A DECADAL OSCILLATION DUE TO THE COUPLING BETWEEN AN OCEAN CIRCULATION MODEL AND A THERMODYNAMIC SEA-ICE MODEL

A 3-dimensional, planetary-geostrophic, ocean general circulation mode l is coupled to a thermodynamic sea-ice model. The thermal coupling ta kes account of the insulating effect of the ice. A simple approach is taken in the case of the freshwater flux by allowing this to pass thro ugh the ice, except that some is used for snow accumulation. It is the n modified by salinity rejection/dilution due to freezing/melting. The model has idealized box geometry extending 60 degrees in both latitud e and longitude, with a horizontal resolution of 2 degrees and 14 vert ical levels. Annual mean surface forcings are used. The coupled system is first spun up using restoring conditions on both surface temperatu re and surface salinity to reach a steady state which includes ice in the high latitudes. A switch of the surface forcing to mixed boundary conditions (restoring on temperature and flux on salinity) leads to an oscillation of period 17 years in the magnitude of the thermohaline c irculation and the ice extent. The oscillation is due to a feedback be tween ice cover and ocean temperature. Since ice forms only in regions where the ocean loses heat to the atmosphere, the thermal insulation of an increased ice cover makes the ocean warmer. The thermohaline cir culation plays a role in transporting this heat polewards, which in tu rn melts the ice. The heat loss over open water at high latitudes then leads to ice formation and the process repeats itself. Salinity rejec tion/dilution associated with ice formation/melting is shown to be of secondary importance in this oscillation. Rather, changes in surface s alinity are dominated by changes in deep convection and the associated vertical mixing, which are themselves associated with the reduction i n surface heat loss due to the insulating effect of the ice. As a cons equence the model exhibits the negative correlation between surface sa linity and ice extent that is observed in the high latitude North Atla ntic.