SEA-ICE INTERACTION AND THE STABILITY OF THE THERMOHALINE CIRCULATION

The role of sea-ice in affecting the stability and long-term variabili ty of the oceanic thermohaline circulation (THC) is studied in this pa per. The emphasis is placed on studying how sea-ice might affect the s tability and the long-term variability of the THC through modulations of the surface heat and freshwater fluxes. A simple box model is analy zed to elucidate qualitatively the distinct physical meanings of these two processes. The analytical solution of this simple model indicates that, for the long timescales considered here, the thermal insulation stabilizes the THC while the freshwater feedback increases the effect ive inertia of the coupled ice-ocean system. Sea-ice insulation lessen s the negative feedback between heat flux and the SST, and therefore, allows the SST to play a greater role in counteracting changes of the THC and high latitude salinity field. The freshwater feedback effectiv ely links the surface heat flux to a freshwater reservoir, and thus, i ncreases the effective inertia of the coupled ocean-ice system. A two- dimensional ocean model coupled with a thermodynamic sea-ice model is used to estimate quantitatively the magnitudes of these two feedbacks. The numerical experiments involve the model's responses both to initi al anomalies and to changes of forcing fields. For the free response c ases (model responses to initial anomalies without changing the forcin g fields), the model shows that the decay rate of an initial anomaly i s greater when sea-ice is included. For small perturbations the therma l insulation effect dominates over the freshwater feedback. The latter becomes increasingly more important for larger perturbations. In resp onse to a change of external forcing, the presence of sea-ice reduces the magnitude and the pace of the model's response. The numerical resu lts are qualitatively consistent with the analytical solution of the b ox model.