A destabilizing thermohaline circulation-atmosphere-sea ice feedback

Some of the interactions and feedbacks between the atmosphere, thermohaline circulation, and sea ice are illustrated using a simple process model. A s implified version of the annual-mean coupled ocean-atmosphere box model of Nakamura, Stone, and Marotzke is modified to include a parameterization of sea ice. The model includes the thermodynamic effects of sea ice and allows for variable coverage. It is found that the addition of sea ice introduces feedbacks that have a destabilizing influence on the thermohaline circulat ion: Sea ice insulates the ocean from the atmosphere, creating colder air t emperatures at high latitudes, which cause larger atmospheric eddy heat and moisture transports and weaker oceanic hear transports. These in turn lead to thicker ice coverage and hence establish a positive feedback. The resul ts indicate that generally in colder climates, the presence of sea ice may lead to a significant destabilization of the thermohaline circulation. Brin e rejection by sea ice plays no important role in this model's dynamics. Th e net destabilizing effect of sea ice in this model is the result of two po sitive feedbacks and one negative feedback and is shown to be model depende nt. To date, the destabilizing feedback between atmospheric and oceanic hea t fluxes, mediated by sea ice, has largely been neglected in conceptual stu dies of thermohaline circulation stability, but it warrants further investi gation in more realistic models.