Testing a Coupled Ice-Mixed-Layer Model Under Subarctic Conditions

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  • 1 Laboratoire d'Océanographie Dynamique et de Climatologie, Université Paris 6, Paris, France
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Abstract

A one-dimensional oceanic mixed-layer model has been coupled with a thermodynamic sea ice model in order to study the seasonal cycle of ice-ocean interaction in the subarctic ocean. The ice thickness is assumed constant and only variations of ice compactness under the effect of the atmospheric and oceanic heat fluxes are considered. The mixed-layer model allows prediction of the rate of entrainment, mainly as the result of the penetrative buoyancy-driven convection. Both heat and salt conservation equations are included so that the respective efficiency, with regard to vertical mixing, of brine rejection below the ice and intense surface cooling in open water can be compared.

The model has been run for ten years on a zonal vertical section across the Greenland Sea. The annual entrainment-retreat cycle of the mixed layer is well reproduced with maximum depths occurring in late winter, even down to the bottom at some locations near the ice edge. Surface cooling is found to be a more efficient mechanism, compared with brine rejection, in driving winter convection: as a result, mixed-layer deepening slows down as soon as surface freezing occurs and the mixed-layer depth tends to decrease as the ice concentration increases westward. The zonal profiles of the mixed-layer temperature are in good agreement with the seasonal climatological distributions. Finally, the model produces realistic ice concentration distributions in winter. The underestimated summer ice concentrations probably result from the oversimplified ice thermodynamics, since the ice distribution appears to be sensitive to the value prescribed as the ice thickness. A more detailed ice thickness distribution is expected to improve the estimation of the ice melting rate.

Abstract

A one-dimensional oceanic mixed-layer model has been coupled with a thermodynamic sea ice model in order to study the seasonal cycle of ice-ocean interaction in the subarctic ocean. The ice thickness is assumed constant and only variations of ice compactness under the effect of the atmospheric and oceanic heat fluxes are considered. The mixed-layer model allows prediction of the rate of entrainment, mainly as the result of the penetrative buoyancy-driven convection. Both heat and salt conservation equations are included so that the respective efficiency, with regard to vertical mixing, of brine rejection below the ice and intense surface cooling in open water can be compared.

The model has been run for ten years on a zonal vertical section across the Greenland Sea. The annual entrainment-retreat cycle of the mixed layer is well reproduced with maximum depths occurring in late winter, even down to the bottom at some locations near the ice edge. Surface cooling is found to be a more efficient mechanism, compared with brine rejection, in driving winter convection: as a result, mixed-layer deepening slows down as soon as surface freezing occurs and the mixed-layer depth tends to decrease as the ice concentration increases westward. The zonal profiles of the mixed-layer temperature are in good agreement with the seasonal climatological distributions. Finally, the model produces realistic ice concentration distributions in winter. The underestimated summer ice concentrations probably result from the oversimplified ice thermodynamics, since the ice distribution appears to be sensitive to the value prescribed as the ice thickness. A more detailed ice thickness distribution is expected to improve the estimation of the ice melting rate.

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