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Formation of the Terra Nova Bay polynya and climatic implications

Antarctica phase III (1993-1997)

Formation of the Terra Nova Bay polynya and climatic implications

Promotor

Professor André Berger
Université Catholique de Louvain
Institut d'Astronomie et de Géophysique G. Lemaître
Chemin du Cyclotron 2
B-1348 LOUVAIN-LA-NEUVE
Phone: +32 (0)10 47 33 03
Fax: +32 (0)10 47 47 22
E-mail: berger@astr.ucl.ac.be

Topics

A preliminary simulation of the Terra Nova Bay polynya was achieved by means of a coupled atmosphere-polynya model.

The three-dimensional Atmospheric Mesoscale Circulation Model (MAR) has been coupled to a wind-driven polynya model, generalized from that of Ou (1988), by including sea-ice dynamics and thermodynamics. The atmospheric model is a hydrostatic primitive equations model that has been validated previously by a simulation of the strong katabatic winds observed in this area. The polynya model includes a representation of the free drift of frazil ice and simple sea-ice dynamics and thermodynamics.

Two-dimensional and three-dimensional experiments have been performed under polar night conditions.

The atmospheric model sensitivity to the presence of a prescribed coastal polynya was analyzed by performing two-dimensional simulations. For sufficiently large areas of open water located in the coastal zone, a strong ice breeze is superimposed on the katabatic flow, causing subsidence of warm maritime air over the ice sheet and a subsequent intensification of the inversion and the katabatic wind. Nevertheless this has little impact on the polynya size predicted by the Pease (1987) model, since stronger katabatic winds reinforce the heat exchange between the polynya and the atmosphere. On the other hand the decay of katabatic winds over the ocean has an impact on the polynya size, owing to internal stresses in consolidated sea ice.

A simulation of the Terra Nova Bay polynya was also performed with the three-dimensional version of the model. The size of the simulated polynya is in qualitative agreement with the observations. As for the two-dimensional experiments, the katabatic flow is significantly reinforced over the ice-sheet slopes in the vicinity of the polynya (i.e. over Reeves Glacier). Observation and simulation of the katabatic wind in this area were in good agreement.

The simulated heat losses from the Terra Nova Bay polynya were shown to be larger by a factor 2 than in previous estimates, so that brine rejection into the ocean could also be more important than expected. Nevertheless processes like the consolidation of frazil ice into pancake ice are still poorly understood and not included in the model, leading to an overestimation of the simulated heat losses. Other missing processes, like frazil ice herding by wind waves, could have a strong impact on the polynya size. This stresses the need for having a better knowledge of the processes governing the frazil ice evolution in the Terra Nova Bay polynya, in order to develop new parameterizations.

Because it provides a realistic atmospheric forcing, the atmospheric model used here could be an useful tool for developing such parameterization.

References:

Ou H.W. (1988): A time-dependent model of coastal polynia,
J. Phys. Oceanogr., 18: 584-590.

Pease C.H. (1987): the size of wind-driven coastal polynias,
J. Geophys. Res., 92: 7049-7059.

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