Prior ocean modeling work suggested that an open central American isthmus would cause a collapse of the North Atlantic thermohaline circulation because of free exchange of low salinity water between the Atlantic and the Pacific. Geological data provide some support for this response, but the data also indicate that some North Atlantic Deep Water formation occurred before final closure of the isthmus. We previously postulated that this early switch on could reflect a more limited exchange of Atlantic waters with the Pacific. In this study we discuss model sensitivity experiments testing that hypothesis and interpret the response in terms of shifts between multiple steady states of the model. Two simulations are conducted with a version of the Hamburg large-scale geostrophic ocean model that is coupled to an atmospheric energy balance model. Constrictions of throughflow through the central American isthmus is mimicked by locally changing the frictional drag coefficient in the ocean model. Results indicate that modest levels of throughflow can maintain some level of thermohaline circulation. These results support the conjecture in our earlier study. However, the overturning cell is about 300 m shallower than in the control run, with deep water production nearly eliminated in the Labrador Sea. These latter responses should be testable with marine data.¿ 1997 American Geophysical Union