theoretical models to realistic ocean and/or coupled general circulation models (GCMs). A few specific objectives of the project are:

  • Design an idealized numerical simulation of the MOC with a GCM and carry out sensitivity experiments with different ocean geometry, physical parameters, and forcing conditions
  • Simulate the MOC with realistic ocean and/or coupled GCM and explore its sensitivity to changes in forcing conditions
  • Analyze the results from both sets of simulations and compare them to theoretical predictions

Results of this project will improve our understanding of the response of the ocean’s overturning circulation to changing climate and hence the role of the ocean in past and future climates.

First potential publication: “The impact of the non zonally-symmetric wind and buoyancy forcing on the overturning circulation of the ocean”.

Skills needed to undertake this project

The student will need an undergraduate Mathematics and/or Physics Degree.