We analyze the sensitivity of the ocean’s thermohaline circulation using ideal- ized models of the North Atlantic. The nonlinear Stommel (1961) box model is solved exactly. Using the analytical solution of a coupled atmosphere-ocean version of Stommel’s model, we derive the critical salinity perturbation, nec- essary to induce a break down of the thermohaline circulation . We find that the glacial oceanic circulation with a weaker meridional overturning is more sensitive than the present one. The model reveals furthermore that the optimal perturbation affecting long-term climate variability is by high lat- itude haline forcing, although this perturbation has little resemblence with the most unstable mode of the system and the leading EOF. The linear stage of the forecast error growth is analyzed in the box model. The amplification of the forecast error ellipsoide is largest for high latitude haline forcing, associated with maximum energy growth on times less than a decade. Sensitivity experiments with a more sophisticated coupled model reveal the basic mechanisms involved in the thermohaline circulation’s re- sponse to sea surface salinity perturbations. The results qualitatively agree with those of the analytical model, although different mechanisms are re- sponsible for the sensitivity. Our results are useful for the interpretation of paleoclimatic records and interdecadal climate variability
