This study uses a box model of the North Atlantic Ocean to test the stability of the thermohaline circulation under two different climatic regimes: a warmer regime where deepwater formation occurs through open ocean convection; and a cooler regime where deepwater formation occurs with the aid of brine rejection from sea ice formation. We find that the brine rejection mechanism produces a more stable modeled thermohaline circulation than the open ocean convection mechanism, i.e. a circulation which is able to withstand a larger high latitude freshwater perturbation. These results demonstrate that the effects of leads and polynyas on brine rejection rates are important and suggest that the presence of open water within modeled sea ice contributes significantly to the sensitivity of the climate response and cannot easily be ignored. ¿ 2001 American Geophysical Union