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Polar
Waters (Ocean Processes)
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Southern Ocean Sector Model
Researchers: Dr Joerg-Olaf Wolff, Dr Nathan BindoffThe parameterization of small scale processes in coarse resolution ocean and sea-ice components of coupled climate models is an important and currently very active field of research. Recently proposed theoretical arguments which link the effect of the meso-scale eddy-field to large scale properties of the mean flow have been shown to dramatically improve certain characteristics of the oceanic circulation in the Southern Ocean (SO), like for example the depth of the mixed layer (McDougall, 1995, Hirst and McDougall, priv. comm.).
These problems are best studied using high resolution models in limited regions that can actually resolve these small scale processes. Such studies will allow us to characterize and develop or verify more detailed parameterizations of these processes in order to improve the current ocean and sea-ice models. Global atmosphere/ocean models still differ significantly in certain aspects of their dynamic and thermodynamic solutions, especially in the Southern Hemisphere.
A recent coupled coarse resolution ocean/atmosphere general circulation model (O/A-GCM) reported by Manabe et al. (1991) with a transient increase in the atmospheric CO2 concentration showed a significantly delayed warming of the Southern Hemisphere due to rapid ventilation through changed water masses in the SO. These changes could have important implications on the Australian regional climate. However, one limitation of coarse resolution ocean models is their inherent inability to reproduce the correct dynamical balance of the SO Circulation, especially the Antarctic Circumpolar Current (ACC).
This balance is intimately linked to the existence of meso-scale eddies, which are thought to be the main carrier of poleward heat transport in this region. Strong small scale mixing and deep overturning within the Southern Ocean probably have a profound influence on the pattern of greenhouse warming in response to transient global change forcing. These processes of water mass formation and transport are critical to the uptake and storage of carbon dioxide and other tracers in the deep ocean.
We are using a state of the art primitive equation ocean model (HOPE, Hamburg Ocean Primitive Equation Model) at high resolution in a sector of the Southern Ocean south of Australia (Figure 1). The model was developed at the Max-Planck-Institute for Meteorology in Hamburg, FRG. Prognostic variables are the three-dimensional velocity fields, sea-surface elevation and the thermohaline variables. The vertical distribution of variables is on prescribed levels and in the horizontal an Arakawa-E-type grid is used. The time discretization uses only two time levels. A simplified sea-ice model (including thermodynamics and dynamics) allows prognostic calculation of sea-ice thickness, compactness and velocity. HOPE is especially useful for altimetry data assimilation purposes because of the prognostic sea-surface elevation.
This is the first time a coupled ocean/sea-ice model will be used in the SO in eddy resolution. Despite restricting the area of interest to a specific sector of the Southern Ocean considerable computer resources will be needed to allow for sensitivity studies and extended integration times.
The resolution of the meso-scale eddy field will allow the investigation of its role in and around the sea-ice zone, and its effect on bottom water formation in a dynamically consistent framework. In addition, the role of the eddies on the mean circulation in the upwelling of heat and transport of salt at the Antarctic Divergence as well as across the ACC will be determined.
This project is aimed at testing, tuning and running a state of the art primitive equation ocean model (HOPE-Model). The first aim of this project is to test the sea-ice model and its interaction with the ocean. Initial problems with a too strong ice build-up in certain areas around Antarctica have been solved and we have completed a 100 year control run where ocean thermohaline fields have been relaxed to climatology with a time scale of about 150 days. The sea-ice model has been improved through consideration of a more complex radiation balance and the annual cycle of sea ice area compares very well with satellite observations with the exception of a too strong heatflux during July/August (see Figure 2). The effect of the heatflux mismatch can be seen in Figure 3 where large areas north-east of the Wedell Sea and north-west of the Ross Sea are basically ice free in contrast to observations.
The second aim of the project is to investigate the relationship between the rates and areas of water-mass formation, sea-ice distribution, deep water pathways to the north, heat- and freshwater fluxes in the SO and the dynamical balance of the ACC.
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Cooperative
Research Centre for Antarctica and the Southern Ocean
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