Principal Investigator Joerg-Olaf Wolff Project g22

Antarctic CRC, Machine VP

University of Tasmania

Co-Investigator Nathan Bindoff

Antarctic CRC, University of Tasmania

Southern Ocean and Atmosphere Process Model

Important problems exist in the parameterization of small scale processes in coarse resolution ocean and sea-ice components of coupled climate models. These problems are best studied using high resolution models in limited regions that can actually resolve these small scale processes. Such studies will allows us to characterize and develop more detailed parameterizations of these processes in order to improve the current ocean and sea-ice models.

A recent coupled coarse resolution GCM 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 Southern Ocean. 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 Southern Ocean (SO) Circulation, especially the Antarctic Circumpolar Current (ACC).

This balance is intimately linked to the existence of mesoscale 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 sequestering carbon dioxide and other tracers into the deep ocean.

We are using a state of the art primitive equation ocean model (HOPE, Hamburg Ocean Primitive Equation Model) at eddy resolution in a sector of the Southern Ocean south of Australia. The model was developed at the Max-Planck-Institute for Meteorology in Hamburg, FRG.

A modified Hibler-type sea-ice model is coupled to the HOPE model and allows prognostic calculation of sea-ice thickness, concentration and velocities. This is the first time a coupled ocean/sea-ice model will be used in the Southern ocean in eddy resolution. Even restricting the area of interest to a specific sector of the Southern Ocean means that considerable computer resources will be needed to allow for sensitivity studies and extended integration times.

What are the basic questions addressed?

The eddy resolution will allow the investigation of their role in and around the sea-ice zone, and their 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 resolved.

What are the results to date and the future of the work?

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. In the initial phase of the project we encountered difficulties with a too strong sea-ice buildup. This was basically in areas where we observe more than 100 meters of shelf ice. The topographic data set we use implies below sea level topography, which is consequently filled with ocean water in the model. The sea ice model cannot cope with ice thicknesses thicker than the upper most ocean layer and we have therefore adjusted the topography in these regions. The massive sea ice buildup increased the ocean salinity through brine release during freezing, which led to an unrealistically strong circulation in the model. We have run the ocean model in an uncoupled control run now for about 26 years (to overcome the initial drift, see Fig. 1) and will start sensitivity studies with the coupled model using this more realistic circulation as an initial condition (Fig. 2).

The second aim 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.

What computational techniques are used and why is a supercomputer required?

HOPE is a primitive equation model of the global ocean circulation, but may be also used for regional oceanographic studies. 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 velocities. HOPE is especially useful for altimetry data assimilation purposes because of the prognostic sea-surface elevation. The code has been specifically designed for vector processing computers. Run time tests at the German Climate Computer Center on a CRAY YMP have shown an average speed of 180 Mflops for this particular code with a resolution similar to that planned for these experiments on the VP2200. Vectorization testing software has indicated none of the code needs further optimization. Production runs on the VP2200 have a 91% (with sea-ice) and 97% (without sea-ice) vector unit usage.

Publications

Climatology and Variability in the ECHO Coupled GCM, Latif, M, T Stockdale, J-O Wolff, G Burgers, E Maier-Reimer, M M Junge, K Arpe and L Bengtsson, Tellus, 46(A), 367-380, (1994).

Some Sensitivities of a coupled Ocean-Atmosphere GCM, Stockdale, T, M Latif, G Burgers and J-O Wolff, Tellus, 46(A), 367-380, (1994).

Ocean Modelling Efforts in the Global Climate System, J-O Wolff, Aust. Met. Mag., 43 (1994), 263-281, (1994).

High Resolution Southern Ocean Sector Model, J-O Wolff, BMRC Report No. 42 on Modelling and observing sea ice in a coupled environment and Physical Oceanography, S Power (Ed), Melbourne, March 1994, Bureau of Meteorology, Dept. of the Environment, Sport and Territories, 29-30, (1994).

HOPE - The Hamburg Ocean Primitive Equation Model. Cycle 1, J-O Wolff, and E Maier-Reimer, German Climate Computer Center (DKRZ) Technical Report, (1994) in press.

Fig. 1: Time series of the mass transport through Drake Passage (identical to the transport between Australia and Antarctica) in Sverdrup. Recent observations indicate about 160 Sverdrup between Tasmania and Antarctica.

Fig. 2: Vertically integrated ocean circulation (barotropic stream function) in the Australian sector of the Southern Ocean after 26 years of model integration.