RESPONSE OF A COUPLED OCEAN ENERGY BALANCE MODEL TO RESTRICTED FLOW-THROUGH THE CENTRAL-AMERICAN ISTHMUS/

Prior ocean modeling work suggested that an open central American isth mus would cause a collapse of the North Atlantic thermohaline circulat ion because of free exchange of low salinity water between the Atlanti c and the Pacific. Geological data provide some support for this respo nse, but the data also indicate that some North Atlantic Deep Water fo rmation occurred before final closure of the isthmus. We previously po stulated that this ''early switch on'' could reflect a more limited ex change of Atlantic waters with the Pacific. In this study we discuss m odel sensitivity experiments testing that hypothesis and interpret the response in terms of shifts between multiple steady states of the mod el. 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 Ameri can isthmus is mimicked by locally changing the frictional drag coeffi cient in the ocean model. Results indicate that modest levels of throu ghflow can maintain some level of thermohaline circulation. These resu lts support the conjecture in our earlier study. However, the overturn ing cell is about 300 m shallower than in the control run, with deep w ater production nearly eliminated in the Labrador Sea. These latter re sponses should be testable with marine data.