My PhD is being done under the supervision of Dr John Fyfe
and Dr Andrew Weaver
in the Climate Modelling Lab at the University of Victoria. The project is within the NSERC CREATE programme, and has two streams:
1. Modelling Southern Ocean Dynamics: The influence of the Agulhas System
The ocean is largest rapidly exchanging carbon reservoir on the planet, and controls the atmospheric CO2 concentration on millennial time-scales (Sigman and Boyle, 2000). Accurate simulations of the global ocean circulation are thus vital to realistic simulations of the global climate system, and the projections of climate change. In particular, the Southern Ocean is of paramount importance because it ventilates the largest volume of the ocean interior (Ganachaud and Wunsch, 2000), and is the principal conduit of exchange between the three major ocean basins (e.g. Lumpkin and Speer, 2007).
The root mean square of the absolute dynamic topography in the Southern Ocean. The Agulhas Return Current in the SW Indian Ocean is the region of highest mesoscale variability in the Southern Ocean. Within the ACC proper, high variability is limited to a few isolated regions downstream of prominent topography. The MADT is a merged product from AVISO.
Despite its importance, the circulation dynamics of the Southern Ocean are still poorly understood. Recent developments suggest that meso-scale eddies may play a prominent role in the dynamics of the Southern Ocean (e.g. Ito et al., 2010; Boning et al., 2008). Meso-scale eddies are small (10 - 100 km wide) features, that can only be resolved by computationally expensive models with a fine grid. However, no current global climate models, and few ocean models currently resolve eddies. The Agulhas Return Current (ARC) in the SW Indian Ocean is the point of highest eddy kinetic energy in the Southern (and probably the global) ocean. It is also know that heat enters the Southern Ocean from the SW Indian Sector, and important modifications of Sub-Antarctic Mode water occur in this region. Little research has been done to evaluate the influence of the Agulhas Current System on the dynamics and thermodynamics of the Southern Ocean. This is the focus of our project.
In order to understand the dynamics of the Southern Ocean, and how this system might respond to variations in future forcing scenarios, we require eddy resolving numerical models (Wolfe and Cessi, 2009). Developing such models, will help to elucidate the dynamics and sensitivity of the Southern Ocean circulation to variations in forcing (Hallberg and Gnanadesikan, 2006). Such information is vital to the development of reliable ocean and global climate models that project future climate change (Spence et al., 2009).
Short project description:
An established ocean circulation model, the MITgcm, will be used to simulate the ocean at a range of resolutions from coarse (2 degrees) to eddy resolving (0.1 degrees). The focus of the effort will be to realistically simulate the circulation of the Southern Ocean, within the global framework. In particular, the sensitivity of the equilibrium ocean circulation to changes in surface forcing and basin geometry will be investigated within the fully eddying regime.
The highly idealized geometries of the one and two basin models. There is a re-entrant channel in the south, which had a 3000 m deep sill, the remainder of the basin is 5000 m deep. The models extend over both hemispheres, and thus have the ability to resolve the influence of northern deep water formation on ACC dynamics. The introduction of an idealized African continent produces an Agulhas Return Current.
The experimental design consists of a control simulation, which is forced using a constant, uniform observed surface forcing for 3000 years, so as to reach an equilibrium state. In particular, a highly idealized single large basin control run will be compared to a two basin model, which includes an Agulhas Return Current. This will allow us to evaluate the influence of the ARC on the Southern Ocean circulation.
Multiple perturbation experiments will then be performed, and compared to the controls. The perturbation experiments will include shifting the man latitude of the Southern Hemisphere westerly wind stress, in accordance with observations and the projections of global climate models. The sensitivity of the model to changes in surface buoyancy forcing (heat and fresh water flux) will also be evaluated.
- Boning, C. W., Dispert, A., Visbeck, M., Rintoul, S. R., Schwarzkopf, F. U., 2008. The response of the antarctic circumpolar current to recent climate change. Nature Geosci 1, 864–869.
- Ganachaud, A., Wunsch, C., 2000. Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data. Nature 408, 453–457.
- Hallberg, R. W., Gnanadesikan, A., 2006. The role of eddies in determining the structure and response of the wind-driven southern hemisphere overturning: Results from the modeling eddies in the southern ocean (meso) project. Journal of Physical Oceanography 36, 2232– 2252.
- Ito, T., Woloszyn, M., Mazloff, M., 2010. Anthropogenic carbon dioxide transport in the Southern Ocean driven by ekman flow. Nature 463, 80–84.
- Lumpkin, R., Speer, K., 2007. Global ocean meridional overturning. Journal of Physical Oceanography 37, 2550–2562.
- Sigman, D., Boyle, E., 2000. Glacial/interglacial variations in atmospheric carbon dioxide. Nature 407, 859–869.
- Spence, P., O.A., S., Eby, M., Weaver, A., 2009. The Southern Ocean overturning: Parameterized versus permitted eddies. Journal of Physical Oceanography 39, 1634–1651.
- Wolfe, C., Cessi, P., 2009. Overturning circulation in an eddy-resolving model: The effect of the pole-to-pole temperature gradient. Journal of Physical Oceanography 39, 125–142.